Android View的工作原理

ViewRoot和DecorView

DecorView作为顶级视图，view的整个绘制流程将从DecorView开始进行下发，DecorView继承FrameLayout，是一个ViewGroup。ViewRoot和DecorView的联系将在另一篇文章中详细阐述。这里只需要知道DecorView是作为一个起点开始整个View绘制的。

MeasureSpec

MeasureSpec是一个32位的int值，是用来存储view规格的对象，这个对象中的内容会影响到view的尺寸，当然父容器的规格也会影响当前view的尺寸，那先从MeasureSpec的构成开始分析。

MeasureSpec组成

MeasureSpec是一个32位的int值，高2位表示SpecMode，低30位表示SpecSize，SpecMode是指测量模式，SpecSize是指view可能的大小。

public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                          @MeasureSpecMode int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }
public static int getMode(int measureSpec) {
            //noinspection ResourceType
            return (measureSpec & MODE_MASK);
        }
public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

MeasureSpec类中提供了拼装和单独拆分的方法，size和mode也分别是int类型，mode有三种类型：

UNSPECIFIED

Measure specification mode: The parent has not imposed any constrainton the child. It can be whatever size it wants.

父容器不对View有任何限制，要多大给多大。这种情况一般是系统内部。

AT_MOST

Measure specification mode: The child can be as large as it wants up to the specified size.

子View可以按照自己的大小确定自身，但是不能超过父容器指定的最大大小。对应wrap_contetn。

EXACTLY

Measure specification mode: The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be.

父容器已经知道view所需要的精确大小，不管子View有多大都会被限制为SpecSize的大小。对应match_parent和准确的数值。

MeasureSpec和LayoutParams

通过上边分析不难发现控制view大小的是MeasureSpec，但是在编码中我们对于View设置宽高的时候并没有直接使用EXACTLY和AT_MOST，那么MeasureSpec和LayoutParams的对应关系是怎样的通过代码来看：

private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

这段代码是顶级DecorView的MeasureSpec的创建过程，可以看到ViewGroup.LayoutParams.MATCH_PARENT对应的MeasureSpec.EXACTLY，大小为窗口大小。ViewGroup.LayoutParams.WRAP_CONTENT对应的MeasureSpec.AT_MOST，大小为窗口大小，但不能超过窗口大小。给出固定数值的对应MeasureSpec.EXACTLY，大小为给定的数值。
当然上边提到的MeasureSpec只是顶层View的，我们知道子View的大小不光由自身决定，父容器也会对子View的大小造成影响，那么父容器与子View是怎么共同作用决定子View的MeasureSpec的我们通过ViewGroup的getChildMeasureSpec代码来看：

 public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

可以看到上边方法中父容器的状态会影响子View，一个状态一个状态的来看。
先看父容器为MeasureSpec.EXACTLY：子View为固定数值时，子View为MeasureSpec.EXACTLY+子view的数值。子View为LayoutParams.MATCH_PARENT时，子View为MeasureSpec.EXACTLY+父容器size。子View为LayoutParams.WRAP_CONTENT时，子View为MeasureSpec.AT_MOST+父容器size。
父容器为MeasureSpec.AT_MOST：子View为固定数值时，子View为MeasureSpec.EXACTLY+子view的数值。子View为LayoutParams.MATCH_PARENT时，子View为MeasureSpec.AT_MOST +父容器size。子View为LayoutParams.WRAP_CONTENT时，子View为MeasureSpec.AT_MOST+父容器size。
父容器为MeasureSpec.UNSPECIFIED：子View为固定数值时，子View为MeasureSpec.EXACTLY+子view的数值。子View为LayoutParams.MATCH_PARENT时，子View为MeasureSpec.UNSPECIFIED +0或父容器size。子View为LayoutParams.WRAP_CONTENT时，子View为MeasureSpec.UNSPECIFIED +0或父容器size。
为了便于查看用一个表格来展示

childLayoutParams	EXACTLY	AT_MOST	UNSPECIFIED
dp／px	EXACTLY+ childSize	EXACTLY+ childSize	EXACTLY+ childSize
match_parent	EXACTLY+parentSize	AT_MOST+parentSize	UNSPECIFIED+0
wrap_content	AT_MOST+parentSize	AT_MOST+parentSize	UNSPECIFIED+0

通过表格可以看到除去UNSPECIFIED这种特殊情况，当子View设定了固定值的时候，都会使用子View的大小作为view的大小，当子View为match_parent或wrap_content时都会按照父容器的大小设定，总是不大于父容器的大小。只是在父容器为EXACTLY和子View为match_parent时子View的SpecMode会为EXACTLY。
得到了MeasureSpec就可以设置具体的大小了。那么进入到具体的view绘制过程。

Measure

measure过程是view确定其大小的过程，ViewGroup会遍历子View的measure，递归的查看每个View，确定大小并最终确定自己的大小。

View的Measure

View的measure方法是一个final的方法不可被修改，方法中会先判断一下view的宽高是否发生改变，来判断是否调用onMeasure方法，宽高发生改变一定会调用onMeasure方法，所以view的measure的关键是onMeasure，那来看onMeasure方法的具体实现：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

看到方法中会调用getDefaultSize的方法：

public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

看到当specMode为AT_MOST和EXACTLY时，得到的大小就是specSize的大小，这是得到的大小并不一定是最终大小（划重点），UNSPECIFIED这种情况下得到的size是通过(mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());得到的，这段代码很好理解，当View没有设置背景时，size就是mMinWidth，当有背景时选择mMinWidth和背景图宽中大的作为最终size。
getSuggestedMinimumWidth方法得到的数值就是UNSPECIFIED情况下获得的size二者都是getSuggestedMinimumWidth得到的返回值。
通过上面的代码可以知道，View的高宽由measureSpec决定，当重写onMeasure方法的时候如果View设置的是wrap_content时，可知如果不重写那么就是父容器给的SpecSize也就等同于match_parent的大小，如果需要在wrap_content时给View一个大小需要重写onMeasure方法给定一个数值当作view的大小，这个值应小于SpecSize的数值，最终通过setMeasuredDimension方法设置。

ViewGroup的measure过程

ViewGroup除了完成自身的measure过程还需要完成子View的measure，ViewGroup是一个抽象类，并没有重写onMeasure，但是提供了一个measureChildren方法来遍历子View并measure每个子View。
来看下measureChildren方法：

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }

上边方法会遍历ViewGroup的每个子View，当子View不为GONE时会调用measureChild方法。

 protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

这个方法中会调用getChildMeasureSpec方法，这个方法之前在分析MeasureSpec的时候分析过了，得到子View的MeasureSpec，调用child.measure方法，之后就会执行view的measure，上边的文章已经分析过这个过程。
经过measure的过程一个view的宽高也就基本确定了，如果需要获取最终的宽高建议在onLayout方法中通过getMeasureWidth／Height获取，因为在某些极端情况下view会多次measure。

layout过程

layout过程是ViewGroup来确定子View的位置的过程，layout会触发自身的onLayout方法，在onLayout方法中遍历子View并调用子View的layout方法，子View方法在调用其onLayout方法。先从View的layout方法看起。

public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;

        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnLayoutChangeListeners != null) {
                ArrayList listenersCopy =
                        (ArrayList)li.mOnLayoutChangeListeners.clone();
                int numListeners = listenersCopy.size();
                for (int i = 0; i < numListeners; ++i) {
                    listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
                }
            }
        }

        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
    }

layout通过setFrame方法来设置上下左右四个位置的坐标，setFrame方法把四个顶点的位置确定，这里如果宽高发生改变会调用sizeChange方法。当设置完当前view的顶点，会调用onLayout方法，这个方法需要具体的view来进行重写，如果是ViewGroup的那么需要遍历子View并调用他们的layout方法。

draw过程

draw过程相比较上两个过程比较简单，直接看代码：

 public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // we're done...
            return;
        }

        /*
         * Here we do the full fledged routine...
         * (this is an uncommon case where speed matters less,
         * this is why we repeat some of the tests that have been
         * done above)
         */

        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;

        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;

        // Step 2, save the canvas' layers
        int paddingLeft = mPaddingLeft;

        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }

        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);

        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }

        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;
        int length = (int) fadeHeight;

        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }

        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }

        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }

        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }

        saveCount = canvas.getSaveCount();

        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }

            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }

            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }

            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }

        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Step 5, draw the fade effect and restore layers
        final Paint p = scrollabilityCache.paint;
        final Matrix matrix = scrollabilityCache.matrix;
        final Shader fade = scrollabilityCache.shader;

        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, right, top + length, p);
        }

        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }

        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, left + length, bottom, p);
        }

        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(right - length, top, right, bottom, p);
        }

        canvas.restoreToCount(saveCount);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);
    }

源码中的注释已经写的很清楚了，draw分成六步，第一步会先绘制背景drawBackground；第二步调用自身的onDraw方法绘制自己；第三步dispatchDraw方法把draw事件传递给子View；第四步绘制自身前景onDrawForeground。

小结

到这里view的整体绘制流程已经讲解完，view的绘制分为measure，layout，draw，我们只需要重写onMeasure，onLayout，onDraw方法。View和ViewGroup作为一种组合模式很巧妙的将view体系转化成树形结构，将每一个绘制流程一级一级的向下专递，最终完成整个View的绘制。