@(Android源码解析)
高级UI系列：
setContentView源码分析_看AppCompatActivity是如何实现兼容的
源码分析_Activity是如何显示的？
源码分析_Android UI何时刷新:Choreographer
requestLayout和invalidate都干了些什么
之前我们在分析Activity是如何显示的时候，看到它调用了requestLayout然后走了重新绘制流程，其实我们在自定义View时经常用到requestLayout，invalidate等方法，我们调用它们的目的就是告诉系统我们要刷新下界面，但是实际上是他们去刷新界面吗？下面我们来看下：
我们之前分析过当我调用requestLayout的时候不管你在哪调最后都会传递到ViewRootImp中，由它来统一调用，其实invalidate也是一样。其实关于页面绘制有关的操作最后都是通过ViewRootImp来实现的。
我们看下ViewRootImp中的requestLayout和invalidate代码:
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void invalidate() {
mDirty.set(0, 0, mWidth, mHeight);
if (!mWillDrawSoon) {
scheduleTraversals();
}
}
他们两个都调用了scheduleTraversals()翻译过来的意思就是安排遍历的意思。
其实我们只要留心会发现很多地方都调了scheduleTraversals();
@Override
public void requestChildFocus(View child, View focused) {
if (DEBUG_INPUT_RESIZE) {
Log.v(mTag, "Request child focus: focus now " + focused);
}
checkThread();
scheduleTraversals();
}
@Override
public void clearChildFocus(View child) {
if (DEBUG_INPUT_RESIZE) {
Log.v(mTag, "Clearing child focus");
}
checkThread();
scheduleTraversals();
}
@Override
public void recomputeViewAttributes(View child) {
checkThread();
if (mView == child) {
mAttachInfo.mRecomputeGlobalAttributes = true;
if (!mWillDrawSoon) {
scheduleTraversals();
}
}
}
@Override
public void requestFitSystemWindows() {
checkThread();
mApplyInsetsRequested = true;
scheduleTraversals();
}
非常多我就不一一贴出来了，大体能看出来都是些跟页面显示状态有关的。所以我们猜测下 scheduleTraversals();他可能就是我们能刷新页面的关键。
总结下：在各处调用的invalidate和requestLayout最终都是调ViewRootImp中的 scheduleTraversals()方法
scheduleTraversals
下面看scheduleTraversals()的代码
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
看到里面有个postCallback方法，并有个mTraversalRunnable看名字可以猜应该是执行遍历的一个线程。不过一看这个就是个Handler那种是等待被执行的而非立即执行。我们先看下这个TraversalRunnable
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
最终执行的performTraversals();这个方法干了些什么我们在源码分析--Activity是如何显示的？中已经分析了大约800行代码核心如下:
private void performTraversals(){
...
performMeasure();
...
performLayout();
...
performDraw();
...
}
正是我们熟悉的三步走，所以performTraversals()是实现页面刷新的逻辑，但是很明显这个刷新是等待被通知的，那么何时被刷新呢？
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
我们看到这个Choreographer.CALLBACK_TRAVERSAL,事件是被Choreographer Post的，所以Choreographer应该就是通知我们何时刷新的类。
Choreographer
我们可以看下这个类的注释：Coordinates the timing of animations, input and drawing。
我翻译为控制输入、动画。绘制的时机。
可以看出这个类是告诉我们何时去执行输入、动画和绘制的。
还有一个注释也很重要：
The choreographer receives timing pulses (such as vertical synchronization) from the display subsystem then schedules work to occur as part of rendering the next display frame.
下面是个人的翻译：
Choreographer从显示的子系统中接受到类似垂直同步的时间脉冲，然后安排在下一帧中要呈现的工作。
这个注释告诉我们Choreographer要干两件事:
- 接收垂直同步的时间脉冲，
- 安排在下一帧中要做的工作。
好，我们回到之前的代码postCallback：
public void postCallback(int callbackType, Runnable action, Object token) {
postCallbackDelayed(callbackType, action, token, 0);
}
public void postCallbackDelayed(int callbackType,
Runnable action, Object token, long delayMillis) {
if (action == null) {
throw new IllegalArgumentException("action must not be null");
}
if (callbackType < 0 || callbackType > CALLBACK_LAST) {
throw new IllegalArgumentException("callbackType is invalid");
}
postCallbackDelayedInternal(callbackType, action, token, delayMillis);
}
postCallbackDelayedInternal();
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
...
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
这里有个是否立即执行的判断，如果立即执行则走scheduleFrameLocked(now);否则使用Handler在等待执行，我们搜下MSG_DO_SCHEDULE_CALLBACK可以找到:
case MSG_DO_SCHEDULE_CALLBACK:
doScheduleCallback(msg.arg1);
break;
void doScheduleCallback(int callbackType) {
synchronized (mLock) {
if (!mFrameScheduled) {
final long now = SystemClock.uptimeMillis();
if (mCallbackQueues[callbackType].hasDueCallbacksLocked(now)) {
scheduleFrameLocked(now);
}
}
}
}
所以都是走scheduleFrameLocked(now);区别在于是否立即执行。接着看scheduleFrameLocked()
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;
if (USE_VSYNC) {
// If running on the Looper thread, then schedule the vsync immediately,
// otherwise post a message to schedule the vsync from the UI thread
// as soon as possible.
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
} else {
Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, nextFrameTime);
}
}
}
USE_VSYNC:
// Enable/disable vsync for animations and drawing.
private static final boolean USE_VSYNC = SystemProperties.getBoolean(
"debug.choreographer.vsync", true);
所以if里面是true，至于false的代码我个人猜测是安排在下一帧再刷新。看还是看true都干了啥，这里跟上面一样不过是对是否为ui线程做了个判断，如果是ui线程立即执行 scheduleVsyncLocked();否则通过Handler切换回ui线程，并将这个事件放在队列最前方然后执行scheduleVsyncLocked();
private void scheduleVsyncLocked() {
mDisplayEventReceiver.scheduleVsync();
}
/**
* Schedules a single vertical sync pulse to be delivered when the next
* display frame begins.
*/
public void scheduleVsync() {
if (mReceiverPtr == 0) {
Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
+ "receiver has already been disposed.");
} else {
nativeScheduleVsync(mReceiverPtr);
}
}
nativeScheduleVsync方式是native的目前看不了，所以下面我们就不跟了。不过我们可以看下scheduleVsync()这个方法的注释:Schedules a single vertical sync pulse to be delivered when the next display frame begins.,我个人翻译为：安排一个垂直同步脉冲在下一次显示帧开始时发送。
其实就是在下一帧的时候安排一个垂直同步脉冲(Vsync)给我，这个我是谁呢?就是这个方法所属的类啊
DisplayEventReceiver。一看就名字：显示事件的接收者。
先总结下：postCallback的核心就是让DisplayEventReceiver注册了个Vsync的通知。
DisplayEventReceiver
上面看到了我们注册了对Vsync的监听，那么在哪接收Vsync呢
我们看到Choreographer中有个FrameDisplayEventReceiver里面重写了onVsync()和run()方法。
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable {
private boolean mHavePendingVsync;
private long mTimestampNanos;
private int mFrame;
public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
super(looper, vsyncSource);
}
@Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
.......
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
}
我们可以看到当我们接收到Vsync消息后，通过Handler执行了run()里面的代码。那么我们接下来就看doFrame()中做了什么：
void doFrame(long frameTimeNanos, int frame) {
.......
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);
mFrameInfo.markInputHandlingStart();
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
mFrameInfo.markAnimationsStart();
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
mFrameInfo.markPerformTraversalsStart();
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
} finally {
AnimationUtils.unlockAnimationClock();
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
....
}
除去一些计算操作，我们看到它主要执行了几个doCallBack,而里面有个Choreographer.CALLBACK_ANIMATION就是PostCallBack里面的。我们看下doCallback();
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
.....
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
.....
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
for (CallbackRecord c = callbacks; c != null; c = c.next) {
....
c.run(frameTimeNanos);
}
} finally {
synchronized (mLock) {
mCallbacksRunning = false;
do {
final CallbackRecord next = callbacks.next;
recycleCallbackLocked(callbacks);
callbacks = next;
} while (callbacks != null);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
doCallbacks就是将CallbackQueues中的的CallbackRecord一个个取出来并执行run()方法,
public void run(long frameTimeNanos) {
if (token == FRAME_CALLBACK_TOKEN) {
((FrameCallback)action).doFrame(frameTimeNanos);
} else {
((Runnable)action).run();
}
}
run()其实就是执行了postCallback放进去的mTraversalRunnable。
现在在回看postCallback：
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
public void postCallback(int callbackType, Runnable action, Object token) {
postCallbackDelayed(callbackType, action, token, 0);
}
public void postCallbackDelayed(int callbackType,
Runnable action, Object token, long delayMillis) {
.....
postCallbackDelayedInternal(callbackType, action, token, delayMillis);
}
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
.....
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
....
}
public void addCallbackLocked(long dueTime, Object action, Object token) {
CallbackRecord callback = obtainCallbackLocked(dueTime, action, token);
.....
}
private CallbackRecord obtainCallbackLocked(long dueTime, Object action, Object token) {
CallbackRecord callback = mCallbackPool;
if (callback == null) {
callback = new CallbackRecord();
} else {
mCallbackPool = callback.next;
callback.next = null;
}
.......
}
ViewRootImp在执行scheduleTraversals()方法时就是把包含Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable的信息封装到了CallbackRecord的对象中，并添加到CallbackQueue，现在doCallback中将其取出然后执行run()方法，它的run()就是执行 doTraversal();
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
至此基本就串上了。
总结
我们调用View的requestLayout或者invalidate时，最终都会触发ViewRootImp执行scheduleTraversals()方法。这个方法中ViewRootImp会通过Choreographer来注册个接收Vsync的监听，当接收到系统体层发送来的Vsync后我们就执行doTraversal()来重新绘制界面。通过上面的分析我们调用invalidate等刷新操作时，系统并不会立即刷新界面，而是等到Vsync消息后才会刷新页面。
当然Choreographer还会通知动画输入等其他事件。至于关于Vsync的分析可以参考一下博客：
破译Android性能优化中的16ms问题
Android垂直同步信号VSync的产生及传播结构详解
