View 绘制流程

绘制流程

View的绘制流程也是从上到下一层层遍历绘制的。我们最顶层的View是DecorView，但DecorView继承自FrameLayout，而ViewGroup的draw方法继承自View，so，所以我们直接看View#draw即可。

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;
    }
    ...
}
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draw方法中，官方对其的步骤进行了清晰的注释，我们来看下流程，在执行流程之前会检查绘制区域是否透明:

• 1、绘制View背景，如果透明则不绘制
• 2、如果需要，则保存画布的图层
• 3、绘制View内容，如果透明则不绘制
• 4、绘制子View————这个很重要
• 5、如果需要，则绘制View的褪色边缘和恢复图层
• 6、绘制装饰滚动条等

这里最重要的步骤是第四步，绘制子View，现在我们来看下这个ViewGroup#dispatchDraw(canvas)方法，注意这里的View是一个DecorView，所以要在ViewGroup中去查看这个方法，View中的这个方法是一个空方法。

protected void dispatchDraw(Canvas canvas) {
    ...
    for (int i = 0; i < childrenCount; i++) {
        while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
            final View transientChild = mTransientViews.get(transientIndex);
            if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                    transientChild.getAnimation() != null) {
                more |= drawChild(canvas, transientChild, drawingTime);
            }
            transientIndex++;
            if (transientIndex >= transientCount) {
                transientIndex = -1;
            }
        }
        int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
        final View child = (preorderedList == null)
                ? children[childIndex] : preorderedList.get(childIndex);
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
            more |= drawChild(canvas, child, drawingTime);
        }
    }
    while (transientIndex >= 0) {
        // there may be additional transient views after the normal views
        final View transientChild = mTransientViews.get(transientIndex);
        if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                transientChild.getAnimation() != null) {
            more |= drawChild(canvas, transientChild, drawingTime);
        }
        transientIndex++;
        if (transientIndex >= transientCount) {
            break;
        }
    }
    ...
}
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上述代码对所有的子View进行遍历，并调用ViewGroup#drawChild方法。

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    return child.draw(canvas, this, drawingTime);
}
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drawChild又调用了子View的draw方法，这样绘制就传递了下去，当然这个draw方法和之前这一小节一开始介绍的View#draw方法并不一样，我们来看看

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
	...
	if (!drawingWithRenderNode) {
    	computeScroll();
    	sx = mScrollX;
    	sy = mScrollY;
	}
	...
    if (!drawingWithDrawingCache) {
        if (drawingWithRenderNode) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            ((DisplayListCanvas) canvas).drawRenderNode(renderNode);
        } else {
            // Fast path for layouts with no backgrounds
            if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                dispatchDraw(canvas);
            } else {
                draw(canvas);
            }
        }
    } else if (cache != null) {
        mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        if (layerType == LAYER_TYPE_NONE) {
            // no layer paint, use temporary paint to draw bitmap
            Paint cachePaint = parent.mCachePaint;
            if (cachePaint == null) {
                cachePaint = new Paint();
                cachePaint.setDither(false);
                parent.mCachePaint = cachePaint;
            }
            cachePaint.setAlpha((int) (alpha * 255));
            canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
        } else {
            // use layer paint to draw the bitmap, merging the two alphas, but also restore
            int layerPaintAlpha = mLayerPaint.getAlpha();
            mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
            canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
            mLayerPaint.setAlpha(layerPaintAlpha);
        }
    }
	...
}
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上述代码会先判断之前是否进行过了绘制，如果没有则进入快速绘制通道，对没有背景的View进行绘制。判断是否需要跳过自身的draw绘制方法，如果跳过则进入dispatchDraw，不跳过则进入当前View的draw方法，即这一小节开头的draw方法，就此形成了循环。