View 和ViewGroup 的draw流程

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本文详细解析了Android中View的绘制流程，包括从ViewRootImpl的performTraversals开始的draw流程，以及View和ViewGroup在绘制过程中的关键步骤。特别关注了如何仅重绘需要更新的部分，以及具体的绘制步骤。

View 的draw流程

View 的3大流程,measure,layout,draw 在上层都是从ViewRootImpl开始的.具体来说是从 performTraversals开始的.

由ViewRoot对象的performTraversals()方法调用draw()方法发起绘制该View树，值得注意的是每次发起绘图时，并不会重新绘制每个View树的视图，而只会重新绘制那些“需要重绘”的视图，View类内部变量包含了一个标志位DRAWN，当该视图需要重绘时，就会为该View添加该标志位。

ViewRootImpl.java

    private void performTraversals() {
      获取Surface对象,用于图形绘制
....
      performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
.........
      performLayout(lp, desiredWindowWidth, desiredWindowHeight);
........
      performDraw();
..........
}

performTraversals 调用了performDraw(), performDraw () 我们暂时需要关注的是它调用了draw(fullRedrawNeeded).

    private void draw(boolean fullRedrawNeeded) {
        Surface surface = mSurface;    //我们千辛万苦获得的surface
        ........
        if (!drawSoftware(surface, attachInfo, yoff, scalingRequired, dirty)) {
               return;
        }
        .......
}

在 draw() 由调用了drawSoftware.

       private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int yoff,
            boolean scalingRequired, Rect dirty) {
       .......
       canvas = mSurface.lockCanvas(dirty);    //通过Surface对象获取并锁定Canvas绘图对象
       .......
       mView.draw(canvas);  //从DecorView开始绘制,也就是整个Window的根视图,这会导致整个树的绘制
       .......
       surface.unlockCanvasAndPost(canvas);    //释放Canvas锁,然后通知SurfaceFlinger更新这个区域
       .......
}

drawSoftware()里面才会调用mView.draw(canvas),到这里才真正开始使用画笔来绘制.

View.java

   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);//绘制背景
	}
	// 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) {//没有设置fadingEdge 属性,渐变投影效果的
		// Step 3, draw the content
		if (!dirtyOpaque) {
			onDraw(canvas);
		}
		// Step 4, draw the children
		dispatchDraw(canvas);
		// Step 6, draw decorations (scrollbars)
		onDrawScrollBars(canvas);
		if (mOverlay != null && !mOverlay.isEmpty()) {
			mOverlay.getOverlayView().dispatchDraw(canvas);
		}
		// we're done...
		return;
	}
	...//处理设置了渐变投影效果的
}

上面的view.draw()方法分为6步:

1.绘制背景.(drawBackground)

2.为第五步绘制fadin投影渐变效果做准备

3.绘制id=content部分,也就是DecorView.(onDraw(canvas);),可以理解绘制视图本身.

4.绘制child view(dispatchDraw(canvas);)

5.绘制fadin投影渐变效果

6.绘制滑动条,(onDrawScrollBars(canvas);)

其中,常见的流程只有1-3-4-6这4部.

其中上面的第3步中的 onDraw() 没有在View 里面具体实现,只是一个空方法.ViewGroup 也没有去重写这个方法.因为每个视图都不一样,需要自己单独去绘制.

其中第4步的 dispatchDraw 没有在View 里面具体实现,只是一个空方法.ViewGroup重写了这个方法.这个方法是用来遍历调用dispatchDraw()来转调child 的draw()方法,这样draw 事件就一层层传递下去了.

下面来说一下ViewGroup 的大致流程,ViewGroup并没有重写View的draw()方法.而是重写了dispatchDraw().其实ViewGroup的这个方法就和View中提到的一样就是用来绘制child view的.实际查看dispatchDraw()也会发现他是通过转调drawChild()来实现调用child 的draw().

实际在查看LinearLayout的源码是发现它虽然实现了onDraw方法,但是至少绘制了Divider,也就是各个child view 之间是否头分割线,及开始和结尾是否有分割线.

   protected void onDraw(Canvas canvas) {
	if (mDivider == null) {
		return;
	}
	if (mOrientation == VERTICAL) {
		drawDividersVertical(canvas);
	} else {
		drawDividersHorizontal(canvas);
	}
}
void drawDividersVertical(Canvas canvas) {
	final int count = getVirtualChildCount();
	for (int i = 0; i < count; i++) {
		final View child = getVirtualChildAt(i);
		if (child != null && child.getVisibility() != GONE) {
			if (hasDividerBeforeChildAt(i)) {
				final LayoutParams lp = (LayoutParams) child.getLayoutParams();
				final int top = child.getTop() - lp.topMargin - mDividerHeight;
				drawHorizontalDivider(canvas, top);
			}
		}
	}
	if (hasDividerBeforeChildAt(count)) {
		final View child = getVirtualChildAt(count - 1);
		int bottom = 0;
		if (child == null) {
			bottom = getHeight() - getPaddingBottom() - mDividerHeight;
		} else {
			final LayoutParams lp = (LayoutParams) child.getLayoutParams();
			bottom = child.getBottom() + lp.bottomMargin;
		}
		drawHorizontalDivider(canvas, bottom);
	}
}