Android学习笔记1

1 关于关于TextView的baseline计算。

baseline如图所示

image

英文显示baseline比中文的更明显一些。可以通过FontMetrics来top等获取属性值，FontMetrics类介绍如下

 /**
     * Class that describes the various metrics for a font at a given text size.
     * Remember, Y values increase going down, so those values will be positive,
     * and values that measure distances going up will be negative. This class
     * is returned by getFontMetrics().
     */
    public static class FontMetrics {
        /**
         * The maximum distance above the baseline for the tallest glyph in
         * the font at a given text size.
         */
        public float   top;
        /**
         * The recommended distance above the baseline for singled spaced text.
         */
        public float   ascent;
        /**
         * The recommended distance below the baseline for singled spaced text.
         */
        public float   descent;
        /**
         * The maximum distance below the baseline for the lowest glyph in
         * the font at a given text size.
         */
        public float   bottom;
        /**
         * The recommended additional space to add between lines of text.
         */
        public float   leading;
    }

通过Paint画笔获取FontMetrics或FontMetricsInt。FontMetrics是float类型，FontMetricsInt是int类型的返回值

    Paint.FontMetrics fontMetrics=mPaint.getFontMetrics();
    Paint.FontMetricsInt fontMetricsInt= mPaint.getFontMetricsInt();

然后就可以计算baseline了，可以通过top和bottom计算，也可以通过ascent和descent来计算。

int dy=(fontMetricsInt.bottom-fontMetricsInt.top)/2-fontMetricsInt.bottom;
int dy=(fontMetricsInt.descent -fontMetricsInt.ascent) / 2 - fontMetricsInt.descent;

canvas的drawText方法如下：

/**
     * Draw the text, with origin at (x,y), using the specified paint. The origin is interpreted
     * based on the Align setting in the paint.
     *
     * @param text The text to be drawn
     * @param x The x-coordinate of the origin of the text being drawn
     * @param y The y-coordinate of the baseline of the text being drawn
     * @param paint The paint used for the text (e.g. color, size, style)
     */
    public void drawText(@NonNull String text, float x, float y, @NonNull Paint paint) {
        super.drawText(text, x, y, paint);
    }

我们要注意第三个参数y与画布的原点位置有关系，比如如果原点在画布左上角，文字要画在画布Y轴中心那么

y=容器高度/2+dy

多试验试验就好了。

2自定义View继承ViewGroup 默认不会调用onDraw方法 为什么？

通过查看了一些资料，知道了是通过ViewRootImpl的performTraversals（）方法依次调用performMeasure（），performLayout（），performDraw（）方法，进而实现了View的measure、layout、draw三大流程。View中调用顺序为measure（），layout（），draw（）；
ViewGroup继承自View ，View的draw方法如下

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

}

主要有6个过程。发现了只有dirtyOpaque是false 才会调用onDraw()方法。而dirtyOpaque这个变量是由 mPrivateFlags控制的。mPrivateFlags 到底是怎么赋值的 通过查找代码， 发现在View的构造函数中调用 computeOpaqueFlags()来设置这个变量

protected void computeOpaqueFlags() {
        // Opaque if:
        //   - Has a background
        //   - Background is opaque
        //   - Doesn't have scrollbars or scrollbars overlay

        if (mBackground != null && mBackground.getOpacity() == PixelFormat.OPAQUE) {
            mPrivateFlags |= PFLAG_OPAQUE_BACKGROUND;
        } else {
            mPrivateFlags &= ~PFLAG_OPAQUE_BACKGROUND;
        }

        final int flags = mViewFlags;
        if (((flags & SCROLLBARS_VERTICAL) == 0 && (flags & SCROLLBARS_HORIZONTAL) == 0) ||
                (flags & SCROLLBARS_STYLE_MASK) == SCROLLBARS_INSIDE_OVERLAY ||
                (flags & SCROLLBARS_STYLE_MASK) == SCROLLBARS_OUTSIDE_OVERLAY) {
            mPrivateFlags |= PFLAG_OPAQUE_SCROLLBARS;
        } else {
            mPrivateFlags &= ~PFLAG_OPAQUE_SCROLLBARS;
        }
    }

ViewGroup 为什么出不来 initViewGroup

 private void initViewGroup() {
        // ViewGroup doesn't draw by default
        if (!debugDraw()) {
            setFlags(WILL_NOT_DRAW, DRAW_MASK);
        }
        导致 mPrivateFlags 会重新赋值
        mGroupFlags |= FLAG_CLIP_CHILDREN;
        mGroupFlags |= FLAG_CLIP_TO_PADDING;
        mGroupFlags |= FLAG_ANIMATION_DONE;
        mGroupFlags |= FLAG_ANIMATION_CACHE;
        mGroupFlags |= FLAG_ALWAYS_DRAWN_WITH_CACHE;

        if (mContext.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB) {
            mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS;
        }

        setDescendantFocusability(FOCUS_BEFORE_DESCENDANTS);

        mChildren = new View[ARRAY_INITIAL_CAPACITY];
        mChildrenCount = 0;

        mPersistentDrawingCache = PERSISTENT_SCROLLING_CACHE;
    }