View的测量、布局和绘制过程中的关键方法
我们这里说的View的测量、布局和绘制,实质上是针对ViewGroup的,简单起见就不区分View和ViewGroup。View的测量、布局和绘制是包含在ViewGroup流程中的。
流程概览
View的测量、布局、绘制是从ViewRootImpl
开始的,它是Activity中的根View。具体是在ViewRootImpl#performTraversals
方法中依次调用performMeasure、performLayout、performDraw。
我们常见的更新UI的方法View#invalidate
最终就是通过调用ViewRootImpl#performTraversals方法来完成的,具体参见View#invalidate方法是如何更新UI的。
如下图所示:
ViewRootImpl#performTraversals图解本文的源码基于android-28
的api。
Measure流程
Measure的具体流程如下:
ViewRootImpl#performTreversals() -->
ViewRootImpl#performMeasure() -->
View#measure() -->
View#onMeasure
如果是ViewGroup,一般会重写View#onMeasure方法,会在ViewGroup#onMeasure方法中循环调用子View的measure方法,如此循环往复,直到最终调用了所有的View#onMeasure方法。
Layout流程
Layout的具体流程如下:
ViewRootImpl#performTreversals() -->
ViewRootImpl#performLayout() -->
View#layout() -->
View#onLayout()
View#onLayout方法默认空实现,如下所示,
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
所以onLayout方法需要View根据需求各自实现。
与onMeasure方法类似,ViewGroup一般会重写View#onLayout方法。
然后在ViewGroup#onLayout方法中循环调用子View的layout方法,如此循环往复,直到最终调用了所有的View#onLayout方法。
这里以LinearLayout为例,
LinearLayout#onLayout -->
Linearlayout#layoutVertical -->
循环遍历每个子View,调用Linearlayout#setChildFrame -->
最终调用View#layout方法。
Draw流程
Draw的具体流程如下:
ViewRootImpl#performTreversals() -->
ViewRootImpl#performDraw() -->
ViewRootImpl#draw(boolean fullRedrawNeeded) -->
View#drawSoftware -->
View#draw(Canvas canvas) -->
绘制当前View:View#onDraw(Canvas canvas) -->
绘制子View:View#dispatchDraw(Canvas canvas)
在View#draw(Canvas canvas)
方法中,有如下注释:
/*
* 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)
*/
绘制流程按照以下六个步骤执行:
①绘制背景
②如果需要,保存图层
③绘制当前View的内容
④绘制子View
⑤如果需要,绘制边界,恢复图层
⑥绘制相关装饰(比如滚动条)
对应的源码为:为了方便阅读,省略了无关代码。
public void draw(Canvas canvas) {
...
/*
* 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
if (!dirtyOpaque) {
drawBackground(canvas);
}
...
// Step 2, save the canvas' layers
...
saveCount = canvas.getSaveCount();
// 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
...
canvas.restoreToCount(saveCount);
drawAutofilledHighlight(canvas);
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
}
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