Android中View绘制过程（六） performDraw

performTraversals()中执行了performmeasure确定了每一个View的width，和height， 就会调用performLayout,确定这些View在屏幕

的位置，调用performDraw()在每一个View将图像画在屏幕上自己layout确定的位置上就可以了。

调用如下performDraw()->draw()->drawSoftware()->从DecorView开始递归调用绘制，代码如下：

mView.draw(canvas);

ViewGroup,FrameLayout中都没有重写此方法，回到View中查看，View中代码也好长，关键是调用了一个diapatchDraw(),在ViewGroup中
重写了这个方法，关键点在于

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;
}
}

final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}

遍历自己的chlidView一次调用DrawChlid，这个方法如下
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}

调用Chlid中的额draw方法，

draw方法会调用onDraw这个回调，在这个回调中可以自定义View的绘制。终于自上而下完成View的绘制。

其他说明draw方法的说明如下：

/*
* 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)
*/

第一步，Draw the background是发生在onDraw之前的，还有剩下的一些画布的处理，应该注意相对回调onDraw的先后的顺序。