View的draw过程源码分析

/**
*
* 原创作者:
* 谷哥的小弟 http://blog.csdn.net/lfdfhl *
* 文档描述:
* View的draw过程源码分析
*
* 绘制过程分为六步:
*
* 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)
*
* 其中第2步和第5步可以忽略,正如源码中所说的:
* skip step 2 & 5 if possible (common case)
*
* 所以关注的是第1,3,4,6步.
*
*/
public void draw(Canvas canvas) {
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);
}

final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;

/**
* 第1步:
* draw the background, if needed
* 绘制View的背景
*/
int saveCount;

if (!dirtyOpaque) {
final Drawable background = mBGDrawable;
if (background != null) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;

if (mBackgroundSizeChanged) {
background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
}

if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
}

/**
* 第2,5步:
* skip step 2 & 5 if possible (common case)
* 忽略这两步.
*/
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
/**
* 第3步:
* draw the content
* 调用onDraw()绘制View的内容
* 在View和ViewGroup的源码中onDraw方法均为空.
* 因为每个View的内容不同,所以需要由具体的子类去实现
*/
if (!dirtyOpaque) onDraw(canvas);

/**
* 第4步:
* draw the children.
* 调用dispatchDraw()绘制View的子View.
* 在View中dispatchDraw()方法是一个空方法,因为View没有孩子.
* 但是ViewGroup重写了该方法,因为ViewGroup有子View
*/
dispatchDraw(canvas);

/**
* 第6步:
* draw decorations (scrollbars)
* 绘制View的滚动条.
* 其实View都是有滚动条的,但一般情况下将其隐藏了,没有显示
*/
onDrawScrollBars(canvas);

return;
}

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}