分析ondraw()过程

(1)从ViewRootImpl的performDraw()方法开始，其调用了ViewRootImpl的draw方法

private void performDraw() {

if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {

return;

}

final boolean fullRedrawNeeded = mFullRedrawNeeded;

mFullRedrawNeeded = false;

mIsDrawing = true;

Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");

try {

draw(fullRedrawNeeded);

} finally {

mIsDrawing = false;

Trace.traceEnd(Trace.TRACE_TAG_VIEW);

}

(3)而ViewRootImpl的draw方法最终调用了本身的drawSoftware方法：

private voiddraw(booleanfullRedrawNeeded) {

...

if(!drawSoftware(surface,mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {

return;

}

(4)ViewRootImpl的drawSoftware方法，最终调用了View的draw方法

private booleandrawSoftware(Surface surface, AttachInfo attachInfo,intxoff, intyoff,

booleanscalingRequired, Rect dirty) {

...

try{

canvas.translate(-xoff, -yoff);

if(mTranslator!= null) {

mTranslator.translateCanvas(canvas);

}

canvas.setScreenDensity(scalingRequired ?mNoncompatDensity: 0);

attachInfo.mSetIgnoreDirtyState =false;

mView.draw(canvas);

drawAccessibilityFocusedDrawableIfNeeded(canvas);

}finally{

if(!attachInfo.mSetIgnoreDirtyState) {

// Only clear the flag if it was not set during the mView.draw() call

attachInfo.mIgnoreDirtyState =false;

}

}

(5) mView.draw调用的是DecorView的draw方法，其会先调用super.draw

@Override

public voiddraw(Canvas canvas) {

super.draw(canvas);

它的super是FrameLayout，其实就是View的draw方法

（6）View的draw方法：

public voiddraw(Canvas canvas) {

final intprivateFlags =mPrivateFlags;

final booleandirtyOpaque = (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

intsaveCount;

//dirtyOpaque:背景不是透明的

//通常情况下ViewGroup不走onDraw方法，但当设置背景的时候，会调用其onDraw方法

if(!dirtyOpaque) {

drawBackground(canvas);

}

...