当我们自定义View的时候，我们会通过invalidate（）来刷新UI，实现重绘。比如最简单的view.setVisibility()的调用路径：
------->view.setVisibility()
----------->view.view.setFlags()
-------------->View.requestLayout()
---------------->ViewGroup.requestLayout()
------------------>ViewRootImpl.requestLayout()
模拟一个非主线程刷新UI的异常，
更能看出最后一步ViewRootImpl.requestLayout()的调用。分析源码ViewRootImpl
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
.........
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
......
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
.....
}
}
Choreographer中文意思是“编舞者”，是一个控制frame（帧）渲染信号的一个类。Choreographer方法doFrame(),处理四种类型的UI刷新的Callback有4种类型:
Input输入、Animation动画、Traversal绘制和布局，Commit是用来解决延迟信号的处理，即丢帧现象.
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
.......
mFrameInfo.markInputHandlingStart();
// Callback type: Input callback. Runs first.
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
//Callback type: Animation callback. Runs before traversals.
mFrameInfo.markAnimationsStart();
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
mFrameInfo.markPerformTraversalsStart();
//CALLBACK_TRAVERSAL:Callback type: Traversal callback. Handles layout and draw. Runs after all other asynchronous messages have been handled.
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
//CALLBACK_COMMIT:Callback type: Commit callback. Handles post-draw operations for the frame.
* Runs after traversal completes.
doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
.......
}
通过mChoreographer这个对象post任务mTraversalRunnable，该任务里只调用一个方法，doTraversal()。这个方法里在调用 performTraversals();
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
.......
performTraversals();
.....
}
}
performTraversals()方法里在调用performMeasure,performLayout,dispatchOnPreDraw实现测量，布局，绘制。
private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
........
if (!mStopped || mReportNextDraw) {
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
......
}
}
}
........
final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
if (didLayout) {
performLayout(lp, mWidth, mHeight);
}
.......
if (!cancelDraw && !newSurface) {
.......
performDraw();
}
}
performDraw()方法中通过调用draw(fullRedrawNeeded);
private void performDraw() {
.......
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
.......
}
draw中绘制的时候按照支持硬件加速，和software两种绘制方式，
private void draw(boolean fullRedrawNeeded) {
if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
.......
mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
} else {
......
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
mHardwareRenderer硬件加速渲染是ThreadedRenderer的一个对象，硬件加速渲染是通过 Display List来刷新的。通过RenderNode的实例化一个DisplayListCanvas.通过画布的DisplayListCanvas实现最终渲染。
private void updateRootDisplayList(View view, HardwareDrawCallbacks callbacks) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Record View#draw()");
updateViewTreeDisplayList(view);
if (mRootNodeNeedsUpdate || !mRootNode.isValid()) {
DisplayListCanvas canvas = mRootNode.start(mSurfaceWidth, mSurfaceHeight);
try {
final int saveCount = canvas.save();
canvas.translate(mInsetLeft, mInsetTop);
callbacks.onHardwarePreDraw(canvas);
canvas.insertReorderBarrier();
canvas.drawRenderNode(view.updateDisplayListIfDirty());
canvas.insertInorderBarrier();
callbacks.onHardwarePostDraw(canvas);
canvas.restoreToCount(saveCount);
mRootNodeNeedsUpdate = false;
} finally {
mRootNode.end(canvas);
}
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
非硬件加速的方式，即drawSoftware的方式，直接在Canvas上绘制，再通过 mView.draw(canvas)回调让子类继续自己的绘制。
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
canvas = mSurface.lockCanvas(dirty);
//略掉
//canvas.setXXX()
// .....
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;
}
}
}
再继续深入，就是RanderNode和Canvas调用地层native本地方法了。
如果你还不太了解硬件加速和非硬件加速渲染的区别，
推荐
Android硬件加速原理与实现简介
总结：
1.所有View的刷新都通过ViewRootImpl的 performTraversals()来实现测量，布局，绘制的。
2.Choreographer是一个控制frame（帧）渲染信号的一个类。Choreographer方法doFrame(),处理四种类型的UI刷新的Callback有4种类型:Input输入、Animation动画、Traversal绘制和布局，Commit是用来解决延迟信号的处理，即丢帧现象.
3.绘制UI的时候分两种方式，
硬件加速绘制条件下：RenderNode构建DisplayListCanvas，CPU用于控制复杂绘制逻辑、构建或更新DisplayList；GPU用于完成图形计算、渲染DisplayList。
软件绘制条件下：Canvas绘制构建Bitmap,CPU用于解码运算；bitmap传到底层，GPU直接渲染。
4.默认是software软件绘制，这种方式渲染效率低，硬件加速绘制优点高，原因是将复杂的图形计算在GPU上实现。避免过度重绘，减少CPU资源和压力。
源码连接：
ViewRootImpl
Choreographer
ThreadedRenderer
RenderNode
