TextView显示丰富多彩的文字（四）——从源码角度解释格式化显示的原理

Layout

Layout是一个抽象类，负责管理文字布局。Layout有三个实现类：BoringLayout、StaticLayout、DynamicLayout。BoringLayout布局用于显示一行文字并且字符从左至右。StaticLayout用于文字布局之后不会再改变。DynamicLayout用于文本编辑时可以自己更新显示效果。并且这三个类均说明了最好不要直接使用这些类，除非你想实现自己显示的控件。

TextView的布局

控件的布局受到父布局和自身设置的共同限制。下面分情况讨论：
- 如果TextView的高度和宽度都是定尺寸，那么不管setText传入的什么内容，都不会重新布局，因为尺寸定死了。
- 如果TextView的宽度是定尺寸，而高度是wrap_content，那么如果在setText重新设置内容之前，高度是两行高度，那么当重新设置的内容如果高于两行或者低于两行，都应该重新布局的，这时会要求父布局重新布局。
- 如果TextView的宽度wrap_content，高度是定尺寸，那么一旦宽度发生变化，就会要求父布局重新布局。

setText

TextView的setText有多个重载，但是最终都会进入到下面这个方法

 private void setText(CharSequence text, BufferType type,
                         boolean notifyBefore, int oldlen) {
        if (text == null) {
            text = "";
        }

        // If suggestions are not enabled, remove the suggestion spans from the text
        if (!isSuggestionsEnabled()) {
            text = removeSuggestionSpans(text);
        }

        if (!mUserSetTextScaleX) mTextPaint.setTextScaleX(1.0f);

        if (text instanceof Spanned &&
            ((Spanned) text).getSpanStart(TextUtils.TruncateAt.MARQUEE) >= 0) {
            if (ViewConfiguration.get(mContext).isFadingMarqueeEnabled()) {
                setHorizontalFadingEdgeEnabled(true);
                mMarqueeFadeMode = MARQUEE_FADE_NORMAL;
            } else {
                setHorizontalFadingEdgeEnabled(false);
                mMarqueeFadeMode = MARQUEE_FADE_SWITCH_SHOW_ELLIPSIS;
            }
            setEllipsize(TextUtils.TruncateAt.MARQUEE);
        }

        int n = mFilters.length;
        for (int i = 0; i < n; i++) {
            CharSequence out = mFilters[i].filter(text, 0, text.length(), EMPTY_SPANNED, 0, 0);
            if (out != null) {
                text = out;
            }
        }

        if (notifyBefore) {
            if (mText != null) {
                oldlen = mText.length();
                sendBeforeTextChanged(mText, 0, oldlen, text.length());
            } else {
                sendBeforeTextChanged("", 0, 0, text.length());
            }
        }

        boolean needEditableForNotification = false;

        if (mListeners != null && mListeners.size() != 0) {
            needEditableForNotification = true;
        }

        if (type == BufferType.EDITABLE || getKeyListener() != null ||
                needEditableForNotification) {
            createEditorIfNeeded();
            mEditor.forgetUndoRedo();
            Editable t = mEditableFactory.newEditable(text);
            text = t;
            setFilters(t, mFilters);
            InputMethodManager imm = InputMethodManager.peekInstance();
            if (imm != null) imm.restartInput(this);
        } else if (type == BufferType.SPANNABLE || mMovement != null) {
            text = mSpannableFactory.newSpannable(text);
        } else if (!(text instanceof CharWrapper)) {
            text = TextUtils.stringOrSpannedString(text);
        }

        if (mAutoLinkMask != 0) {
            Spannable s2;

            if (type == BufferType.EDITABLE || text instanceof Spannable) {
                s2 = (Spannable) text;
            } else {
                s2 = mSpannableFactory.newSpannable(text);
            }

            if (Linkify.addLinks(s2, mAutoLinkMask)) {
                text = s2;
                type = (type == BufferType.EDITABLE) ? BufferType.EDITABLE : BufferType.SPANNABLE;

                /*
                 * We must go ahead and set the text before changing the
                 * movement method, because setMovementMethod() may call
                 * setText() again to try to upgrade the buffer type.
                 */
                mText = text;

                // Do not change the movement method for text that support text selection as it
                // would prevent an arbitrary cursor displacement.
                if (mLinksClickable && !textCanBeSelected()) {
                    setMovementMethod(LinkMovementMethod.getInstance());
                }
            }
        }

        mBufferType = type;
        mText = text;

        if (mTransformation == null) {
            mTransformed = text;
        } else {
            mTransformed = mTransformation.getTransformation(text, this);
        }

        final int textLength = text.length();

        if (text instanceof Spannable && !mAllowTransformationLengthChange) {
            Spannable sp = (Spannable) text;

            // Remove any ChangeWatchers that might have come from other TextViews.
            final ChangeWatcher[] watchers = sp.getSpans(0, sp.length(), ChangeWatcher.class);
            final int count = watchers.length;
            for (int i = 0; i < count; i++) {
                sp.removeSpan(watchers[i]);
            }

            if (mChangeWatcher == null) mChangeWatcher = new ChangeWatcher();

            sp.setSpan(mChangeWatcher, 0, textLength, Spanned.SPAN_INCLUSIVE_INCLUSIVE |
                       (CHANGE_WATCHER_PRIORITY << Spanned.SPAN_PRIORITY_SHIFT));

            if (mEditor != null) mEditor.addSpanWatchers(sp);

            if (mTransformation != null) {
                sp.setSpan(mTransformation, 0, textLength, Spanned.SPAN_INCLUSIVE_INCLUSIVE);
            }

            if (mMovement != null) {
                mMovement.initialize(this, (Spannable) text);

                /*
                 * Initializing the movement method will have set the
                 * selection, so reset mSelectionMoved to keep that from
                 * interfering with the normal on-focus selection-setting.
                 */
                if (mEditor != null) mEditor.mSelectionMoved = false;
            }
        }

        if (mLayout != null) {
            checkForRelayout();
        }

        sendOnTextChanged(text, 0, oldlen, textLength);
        onTextChanged(text, 0, oldlen, textLength);

        notifyViewAccessibilityStateChangedIfNeeded(AccessibilityEvent.CONTENT_CHANGE_TYPE_TEXT);

        if (needEditableForNotification) {
            sendAfterTextChanged((Editable) text);
        }

        // SelectionModifierCursorController depends on textCanBeSelected, which depends on text
        if (mEditor != null) mEditor.prepareCursorControllers();
    }

setText负责将text进行转换，并且如果设置了TextWatch监听器，那么需要调用接口。其中70-97行的代码是用于XML中如果设置了autoLink，那么需要对text进行处理，可以看到首先将text转换为Spannable，然后再调用Linkify.addLinks方法为text添加效果，最后会自动调用setMovementMethod，所以一个autoLink属性就解决所有问题了。下面是addLinks的方法实现：

public static final boolean addLinks(Spannable text, int mask) {
        if (mask == 0) {
            return false;
        }

        URLSpan[] old = text.getSpans(0, text.length(), URLSpan.class);

        for (int i = old.length - 1; i >= 0; i--) {
            text.removeSpan(old[i]);
        }

        ArrayList links = new ArrayList();

        if ((mask & WEB_URLS) != 0) {
            gatherLinks(links, text, Patterns.WEB_URL,
                new String[] { "http://", "https://", "rtsp://" },
                sUrlMatchFilter, null);
        }

        if ((mask & EMAIL_ADDRESSES) != 0) {
            gatherLinks(links, text, Patterns.EMAIL_ADDRESS,
                new String[] { "mailto:" },
                null, null);
        }

        if ((mask & PHONE_NUMBERS) != 0) {
            gatherTelLinks(links, text);
        }

        if ((mask & MAP_ADDRESSES) != 0) {
            gatherMapLinks(links, text);
        }

        pruneOverlaps(links);

        if (links.size() == 0) {
            return false;
        }

        for (LinkSpec link: links) {
            applyLink(link.url, link.start, link.end, text);
        }

        return true;
    }

该方法扫描文本然后将其中的链接更改显示效果并添加点击事件。从方法中可以看出，首先移除text中已经设置的URLSpan，也就是说如果即通过了代码添加了URLSpan，也在属性中使用了autoLink属性，那么代码中的添加的URLSpan全部失效。接下来就是判断autoLink的类型，扫描出符合条件的LinkSpec，再加入到列表中；然后再调用pruneOverlapas用于删除重复的效果；最后调用applyLink产生效果。下面是gatherLinks方法实现：

 private static final void gatherLinks(ArrayList links,
            Spannable s, Pattern pattern, String[] schemes,
            MatchFilter matchFilter, TransformFilter transformFilter) {
        Matcher m = pattern.matcher(s);

        while (m.find()) {
            int start = m.start();
            int end = m.end();

            if (matchFilter == null || matchFilter.acceptMatch(s, start, end)) {
                LinkSpec spec = new LinkSpec();
                String url = makeUrl(m.group(0), schemes, m, transformFilter);

                spec.url = url;
                spec.start = start;
                spec.end = end;

                links.add(spec);
            }
        }
    }

可以看到该方法就是调用正则表达式匹配符合条件的链接，然后加入到列表中。其中LinkSpec类的定义如下：

class LinkSpec {
    String url;    
    int start;
    int end;
}

LinkSpec就是封装了URL、以及text中的开启位置和结束位置。下面是applyLink方法的实现，该方法用于将找到的LinkSpec真正产生效果：

 private static final void applyLink(String url, int start, int end, Spannable text) {
        URLSpan span = new URLSpan(url);

        text.setSpan(span, start, end, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
    }

可以看到该方法也是通过给text设置URLSpan实现效果的，所以最终的效果都是依赖URLSpan实现的。

onDraw

Spannable的各种效果，最终都是通过onDraw方法绘制出来的，下面是onDraw()方法的实现。

@Override
    protected void onDraw(Canvas canvas) {
        restartMarqueeIfNeeded();

        // Draw the background for this view
        super.onDraw(canvas);

        final int compoundPaddingLeft = getCompoundPaddingLeft();
        final int compoundPaddingTop = getCompoundPaddingTop();
        final int compoundPaddingRight = getCompoundPaddingRight();
        final int compoundPaddingBottom = getCompoundPaddingBottom();
        final int scrollX = mScrollX;
        final int scrollY = mScrollY;
        final int right = mRight;
        final int left = mLeft;
        final int bottom = mBottom;
        final int top = mTop;
        final boolean isLayoutRtl = isLayoutRtl();
        final int offset = getHorizontalOffsetForDrawables();
        final int leftOffset = isLayoutRtl ? 0 : offset;
        final int rightOffset = isLayoutRtl ? offset : 0 ;

        final Drawables dr = mDrawables;
        if (dr != null) {
            /*
             * Compound, not extended, because the icon is not clipped
             * if the text height is smaller.
             */

            int vspace = bottom - top - compoundPaddingBottom - compoundPaddingTop;
            int hspace = right - left - compoundPaddingRight - compoundPaddingLeft;

            // IMPORTANT: The coordinates computed are also used in invalidateDrawable()
            // Make sure to update invalidateDrawable() when changing this code.
            if (dr.mShowing[Drawables.LEFT] != null) {
                canvas.save();
                canvas.translate(scrollX + mPaddingLeft + leftOffset,
                                 scrollY + compoundPaddingTop +
                                 (vspace - dr.mDrawableHeightLeft) / 2);
                dr.mShowing[Drawables.LEFT].draw(canvas);
                canvas.restore();
            }

            // IMPORTANT: The coordinates computed are also used in invalidateDrawable()
            // Make sure to update invalidateDrawable() when changing this code.
            if (dr.mShowing[Drawables.RIGHT] != null) {
                canvas.save();
                canvas.translate(scrollX + right - left - mPaddingRight
                        - dr.mDrawableSizeRight - rightOffset,
                         scrollY + compoundPaddingTop + (vspace - dr.mDrawableHeightRight) / 2);
                dr.mShowing[Drawables.RIGHT].draw(canvas);
                canvas.restore();
            }

            // IMPORTANT: The coordinates computed are also used in invalidateDrawable()
            // Make sure to update invalidateDrawable() when changing this code.
            if (dr.mShowing[Drawables.TOP] != null) {
                canvas.save();
                canvas.translate(scrollX + compoundPaddingLeft +
                        (hspace - dr.mDrawableWidthTop) / 2, scrollY + mPaddingTop);
                dr.mShowing[Drawables.TOP].draw(canvas);
                canvas.restore();
            }

            // IMPORTANT: The coordinates computed are also used in invalidateDrawable()
            // Make sure to update invalidateDrawable() when changing this code.
            if (dr.mShowing[Drawables.BOTTOM] != null) {
                canvas.save();
                canvas.translate(scrollX + compoundPaddingLeft +
                        (hspace - dr.mDrawableWidthBottom) / 2,
                         scrollY + bottom - top - mPaddingBottom - dr.mDrawableSizeBottom);
                dr.mShowing[Drawables.BOTTOM].draw(canvas);
                canvas.restore();
            }
        }

        int color = mCurTextColor;

        if (mLayout == null) {
            assumeLayout();
        }

        Layout layout = mLayout;

        if (mHint != null && mText.length() == 0) {
            if (mHintTextColor != null) {
                color = mCurHintTextColor;
            }

            layout = mHintLayout;
        }

        mTextPaint.setColor(color);
        mTextPaint.drawableState = getDrawableState();

        canvas.save();
        /*  Would be faster if we didn't have to do this. Can we chop the
            (displayable) text so that we don't need to do this ever?
        */

        int extendedPaddingTop = getExtendedPaddingTop();
        int extendedPaddingBottom = getExtendedPaddingBottom();

        final int vspace = mBottom - mTop - compoundPaddingBottom - compoundPaddingTop;
        final int maxScrollY = mLayout.getHeight() - vspace;

        float clipLeft = compoundPaddingLeft + scrollX;
        float clipTop = (scrollY == 0) ? 0 : extendedPaddingTop + scrollY;
        float clipRight = right - left - getFudgedPaddingRight() + scrollX;
        float clipBottom = bottom - top + scrollY -
                ((scrollY == maxScrollY) ? 0 : extendedPaddingBottom);

        if (mShadowRadius != 0) {
            clipLeft += Math.min(0, mShadowDx - mShadowRadius);
            clipRight += Math.max(0, mShadowDx + mShadowRadius);

            clipTop += Math.min(0, mShadowDy - mShadowRadius);
            clipBottom += Math.max(0, mShadowDy + mShadowRadius);
        }

        canvas.clipRect(clipLeft, clipTop, clipRight, clipBottom);

        int voffsetText = 0;
        int voffsetCursor = 0;

        // translate in by our padding
        /* shortcircuit calling getVerticaOffset() */
        if ((mGravity & Gravity.VERTICAL_GRAVITY_MASK) != Gravity.TOP) {
            voffsetText = getVerticalOffset(false);
            voffsetCursor = getVerticalOffset(true);
        }
        canvas.translate(compoundPaddingLeft, extendedPaddingTop + voffsetText);

        final int layoutDirection = getLayoutDirection();
        final int absoluteGravity = Gravity.getAbsoluteGravity(mGravity, layoutDirection);
        if (mEllipsize == TextUtils.TruncateAt.MARQUEE &&
                mMarqueeFadeMode != MARQUEE_FADE_SWITCH_SHOW_ELLIPSIS) {
            if (!mSingleLine && getLineCount() == 1 && canMarquee() &&
                    (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) != Gravity.LEFT) {
                final int width = mRight - mLeft;
                final int padding = getCompoundPaddingLeft() + getCompoundPaddingRight();
                final float dx = mLayout.getLineRight(0) - (width - padding);
                canvas.translate(layout.getParagraphDirection(0) * dx, 0.0f);
            }

            if (mMarquee != null && mMarquee.isRunning()) {
                final float dx = -mMarquee.getScroll();
                canvas.translate(layout.getParagraphDirection(0) * dx, 0.0f);
            }
        }

        final int cursorOffsetVertical = voffsetCursor - voffsetText;

        Path highlight = getUpdatedHighlightPath();
        if (mEditor != null) {
            mEditor.onDraw(canvas, layout, highlight, mHighlightPaint, cursorOffsetVertical);
        } else {
            layout.draw(canvas, highlight, mHighlightPaint, cursorOffsetVertical);
        }

        if (mMarquee != null && mMarquee.shouldDrawGhost()) {
            final float dx = mMarquee.getGhostOffset();
            canvas.translate(layout.getParagraphDirection(0) * dx, 0.0f);
            layout.draw(canvas, highlight, mHighlightPaint, cursorOffsetVertical);
        }

        canvas.restore();
    }

TextView是可以设置Drawable，上述代码首先先绘制Drawable。然后获取Layout对象，该对象用于绘制文本。接下来就是各种控制属性的计算，最后在409-413行会绘制文本，并且通过区分mEditor是不是为null来判断，TextView中mEditor是为null的，而EditText中mEditor是不为空的，所以TextView的绘制最终是通过Layout的draw方法绘制的，而EditText是通过Editor的绘制形成的。最终是判断是不是处于“跑马灯”的设置（暂且不考虑）。下面是assumeLayout方法，该方法根据文本内容返回Layout对象。

/**
     * Make a new Layout based on the already-measured size of the view,
     * on the assumption that it was measured correctly at some point.
     */
    private void assumeLayout() {
        int width = mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight();

        if (width < 1) {
            width = 0;
        }

        int physicalWidth = width;

        if (mHorizontallyScrolling) {
            width = VERY_WIDE;
        }

        makeNewLayout(width, physicalWidth, UNKNOWN_BORING, UNKNOWN_BORING,
                      physicalWidth, false);
    }

可以看到该方法又是调用了makeNewLayout方法，而makeNewLayout的关键方法是makeSingleLayout，下面是makeSingleLayout方法的实现：

 private Layout makeSingleLayout(int wantWidth, BoringLayout.Metrics boring, int ellipsisWidth,
            Layout.Alignment alignment, boolean shouldEllipsize, TruncateAt effectiveEllipsize,
            boolean useSaved) {
        Layout result = null;
        if (mText instanceof Spannable) {
            result = new DynamicLayout(mText, mTransformed, mTextPaint, wantWidth,
                    alignment, mTextDir, mSpacingMult, mSpacingAdd, mIncludePad,
                    mBreakStrategy, mHyphenationFrequency,
                    getKeyListener() == null ? effectiveEllipsize : null, ellipsisWidth);
        } else {
            if (boring == UNKNOWN_BORING) {
                boring = BoringLayout.isBoring(mTransformed, mTextPaint, mTextDir, mBoring);
                if (boring != null) {
                    mBoring = boring;
                }
            }

            if (boring != null) {
                if (boring.width <= wantWidth &&
                        (effectiveEllipsize == null || boring.width <= ellipsisWidth)) {
                    if (useSaved && mSavedLayout != null) {
                        result = mSavedLayout.replaceOrMake(mTransformed, mTextPaint,
                                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                                boring, mIncludePad);
                    } else {
                        result = BoringLayout.make(mTransformed, mTextPaint,
                                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                                boring, mIncludePad);
                    }

                    if (useSaved) {
                        mSavedLayout = (BoringLayout) result;
                    }
                } else if (shouldEllipsize && boring.width <= wantWidth) {
                    if (useSaved && mSavedLayout != null) {
                        result = mSavedLayout.replaceOrMake(mTransformed, mTextPaint,
                                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                                boring, mIncludePad, effectiveEllipsize,
                                ellipsisWidth);
                    } else {
                        result = BoringLayout.make(mTransformed, mTextPaint,
                                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                                boring, mIncludePad, effectiveEllipsize,
                                ellipsisWidth);
                    }
                }
            }
        }
        if (result == null) {
            StaticLayout.Builder builder = StaticLayout.Builder.obtain(mTransformed,
                    0, mTransformed.length(), mTextPaint, wantWidth)
                    .setAlignment(alignment)
                    .setTextDirection(mTextDir)
                    .setLineSpacing(mSpacingAdd, mSpacingMult)
                    .setIncludePad(mIncludePad)
                    .setBreakStrategy(mBreakStrategy)
                    .setHyphenationFrequency(mHyphenationFrequency);
            if (shouldEllipsize) {
                builder.setEllipsize(effectiveEllipsize)
                        .setEllipsizedWidth(ellipsisWidth)
                        .setMaxLines(mMaxMode == LINES ? mMaximum : Integer.MAX_VALUE);
            }
            // TODO: explore always setting maxLines
            result = builder.build();
        }
        return result;
    }

可以看到根据不同的情况，返回不同的Layout具体实现。如果TextView目前显示的文本是Spannable类型的话，那么返回DynamicLayout；如果文本不是Spannable类型的话，并且是boring的话，那么返回BoringLayout;如果文本既不是Spannable类型的话也不是boring的话，那么返回StaticLayout。那么什么样的文本是boring呢？根据BoringLayout文档来说，就是可以在一行内显示的文本并且文本方向是从左向右的，那么就是Boring的，否则就不是。那么下面总结一下，什么情况下会返回什么Layout。
- 如果文本属于Spannable类型，那么返回DynamicLayout；
- 如果文本不属于Spannable类型，并且文本可以在一行内显示完并且顺序从左至右，那么返回BoringLayout；
- 如果文本不属于Spannable类型，文本需多行显示或文本方向是从右至左，那么返回StaticLayout。

从前面知道TextView的绘制最终是通过调用layout的draw方法，因为我们考虑的文本是Spannbale类型的，所以直接看DynamicLayout的draw方法，而DynamicLayout没有覆写父类的draw方法，Layout有几个draw方法，但最终都是调用下面的draw方法，下面是Layout的draw方法实现：

/**
     * Draw this Layout on the specified canvas, with the highlight path drawn
     * between the background and the text.
     *
     * @param canvas the canvas
     * @param highlight the path of the highlight or cursor; can be null
     * @param highlightPaint the paint for the highlight
     * @param cursorOffsetVertical the amount to temporarily translate the
     *        canvas while rendering the highlight
     */
    public void draw(Canvas canvas, Path highlight, Paint highlightPaint,
            int cursorOffsetVertical) {
        final long lineRange = getLineRangeForDraw(canvas);
        int firstLine = TextUtils.unpackRangeStartFromLong(lineRange);
        int lastLine = TextUtils.unpackRangeEndFromLong(lineRange);
        if (lastLine < 0) return;

        drawBackground(canvas, highlight, highlightPaint, cursorOffsetVertical,
                firstLine, lastLine);
        drawText(canvas, firstLine, lastLine);
    }

可以看到draw方法就是获取绘制文本的行数，然后得到第一行和最后一行，先去画背景，再去画文字。下面首先看画背景的drawBackground方法：

 /**
     * @hide
     */
    public void drawBackground(Canvas canvas, Path highlight, Paint highlightPaint,
            int cursorOffsetVertical, int firstLine, int lastLine) {
        // First, draw LineBackgroundSpans.
        // LineBackgroundSpans know nothing about the alignment, margins, or
        // direction of the layout or line.  XXX: Should they?
        // They are evaluated at each line.
        if (mSpannedText) {
            if (mLineBackgroundSpans == null) {
                mLineBackgroundSpans = new SpanSet(LineBackgroundSpan.class);
            }

            Spanned buffer = (Spanned) mText;
            int textLength = buffer.length();
            mLineBackgroundSpans.init(buffer, 0, textLength);

            if (mLineBackgroundSpans.numberOfSpans > 0) {
                int previousLineBottom = getLineTop(firstLine);
                int previousLineEnd = getLineStart(firstLine);
                ParagraphStyle[] spans = NO_PARA_SPANS;
                int spansLength = 0;
                TextPaint paint = mPaint;
                int spanEnd = 0;
                final int width = mWidth;
                for (int i = firstLine; i <= lastLine; i++) {
                    int start = previousLineEnd;
                    int end = getLineStart(i + 1);
                    previousLineEnd = end;

                    int ltop = previousLineBottom;
                    int lbottom = getLineTop(i + 1);
                    previousLineBottom = lbottom;
                    int lbaseline = lbottom - getLineDescent(i);

                    if (start >= spanEnd) {
                        // These should be infrequent, so we'll use this so that
                        // we don't have to check as often.
                        spanEnd = mLineBackgroundSpans.getNextTransition(start, textLength);
                        // All LineBackgroundSpans on a line contribute to its background.
                        spansLength = 0;
                        // Duplication of the logic of getParagraphSpans
                        if (start != end || start == 0) {
                            // Equivalent to a getSpans(start, end), but filling the 'spans' local
                            // array instead to reduce memory allocation
                            for (int j = 0; j < mLineBackgroundSpans.numberOfSpans; j++) {
                                // equal test is valid since both intervals are not empty by
                                // construction
                                if (mLineBackgroundSpans.spanStarts[j] >= end ||
                                        mLineBackgroundSpans.spanEnds[j] <= start) continue;
                                spans = GrowingArrayUtils.append(
                                        spans, spansLength, mLineBackgroundSpans.spans[j]);
                                spansLength++;
                            }
                        }
                    }

                    for (int n = 0; n < spansLength; n++) {
                        LineBackgroundSpan lineBackgroundSpan = (LineBackgroundSpan) spans[n];
                        lineBackgroundSpan.drawBackground(canvas, paint, 0, width,
                                ltop, lbaseline, lbottom,
                                buffer, start, end, i);
                    }
                }
            }
            mLineBackgroundSpans.recycle();
        }

        // There can be a highlight even without spans if we are drawing
        // a non-spanned transformation of a spanned editing buffer.
        if (highlight != null) {
            if (cursorOffsetVertical != 0) canvas.translate(0, cursorOffsetVertical);
            canvas.drawPath(highlight, highlightPaint);
            if (cursorOffsetVertical != 0) canvas.translate(0, -cursorOffsetVertical);
        }
    }

从注释中可以看到，首先绘制LineBackgrounSpan。首先判断是否是Spannable类型，如果是，那么绘制LineBackgrounSpan。而具体的绘制工作又是LineBackgroundSpan的drawBackgroun方法，在TextView显示丰富多彩的文字（二）——如何使用ParagraphStyle格式化段落中，我们知道Android并没有提供LineBackgroundSpan的实现类，所以这段代码基本是不会做什么的。下面再判断是否有高亮，绘制高亮。总结一下，在现有的代码中，drawBackground基本是没有绘制任何东西的。下面是drawText方法的实现，drawText用于绘制文字：

 /**
     * @hide
     */
    public void drawText(Canvas canvas, int firstLine, int lastLine) {
        int previousLineBottom = getLineTop(firstLine);
        int previousLineEnd = getLineStart(firstLine);
        ParagraphStyle[] spans = NO_PARA_SPANS;
        int spanEnd = 0;
        TextPaint paint = mPaint;
        CharSequence buf = mText;

        Alignment paraAlign = mAlignment;
        TabStops tabStops = null;
        boolean tabStopsIsInitialized = false;

        TextLine tl = TextLine.obtain();

        // Draw the lines, one at a time.
        // The baseline is the top of the following line minus the current line's descent.
        for (int lineNum = firstLine; lineNum <= lastLine; lineNum++) {
            int start = previousLineEnd;
            previousLineEnd = getLineStart(lineNum + 1);
            int end = getLineVisibleEnd(lineNum, start, previousLineEnd);

            int ltop = previousLineBottom;
            int lbottom = getLineTop(lineNum + 1);
            previousLineBottom = lbottom;
            int lbaseline = lbottom - getLineDescent(lineNum);

            int dir = getParagraphDirection(lineNum);
            int left = 0;
            int right = mWidth;

            if (mSpannedText) {
                Spanned sp = (Spanned) buf;
                int textLength = buf.length();
                boolean isFirstParaLine = (start == 0 || buf.charAt(start - 1) == '\n');

                // New batch of paragraph styles, collect into spans array.
                // Compute the alignment, last alignment style wins.
                // Reset tabStops, we'll rebuild if we encounter a line with
                // tabs.
                // We expect paragraph spans to be relatively infrequent, use
                // spanEnd so that we can check less frequently.  Since
                // paragraph styles ought to apply to entire paragraphs, we can
                // just collect the ones present at the start of the paragraph.
                // If spanEnd is before the end of the paragraph, that's not
                // our problem.
                if (start >= spanEnd && (lineNum == firstLine || isFirstParaLine)) {
                    spanEnd = sp.nextSpanTransition(start, textLength,
                                                    ParagraphStyle.class);
                    spans = getParagraphSpans(sp, start, spanEnd, ParagraphStyle.class);

                    paraAlign = mAlignment;
                    for (int n = spans.length - 1; n >= 0; n--) {
                        if (spans[n] instanceof AlignmentSpan) {
                            paraAlign = ((AlignmentSpan) spans[n]).getAlignment();
                            break;
                        }
                    }

                    tabStopsIsInitialized = false;
                }

                // Draw all leading margin spans.  Adjust left or right according
                // to the paragraph direction of the line.
                final int length = spans.length;
                boolean useFirstLineMargin = isFirstParaLine;
                for (int n = 0; n < length; n++) {
                    if (spans[n] instanceof LeadingMarginSpan2) {
                        int count = ((LeadingMarginSpan2) spans[n]).getLeadingMarginLineCount();
                        int startLine = getLineForOffset(sp.getSpanStart(spans[n]));
                        // if there is more than one LeadingMarginSpan2, use
                        // the count that is greatest
                        if (lineNum < startLine + count) {
                            useFirstLineMargin = true;
                            break;
                        }
                    }
                }
                for (int n = 0; n < length; n++) {
                    if (spans[n] instanceof LeadingMarginSpan) {
                        LeadingMarginSpan margin = (LeadingMarginSpan) spans[n];
                        if (dir == DIR_RIGHT_TO_LEFT) {
                            margin.drawLeadingMargin(canvas, paint, right, dir, ltop,
                                                     lbaseline, lbottom, buf,
                                                     start, end, isFirstParaLine, this);
                            right -= margin.getLeadingMargin(useFirstLineMargin);
                        } else {
                            margin.drawLeadingMargin(canvas, paint, left, dir, ltop,
                                                     lbaseline, lbottom, buf,
                                                     start, end, isFirstParaLine, this);
                            left += margin.getLeadingMargin(useFirstLineMargin);
                        }
                    }
                }
            }

            boolean hasTabOrEmoji = getLineContainsTab(lineNum);
            // Can't tell if we have tabs for sure, currently
            if (hasTabOrEmoji && !tabStopsIsInitialized) {
                if (tabStops == null) {
                    tabStops = new TabStops(TAB_INCREMENT, spans);
                } else {
                    tabStops.reset(TAB_INCREMENT, spans);
                }
                tabStopsIsInitialized = true;
            }

            // Determine whether the line aligns to normal, opposite, or center.
            Alignment align = paraAlign;
            if (align == Alignment.ALIGN_LEFT) {
                align = (dir == DIR_LEFT_TO_RIGHT) ?
                    Alignment.ALIGN_NORMAL : Alignment.ALIGN_OPPOSITE;
            } else if (align == Alignment.ALIGN_RIGHT) {
                align = (dir == DIR_LEFT_TO_RIGHT) ?
                    Alignment.ALIGN_OPPOSITE : Alignment.ALIGN_NORMAL;
            }

            int x;
            if (align == Alignment.ALIGN_NORMAL) {
                if (dir == DIR_LEFT_TO_RIGHT) {
                    x = left + getIndentAdjust(lineNum, Alignment.ALIGN_LEFT);
                } else {
                    x = right + getIndentAdjust(lineNum, Alignment.ALIGN_RIGHT);
                }
            } else {
                int max = (int)getLineExtent(lineNum, tabStops, false);
                if (align == Alignment.ALIGN_OPPOSITE) {
                    if (dir == DIR_LEFT_TO_RIGHT) {
                        x = right - max + getIndentAdjust(lineNum, Alignment.ALIGN_RIGHT);
                    } else {
                        x = left - max + getIndentAdjust(lineNum, Alignment.ALIGN_LEFT);
                    }
                } else { // Alignment.ALIGN_CENTER
                    max = max & ~1;
                    x = ((right + left - max) >> 1) +
                            getIndentAdjust(lineNum, Alignment.ALIGN_CENTER);
                }
            }

            paint.setHyphenEdit(getHyphen(lineNum));
            Directions directions = getLineDirections(lineNum);
            if (directions == DIRS_ALL_LEFT_TO_RIGHT && !mSpannedText && !hasTabOrEmoji) {
                // XXX: assumes there's nothing additional to be done
                canvas.drawText(buf, start, end, x, lbaseline, paint);
            } else {
                tl.set(paint, buf, start, end, dir, directions, hasTabOrEmoji, tabStops);
                tl.draw(canvas, x, ltop, lbaseline, lbottom);
            }
            paint.setHyphenEdit(0);
        }

        TextLine.recycle(tl);
    }

关键部分是for循环，一次绘制一行，内部如果是Spannable类型的文本，需要进行特别处理。具体的操作是首先获取所有ParagraphStyle对象，然后绘制。首先获取LeadingMarginSpan2对象，判断当前行是否具备第一行的属性，然后再根据是否是第一行，调用LeadingMarginSpan计算文本绘制的起始位置。不了解这两个对象的可以参考TextView显示丰富多彩的文字（三）——自定义CharacterStyle和ParagraphStyle显示效果。接下来判断有没有tab或者emoji。如果有，就会创建TabStops对象，接下来再决定这一行是靠右对齐还是居中对齐，还是靠左对齐。最后是绘制一行，又分为两种情况，如果方向是从左至右并且不是Spannable类型且没有tab或者emoji，那么直接调用canvas的drawText方法，如果不是上述情况，就会调用TextLine的draw方法。
总结一下：如果该行文本从左至右且没有特殊格式，那么就可以调用drawText绘制纯文本；如果该行由特殊格式，那么就会调用TextLine的draw方法。再使用TextLine的draw方法之前，需要先调用set方法将各种参数设置进入。下面看TextLine的draw方法：

/**
     * Renders the TextLine.
     *
     * @param c the canvas to render on
     * @param x the leading margin position
     * @param top the top of the line
     * @param y the baseline
     * @param bottom the bottom of the line
     */
    void draw(Canvas c, float x, int top, int y, int bottom) {
        if (!mHasTabs) {
            if (mDirections == Layout.DIRS_ALL_LEFT_TO_RIGHT) {
                drawRun(c, 0, mLen, false, x, top, y, bottom, false);
                return;
            }
            if (mDirections == Layout.DIRS_ALL_RIGHT_TO_LEFT) {
                drawRun(c, 0, mLen, true, x, top, y, bottom, false);
                return;
            }
        }

        float h = 0;
        int[] runs = mDirections.mDirections;
        RectF emojiRect = null;

        int lastRunIndex = runs.length - 2;
        for (int i = 0; i < runs.length; i += 2) {
            int runStart = runs[i];
            int runLimit = runStart + (runs[i+1] & Layout.RUN_LENGTH_MASK);
            if (runLimit > mLen) {
                runLimit = mLen;
            }
            boolean runIsRtl = (runs[i+1] & Layout.RUN_RTL_FLAG) != 0;

            int segstart = runStart;
            for (int j = mHasTabs ? runStart : runLimit; j <= runLimit; j++) {
                int codept = 0;
                Bitmap bm = null;

                if (mHasTabs && j < runLimit) {
                    codept = mChars[j];
                    if (codept >= 0xd800 && codept < 0xdc00 && j + 1 < runLimit) {
                        codept = Character.codePointAt(mChars, j);
                        if (codept >= Layout.MIN_EMOJI && codept <= Layout.MAX_EMOJI) {
                            bm = Layout.EMOJI_FACTORY.getBitmapFromAndroidPua(codept);
                        } else if (codept > 0xffff) {
                            ++j;
                            continue;
                        }
                    }
                }

                if (j == runLimit || codept == '\t' || bm != null) {
                    h += drawRun(c, segstart, j, runIsRtl, x+h, top, y, bottom,
                            i != lastRunIndex || j != mLen);

                    if (codept == '\t') {
                        h = mDir * nextTab(h * mDir);
                    } else if (bm != null) {
                        float bmAscent = ascent(j);
                        float bitmapHeight = bm.getHeight();
                        float scale = -bmAscent / bitmapHeight;
                        float width = bm.getWidth() * scale;

                        if (emojiRect == null) {
                            emojiRect = new RectF();
                        }
                        emojiRect.set(x + h, y + bmAscent,
                                x + h + width, y);
                        c.drawBitmap(bm, null, emojiRect, mPaint);
                        h += width;
                        j++;
                    }
                    segstart = j + 1;
                }
            }
        }
    }

可以看到，如果没有tab或者emoji，就会调用drawRun方法。而如果有tab或者emoji，如果有emoji，那么会获取Bitmap，再绘制Bitmap，其中也会调用drawRun方法。所以如果绘制文本，那么都会调用drawRun方法，如果有图片，那么就会调用Canvas的drawBitmap方法。下面是drawRun方法，

 /**
     * Draws a unidirectional (but possibly multi-styled) run of text.
     *
     *
     * @param c the canvas to draw on
     * @param start the line-relative start
     * @param limit the line-relative limit
     * @param runIsRtl true if the run is right-to-left
     * @param x the position of the run that is closest to the leading margin
     * @param top the top of the line
     * @param y the baseline
     * @param bottom the bottom of the line
     * @param needWidth true if the width value is required.
     * @return the signed width of the run, based on the paragraph direction.
     * Only valid if needWidth is true.
     */
    private float drawRun(Canvas c, int start,
            int limit, boolean runIsRtl, float x, int top, int y, int bottom,
            boolean needWidth) {

        if ((mDir == Layout.DIR_LEFT_TO_RIGHT) == runIsRtl) {
            float w = -measureRun(start, limit, limit, runIsRtl, null);
            handleRun(start, limit, limit, runIsRtl, c, x + w, top,
                    y, bottom, null, false);
            return w;
        }

        return handleRun(start, limit, limit, runIsRtl, c, x, top,
                y, bottom, null, needWidth);
    }

可以看到上面的方法会根据方向调用handleRun方法，下面是handleRun方法：

= & <= tests
                if ((mMetricAffectingSpanSpanSet.spanStarts[j] >= mStart + mlimit) ||
                        (mMetricAffectingSpanSpanSet.spanEnds[j] <= mStart + i)) continue;
                MetricAffectingSpan span = mMetricAffectingSpanSpanSet.spans[j];
                if (span instanceof ReplacementSpan) {
                    replacement = (ReplacementSpan)span;
                } else {
                    // We might have a replacement that uses the draw
                    // state, otherwise measure state would suffice.
                    span.updateDrawState(wp);
                }
            }

            if (replacement != null) {
                x += handleReplacement(replacement, wp, i, mlimit, runIsRtl, c, x, top, y,
                        bottom, fmi, needWidth || mlimit < measureLimit);
                continue;
            }

            for (int j = i, jnext; j < mlimit; j = jnext) {
                jnext = mCharacterStyleSpanSet.getNextTransition(mStart + j, mStart + mlimit) -
                        mStart;

                wp.set(mPaint);
                for (int k = 0; k < mCharacterStyleSpanSet.numberOfSpans; k++) {
                    // Intentionally using >= and <= as explained above
                    if ((mCharacterStyleSpanSet.spanStarts[k] >= mStart + jnext) ||
                            (mCharacterStyleSpanSet.spanEnds[k] <= mStart + j)) continue;

                    CharacterStyle span = mCharacterStyleSpanSet.spans[k];
                    span.updateDrawState(wp);
                }

                // Only draw hyphen on last run in line
                if (jnext < mLen) {
                    wp.setHyphenEdit(0);
                }
                x += handleText(wp, j, jnext, i, inext, runIsRtl, c, x,
                        top, y, bottom, fmi, needWidth || jnext < measureLimit);
            }
        }

        return x - originalX;
    }" data-snippet-id="ext.243646ebd8d3c454518a70e3769518ee" data-snippet-saved="false" data-codota-status="done">/**
     * Utility function for handling a unidirectional run.  The run must not
     * contain tabs or emoji but can contain styles.
     *
     *
     * @param start the line-relative start of the run
     * @param measureLimit the offset to measure to, between start and limit inclusive
     * @param limit the limit of the run
     * @param runIsRtl true if the run is right-to-left
     * @param c the canvas, can be null
     * @param x the end of the run closest to the leading margin
     * @param top the top of the line
     * @param y the baseline
     * @param bottom the bottom of the line
     * @param fmi receives metrics information, can be null
     * @param needWidth true if the width is required
     * @return the signed width of the run based on the run direction; only
     * valid if needWidth is true
     */
    private float handleRun(int start, int measureLimit,
            int limit, boolean runIsRtl, Canvas c, float x, int top, int y,
            int bottom, FontMetricsInt fmi, boolean needWidth) {

        // Case of an empty line, make sure we update fmi according to mPaint
        if (start == measureLimit) {
            TextPaint wp = mWorkPaint;
            wp.set(mPaint);
            if (fmi != null) {
                expandMetricsFromPaint(fmi, wp);
            }
            return 0f;
        }

        if (mSpanned == null) {
            TextPaint wp = mWorkPaint;
            wp.set(mPaint);
            final int mlimit = measureLimit;
            return handleText(wp, start, mlimit, start, limit, runIsRtl, c, x, top,
                    y, bottom, fmi, needWidth || mlimit < measureLimit);
        }

        mMetricAffectingSpanSpanSet.init(mSpanned, mStart + start, mStart + limit);
        mCharacterStyleSpanSet.init(mSpanned, mStart + start, mStart + limit);

        // Shaping needs to take into account context up to metric boundaries,
        // but rendering needs to take into account character style boundaries.
        // So we iterate through metric runs to get metric bounds,
        // then within each metric run iterate through character style runs
        // for the run bounds.
        final float originalX = x;
        for (int i = start, inext; i < measureLimit; i = inext) {
            TextPaint wp = mWorkPaint;
            wp.set(mPaint);

            inext = mMetricAffectingSpanSpanSet.getNextTransition(mStart + i, mStart + limit) -
                    mStart;
            int mlimit = Math.min(inext, measureLimit);

            ReplacementSpan replacement = null;

            for (int j = 0; j < mMetricAffectingSpanSpanSet.numberOfSpans; j++) {
                // Both intervals [spanStarts..spanEnds] and [mStart + i..mStart + mlimit] are NOT
                // empty by construction. This special case in getSpans() explains the >= & <= tests
                if ((mMetricAffectingSpanSpanSet.spanStarts[j] >= mStart + mlimit) ||
                        (mMetricAffectingSpanSpanSet.spanEnds[j] <= mStart + i)) continue;
                MetricAffectingSpan span = mMetricAffectingSpanSpanSet.spans[j];
                if (span instanceof ReplacementSpan) {
                    replacement = (ReplacementSpan)span;
                } else {
                    // We might have a replacement that uses the draw
                    // state, otherwise measure state would suffice.
                    span.updateDrawState(wp);
                }
            }

            if (replacement != null) {
                x += handleReplacement(replacement, wp, i, mlimit, runIsRtl, c, x, top, y,
                        bottom, fmi, needWidth || mlimit < measureLimit);
                continue;
            }

            for (int j = i, jnext; j < mlimit; j = jnext) {
                jnext = mCharacterStyleSpanSet.getNextTransition(mStart + j, mStart + mlimit) -
                        mStart;

                wp.set(mPaint);
                for (int k = 0; k < mCharacterStyleSpanSet.numberOfSpans; k++) {
                    // Intentionally using >= and <= as explained above
                    if ((mCharacterStyleSpanSet.spanStarts[k] >= mStart + jnext) ||
                            (mCharacterStyleSpanSet.spanEnds[k] <= mStart + j)) continue;

                    CharacterStyle span = mCharacterStyleSpanSet.spans[k];
                    span.updateDrawState(wp);
                }

                // Only draw hyphen on last run in line
                if (jnext < mLen) {
                    wp.setHyphenEdit(0);
                }
                x += handleText(wp, j, jnext, i, inext, runIsRtl, c, x,
                        top, y, bottom, fmi, needWidth || jnext < measureLimit);
            }
        }

        return x - originalX;
    }

从注释中可以看到该方法用于绘制单方向的文字，并且文字中不能包含制表符或者图片，但是可以包含其他Span。首先看一下TextLine的set方法：

 /**
     * Initializes a TextLine and prepares it for use.
     *
     * @param paint the base paint for the line
     * @param text the text, can be Styled
     * @param start the start of the line relative to the text
     * @param limit the limit of the line relative to the text
     * @param dir the paragraph direction of this line
     * @param directions the directions information of this line
     * @param hasTabs true if the line might contain tabs or emoji
     * @param tabStops the tabStops. Can be null.
     */
    void set(TextPaint paint, CharSequence text, int start, int limit, int dir,
            Directions directions, boolean hasTabs, TabStops tabStops) {
        mPaint = paint;
        mText = text;
        mStart = start;
        mLen = limit - start;
        mDir = dir;
        mDirections = directions;
        if (mDirections == null) {
            throw new IllegalArgumentException("Directions cannot be null");
        }
        mHasTabs = hasTabs;
        mSpanned = null;

        boolean hasReplacement = false;
        if (text instanceof Spanned) {
            mSpanned = (Spanned) text;
            mReplacementSpanSpanSet.init(mSpanned, start, limit);
            hasReplacement = mReplacementSpanSpanSet.numberOfSpans > 0;
        }

        mCharsValid = hasReplacement || hasTabs || directions != Layout.DIRS_ALL_LEFT_TO_RIGHT;

        if (mCharsValid) {
            if (mChars == null || mChars.length < mLen) {
                mChars = ArrayUtils.newUnpaddedCharArray(mLen);
            }
            TextUtils.getChars(text, start, limit, mChars, 0);
            if (hasReplacement) {
                // Handle these all at once so we don't have to do it as we go.
                // Replace the first character of each replacement run with the
                // object-replacement character and the remainder with zero width
                // non-break space aka BOM.  Cursor movement code skips these
                // zero-width characters.
                char[] chars = mChars;
                for (int i = start, inext; i < limit; i = inext) {
                    inext = mReplacementSpanSpanSet.getNextTransition(i, limit);
                    if (mReplacementSpanSpanSet.hasSpansIntersecting(i, inext)) {
                        // transition into a span
                        chars[i - start] = '\ufffc';
                        for (int j = i - start + 1, e = inext - start; j < e; ++j) {
                            chars[j] = '\ufeff'; // used as ZWNBS, marks positions to skip
                        }
                    }
                }
            }
        }
        mTabs = tabStops;
    }

可以看到该方法根据传入的参数做了一些初始化工作，包含判断文字是否是Spanned对象，如果是的话，还判断其中有没有ReplacementSpan。然后再根据当前是否属于特殊情况（有ReplacementSpan或者有支付表或者方向不是从左至右），那么需要需要获取文字的mChars数组类型，如果有ReplacementSpan，还需要mChars进行转换。
下面再看之前的handleRun方法，可以看到如果文字不是Spanned类型的话，那么会调用handleText方法；如果是Spanned的话，那么首先获取文本中所有的MetricAffectingSpan和CharacterStyle对象，在TextView显示丰富多彩的文字（二）——如何使用ParagraphStyle格式化段落中我们知道MetricAffectingSpan会影响文字的外观，接下来会调用每个MetricAffectingSpan的updateDrawState方法
设置TextPaint对象，并且由于set方法中已经替换了Replacement的字符并且在handleRun方法中已经将Replacement的图片绘制出来了，所以这儿就忽略了ReplacementSpan。接下来，就是如果ReplacementSpan不为null，那么需要把位置预留出来。接下来再对所有CharacterStyle调用updateDrawState设置TextPaint。然后调用handleText方法。handleText方法读者测量和渲染文本，文本必须不包含制表符和emoji。handleText方法如下：

/**
     * Utility function for measuring and rendering text.  The text must
     * not include a tab or emoji.
     *
     * @param wp the working paint
     * @param start the start of the text
     * @param end the end of the text
     * @param runIsRtl true if the run is right-to-left
     * @param c the canvas, can be null if rendering is not needed
     * @param x the edge of the run closest to the leading margin
     * @param top the top of the line
     * @param y the baseline
     * @param bottom the bottom of the line
     * @param fmi receives metrics information, can be null
     * @param needWidth true if the width of the run is needed
     * @return the signed width of the run based on the run direction; only
     * valid if needWidth is true
     */
    private float handleText(TextPaint wp, int start, int end,
            int contextStart, int contextEnd, boolean runIsRtl,
            Canvas c, float x, int top, int y, int bottom,
            FontMetricsInt fmi, boolean needWidth) {

        // Get metrics first (even for empty strings or "0" width runs)
        if (fmi != null) {
            expandMetricsFromPaint(fmi, wp);
        }

        int runLen = end - start;
        // No need to do anything if the run width is "0"
        if (runLen == 0) {
            return 0f;
        }

        float ret = 0;

        if (needWidth || (c != null && (wp.bgColor != 0 || wp.underlineColor != 0 || runIsRtl))) {
            if (mCharsValid) {
                ret = wp.getRunAdvance(mChars, start, end, contextStart, contextEnd,
                        runIsRtl, end);
            } else {
                int delta = mStart;
                ret = wp.getRunAdvance(mText, delta + start, delta + end,
                        delta + contextStart, delta + contextEnd, runIsRtl, delta + end);
            }
        }

        if (c != null) {
            if (runIsRtl) {
                x -= ret;
            }

            if (wp.bgColor != 0) {
                int previousColor = wp.getColor();
                Paint.Style previousStyle = wp.getStyle();

                wp.setColor(wp.bgColor);
                wp.setStyle(Paint.Style.FILL);
                c.drawRect(x, top, x + ret, bottom, wp);

                wp.setStyle(previousStyle);
                wp.setColor(previousColor);
            }

            if (wp.underlineColor != 0) {
                // kStdUnderline_Offset = 1/9, defined in SkTextFormatParams.h
                float underlineTop = y + wp.baselineShift + (1.0f / 9.0f) * wp.getTextSize();

                int previousColor = wp.getColor();
                Paint.Style previousStyle = wp.getStyle();
                boolean previousAntiAlias = wp.isAntiAlias();

                wp.setStyle(Paint.Style.FILL);
                wp.setAntiAlias(true);

                wp.setColor(wp.underlineColor);
                c.drawRect(x, underlineTop, x + ret, underlineTop + wp.underlineThickness, wp);

                wp.setStyle(previousStyle);
                wp.setColor(previousColor);
                wp.setAntiAlias(previousAntiAlias);
            }

            drawTextRun(c, wp, start, end, contextStart, contextEnd, runIsRtl,
                    x, y + wp.baselineShift);
        }

        return runIsRtl ? -ret : ret;
    }

如果Canvas不为空，那么首先画背景色，再画下划线，最后调用drawTextRun方法。drawTextRun方法用已经设置号的TextPaint渲染文本.下面是drawTextRun方法的实现：

 /**
     * Render a text run with the set-up paint.
     *
     * @param c the canvas
     * @param wp the paint used to render the text
     * @param start the start of the run
     * @param end the end of the run
     * @param contextStart the start of context for the run
     * @param contextEnd the end of the context for the run
     * @param runIsRtl true if the run is right-to-left
     * @param x the x position of the left edge of the run
     * @param y the baseline of the run
     */
    private void drawTextRun(Canvas c, TextPaint wp, int start, int end,
            int contextStart, int contextEnd, boolean runIsRtl, float x, int y) {

        if (mCharsValid) {
            int count = end - start;
            int contextCount = contextEnd - contextStart;
            c.drawTextRun(mChars, start, count, contextStart, contextCount,
                    x, y, runIsRtl, wp);
        } else {
            int delta = mStart;
            c.drawTextRun(mText, delta + start, delta + end,
                    delta + contextStart, delta + contextEnd, x, y, runIsRtl, wp);
        }
    }

从上述方法可以看到最终调用的是Canvas的drawTextRun方法，

 /**
     * Draw a run of text, all in a single direction, with optional context for complex text
     * shaping.
     *
     * 
See {@link #drawTextRun(CharSequence, int, int, int, int, float, float, boolean, Paint)}
     * for more details. This method uses a character array rather than CharSequence to
     * represent the string. Also, to be consistent with the pattern established in
     * {@link #drawText}, in this method {@code count} and {@code contextCount} are used rather
     * than offsets of the end position; {@code count = end - start, contextCount = contextEnd -
     * contextStart}.
     *
     * @param text the text to render
     * @param index the start of the text to render
     * @param count the count of chars to render
     * @param contextIndex the start of the context for shaping.  Must be
     *         no greater than index.
     * @param contextCount the number of characters in the context for shaping.
     *         contexIndex + contextCount must be no less than index + count.
     * @param x the x position at which to draw the text
     * @param y the y position at which to draw the text
     * @param isRtl whether the run is in RTL direction
     * @param paint the paint
     */
    public void drawTextRun(@NonNull char[] text, int index, int count, int contextIndex,
            int contextCount, float x, float y, boolean isRtl, @NonNull Paint paint) {

        if (text == null) {
            throw new NullPointerException("text is null");
        }
        if (paint == null) {
            throw new NullPointerException("paint is null");
        }
        if ((index | count | contextIndex | contextCount | index - contextIndex
                | (contextIndex + contextCount) - (index + count)
                | text.length - (contextIndex + contextCount)) < 0) {
            throw new IndexOutOfBoundsException();
        }

        native_drawTextRun(mNativeCanvasWrapper, text, index, count, contextIndex, contextCount,
                x, y, isRtl, paint.getNativeInstance(), paint.mNativeTypeface);
    }

先经过一系列的判定，最后调用native_drawTextRun方法，该方法是native的。上面还有一个handleReplacement方法，用于测量和渲染一个replacement。下面是handleReplacement方法的实现：

/**
     * Utility function for measuring and rendering a replacement.
     *
     *
     * @param replacement the replacement
     * @param wp the work paint
     * @param start the start of the run
     * @param limit the limit of the run
     * @param runIsRtl true if the run is right-to-left
     * @param c the canvas, can be null if not rendering
     * @param x the edge of the replacement closest to the leading margin
     * @param top the top of the line
     * @param y the baseline
     * @param bottom the bottom of the line
     * @param fmi receives metrics information, can be null
     * @param needWidth true if the width of the replacement is needed
     * @return the signed width of the run based on the run direction; only
     * valid if needWidth is true
     */
    private float handleReplacement(ReplacementSpan replacement, TextPaint wp,
            int start, int limit, boolean runIsRtl, Canvas c,
            float x, int top, int y, int bottom, FontMetricsInt fmi,
            boolean needWidth) {

        float ret = 0;

        int textStart = mStart + start;
        int textLimit = mStart + limit;

        if (needWidth || (c != null && runIsRtl)) {
            int previousTop = 0;
            int previousAscent = 0;
            int previousDescent = 0;
            int previousBottom = 0;
            int previousLeading = 0;

            boolean needUpdateMetrics = (fmi != null);

            if (needUpdateMetrics) {
                previousTop     = fmi.top;
                previousAscent  = fmi.ascent;
                previousDescent = fmi.descent;
                previousBottom  = fmi.bottom;
                previousLeading = fmi.leading;
            }

            ret = replacement.getSize(wp, mText, textStart, textLimit, fmi);

            if (needUpdateMetrics) {
                updateMetrics(fmi, previousTop, previousAscent, previousDescent, previousBottom,
                        previousLeading);
            }
        }

        if (c != null) {
            if (runIsRtl) {
                x -= ret;
            }
            replacement.draw(c, mText, textStart, textLimit,
                    x, top, y, bottom, wp);
        }

        return runIsRtl ? -ret : ret;
    }

该方法调用replcement的draw方法进行绘画，并返回宽度。按照Android目前的类库，ReplacementSpan目前只有一个具体的实现类，就是ImageSpan，ImageSpan的draw方法就是绘画设置的图片。
从上可以看到，当TextView设置的文本样式是Spannable类型时，会使用DynamicLayout负责文本的布局，而DynamicLayout会首先绘制背景，绘制背景过程中会调用设置的LineBackgrounSpan绘制背景；接着绘制文本，绘制文本中会首先获取所有的ParagraphStyle，重点就是LeadingMarginSpan，然后逐个绘制Margin，急着就是根据是否是纯文本，如果是纯文本，就直接使用Canvas的drawText方法，如果不是的话，就需要使用TextLine的draw方法。由此可以看出来，Layout绘制文本会将ParagraphStyle绘制，除了TabStopSpan。TextLine负责绘制每一行，会首先获取所有的CharacterStyle对象，再设置TextPaint属性，最后再调用canvas的native的draw方法。

下面看DynamicLayout的构造方法，DynamicLayout有几个构造方法，但最终都会调用下面这一个：

 /**
     * Make a layout for the transformed text (password transformation
     * being the primary example of a transformation)
     * that will be updated as the base text is changed.
     * If ellipsize is non-null, the Layout will ellipsize the text
     * down to ellipsizedWidth.
     * *
     * *@hide
     */
    public DynamicLayout(CharSequence base, CharSequence display,
                         TextPaint paint,
                         int width, Alignment align, TextDirectionHeuristic textDir,
                         float spacingmult, float spacingadd,
                         boolean includepad, int breakStrategy, int hyphenationFrequency,
                         TextUtils.TruncateAt ellipsize, int ellipsizedWidth) {
        super((ellipsize == null)
                ? display
                : (display instanceof Spanned)
                    ? new SpannedEllipsizer(display)
                    : new Ellipsizer(display),
              paint, width, align, textDir, spacingmult, spacingadd);

        mBase = base;
        mDisplay = display;

        if (ellipsize != null) {
            mInts = new PackedIntVector(COLUMNS_ELLIPSIZE);
            mEllipsizedWidth = ellipsizedWidth;
            mEllipsizeAt = ellipsize;
        } else {
            mInts = new PackedIntVector(COLUMNS_NORMAL);
            mEllipsizedWidth = width;
            mEllipsizeAt = null;
        }

        mObjects = new PackedObjectVector(1);

        mIncludePad = includepad;
        mBreakStrategy = breakStrategy;
        mHyphenationFrequency = hyphenationFrequency;

        /*
         * This is annoying, but we can't refer to the layout until
         * superclass construction is finished, and the superclass
         * constructor wants the reference to the display text.
         *
         * This will break if the superclass constructor ever actually
         * cares about the content instead of just holding the reference.
         */
        if (ellipsize != null) {
            Ellipsizer e = (Ellipsizer) getText();

            e.mLayout = this;
            e.mWidth = ellipsizedWidth;
            e.mMethod = ellipsize;
            mEllipsize = true;
        }

        // Initial state is a single line with 0 characters (0 to 0),
        // with top at 0 and bottom at whatever is natural, and
        // undefined ellipsis.

        int[] start;

        if (ellipsize != null) {
            start = new int[COLUMNS_ELLIPSIZE];
            start[ELLIPSIS_START] = ELLIPSIS_UNDEFINED;
        } else {
            start = new int[COLUMNS_NORMAL];
        }

        Directions[] dirs = new Directions[] { DIRS_ALL_LEFT_TO_RIGHT };

        Paint.FontMetricsInt fm = paint.getFontMetricsInt();
        int asc = fm.ascent;
        int desc = fm.descent;

        start[DIR] = DIR_LEFT_TO_RIGHT << DIR_SHIFT;
        start[TOP] = 0;
        start[DESCENT] = desc;
        mInts.insertAt(0, start);

        start[TOP] = desc - asc;
        mInts.insertAt(1, start);

        mObjects.insertAt(0, dirs);

        // Update from 0 characters to whatever the real text is
        reflow(base, 0, 0, base.length());

        if (base instanceof Spannable) {
            if (mWatcher == null)
                mWatcher = new ChangeWatcher(this);

            // Strip out any watchers for other DynamicLayouts.
            Spannable sp = (Spannable) base;
            ChangeWatcher[] spans = sp.getSpans(0, sp.length(), ChangeWatcher.class);
            for (int i = 0; i < spans.length; i++)
                sp.removeSpan(spans[i]);

            sp.setSpan(mWatcher, 0, base.length(),
                       Spannable.SPAN_INCLUSIVE_INCLUSIVE |
                       (PRIORITY << Spannable.SPAN_PRIORITY_SHIFT));
        }
    }

从上面的代码可以看出，首先调用了父类Layout的构造方法，接着就是自己的处理。构造方法内部会调用reflow方法更新。下面是reflow方法

 private void reflow(CharSequence s, int where, int before, int after) {
        if (s != mBase)
            return;

        CharSequence text = mDisplay;
        int len = text.length();

        // seek back to the start of the paragraph

        int find = TextUtils.lastIndexOf(text, '\n', where - 1);
        if (find < 0)
            find = 0;
        else
            find = find + 1;

        {
            int diff = where - find;
            before += diff;
            after += diff;
            where -= diff;
        }

        // seek forward to the end of the paragraph

        int look = TextUtils.indexOf(text, '\n', where + after);
        if (look < 0)
            look = len;
        else
            look++; // we want the index after the \n

        int change = look - (where + after);
        before += change;
        after += change;

        // seek further out to cover anything that is forced to wrap together

        if (text instanceof Spanned) {
            Spanned sp = (Spanned) text;
            boolean again;

            do {
                again = false;

                Object[] force = sp.getSpans(where, where + after,
                                             WrapTogetherSpan.class);

                for (int i = 0; i < force.length; i++) {
                    int st = sp.getSpanStart(force[i]);
                    int en = sp.getSpanEnd(force[i]);

                    if (st < where) {
                        again = true;

                        int diff = where - st;
                        before += diff;
                        after += diff;
                        where -= diff;
                    }

                    if (en > where + after) {
                        again = true;

                        int diff = en - (where + after);
                        before += diff;
                        after += diff;
                    }
                }
            } while (again);
        }

        // find affected region of old layout

        int startline = getLineForOffset(where);
        int startv = getLineTop(startline);

        int endline = getLineForOffset(where + before);
        if (where + after == len)
            endline = getLineCount();
        int endv = getLineTop(endline);
        boolean islast = (endline == getLineCount());

        // generate new layout for affected text

        StaticLayout reflowed;
        StaticLayout.Builder b;

        synchronized (sLock) {
            reflowed = sStaticLayout;
            b = sBuilder;
            sStaticLayout = null;
            sBuilder = null;
        }

        if (reflowed == null) {
            reflowed = new StaticLayout(null);
            b = StaticLayout.Builder.obtain(text, where, where + after, getPaint(), getWidth());
        }

        b.setText(text, where, where + after)
                .setPaint(getPaint())
                .setWidth(getWidth())
                .setTextDirection(getTextDirectionHeuristic())
                .setLineSpacing(getSpacingAdd(), getSpacingMultiplier())
                .setEllipsizedWidth(mEllipsizedWidth)
                .setEllipsize(mEllipsizeAt)
                .setBreakStrategy(mBreakStrategy)
                .setHyphenationFrequency(mHyphenationFrequency);
        reflowed.generate(b, false, true);
        int n = reflowed.getLineCount();

        // If the new layout has a blank line at the end, but it is not
        // the very end of the buffer, then we already have a line that
        // starts there, so disregard the blank line.

        if (where + after != len && reflowed.getLineStart(n - 1) == where + after)
            n--;

        // remove affected lines from old layout
        mInts.deleteAt(startline, endline - startline);
        mObjects.deleteAt(startline, endline - startline);

        // adjust offsets in layout for new height and offsets

        int ht = reflowed.getLineTop(n);
        int toppad = 0, botpad = 0;

        if (mIncludePad && startline == 0) {
            toppad = reflowed.getTopPadding();
            mTopPadding = toppad;
            ht -= toppad;
        }
        if (mIncludePad && islast) {
            botpad = reflowed.getBottomPadding();
            mBottomPadding = botpad;
            ht += botpad;
        }

        mInts.adjustValuesBelow(startline, START, after - before);
        mInts.adjustValuesBelow(startline, TOP, startv - endv + ht);

        // insert new layout

        int[] ints;

        if (mEllipsize) {
            ints = new int[COLUMNS_ELLIPSIZE];
            ints[ELLIPSIS_START] = ELLIPSIS_UNDEFINED;
        } else {
            ints = new int[COLUMNS_NORMAL];
        }

        Directions[] objects = new Directions[1];

        for (int i = 0; i < n; i++) {
            ints[START] = reflowed.getLineStart(i) |
                          (reflowed.getParagraphDirection(i) << DIR_SHIFT) |
                          (reflowed.getLineContainsTab(i) ? TAB_MASK : 0);

            int top = reflowed.getLineTop(i) + startv;
            if (i > 0)
                top -= toppad;
            ints[TOP] = top;

            int desc = reflowed.getLineDescent(i);
            if (i == n - 1)
                desc += botpad;

            ints[DESCENT] = desc;
            objects[0] = reflowed.getLineDirections(i);

            ints[HYPHEN] = reflowed.getHyphen(i);

            if (mEllipsize) {
                ints[ELLIPSIS_START] = reflowed.getEllipsisStart(i);
                ints[ELLIPSIS_COUNT] = reflowed.getEllipsisCount(i);
            }

            mInts.insertAt(startline + i, ints);
            mObjects.insertAt(startline + i, objects);
        }

        updateBlocks(startline, endline - 1, n);

        b.finish();
        synchronized (sLock) {
            sStaticLayout = reflowed;
            sBuilder = b;
        }
    }

reflow方法会首先将前去查找段落的开始，然后再向后查找段落的结尾。然后如果文本是Spanned类型的，则查找所有WrapTogetherSpan类型，然后再找出影响布局的部分，再根据影响的文本创建新的布局，可以看到接下来是使用StaticLayout的generate方法，StaticLayout布局的generate方法中有下面一段：

 // tab stop locations
            int[] variableTabStops = null;
            if (spanned != null) {
                TabStopSpan[] spans = getParagraphSpans(spanned, paraStart,
                        paraEnd, TabStopSpan.class);
                if (spans.length > 0) {
                    int[] stops = new int[spans.length];
                    for (int i = 0; i < spans.length; i++) {
                        stops[i] = spans[i].getTabStop();
                    }
                    Arrays.sort(stops, 0, stops.length);
                    variableTabStops = stops;
                }
            }

onTouchEvent方法

在TextView显示丰富多彩的文字（一）——如何使用CharacterStyle格式化字符中留有一个问题，就是对同一位置的文字设置两个URLSpan，点击事件响应的是第一个设置的URLSpan的URL，这与我们平常想的覆盖有所不同，而具体的解释可以看onTouchEvent方法中的这段代码：

if ((mMovement != null || onCheckIsTextEditor()) && isEnabled()
                && mText instanceof Spannable && mLayout != null) {
            boolean handled = false;

            if (mMovement != null) {
                handled |= mMovement.onTouchEvent(this, (Spannable) mText, event);
            }

            final boolean textIsSelectable = isTextSelectable();
            if (touchIsFinished && mLinksClickable && mAutoLinkMask != 0 && textIsSelectable) {
                // The LinkMovementMethod which should handle taps on links has not been installed
                // on non editable text that support text selection.
                // We reproduce its behavior here to open links for these.
                ClickableSpan[] links = ((Spannable) mText).getSpans(getSelectionStart(),
                        getSelectionEnd(), ClickableSpan.class);

                if (links.length > 0) {
                    links[0].onClick(this);
                    handled = true;
                }
            }

            if (touchIsFinished && (isTextEditable() || textIsSelectable)) {
                // Show the IME, except when selecting in read-only text.
                final InputMethodManager imm = InputMethodManager.peekInstance();
                viewClicked(imm);
                if (!textIsSelectable && mEditor.mShowSoftInputOnFocus) {
                    handled |= imm != null && imm.showSoftInput(this, 0);
                }

                // The above condition ensures that the mEditor is not null
                mEditor.onTouchUpEvent(event);

                handled = true;
            }

            if (handled) {
                return true;
            }
        }

上述代码会获取所有的ClickableSpan对象，然后会调用第一个ClickableSpan对象的click方法，这也就解释了如果设置多个URLSpan的话，为什么点击事件会显示第一个设置的URL。

总结

上面基本就是从TextView的源码解释为什么可以显示丰富多彩的文字，第一次分析源码，有很多看的不太懂，可能会有错误，欢迎讨论。