public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
//1 检测线程
checkThread();
mLayoutRequested = true;
//2
scheduleTraversals();
}
}
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注释1 是检测当前是否是在主线程
2.1 ViewRootImpl#checkThread
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
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Message next() {
...
for (;;) {
...
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) { //若是msg不为空而且target为空
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
...
}
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// Enable/disable vsync for animations and drawing. 系统属性参数，默认true
private static final boolean USE_VSYNC = SystemProperties.getBoolean(
"debug.choreographer.vsync", true);
...
private void scheduleFrameLocked(long now) {
//标志位，避免没必要要的屡次调用
if (!mFrameScheduled) {
mFrameScheduled = true;
if (USE_VSYNC) {
if (DEBUG_FRAMES) {
Log.d(TAG, "Scheduling next frame on 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.
//1 若是当前线程是UI线程，直接执行scheduleFrameLocked，不然经过Handler处理
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
} else {
final long nextFrameTime = Math.max(
mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now);
if (DEBUG_FRAMES) {
Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");
}
Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, nextFrameTime);
}
}
}
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/**
* 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); //一、请求vsync
}
}
// Called from native code. //二、vsync来的时候底层会经过JNI回调这个方法
@SuppressWarnings("unused")
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
onVsync(timestampNanos, builtInDisplayId, frame);
}
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/**
* Called when a vertical sync pulse is received.
* The recipient should render a frame and then call {@link #scheduleVsync}
* to schedule the next vertical sync pulse.
*
* @param timestampNanos The timestamp of the pulse, in the {@link System#nanoTime()}
* timebase.
* @param builtInDisplayId The surface flinger built-in display id such as
* {@link SurfaceControl#BUILT_IN_DISPLAY_ID_MAIN}.
* @param frame The frame number. Increases by one for each vertical sync interval.
*/
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
}
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private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable {
private boolean mHavePendingVsync;
private long mTimestampNanos;
private int mFrame;
public FrameDisplayEventReceiver(Looper looper) {
super(looper);
}
@Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
// Post the vsync event to the Handler.
// The idea is to prevent incoming vsync events from completely starving
// the message queue. If there are no messages in the queue with timestamps
// earlier than the frame time, then the vsync event will be processed immediately.
// Otherwise, messages that predate the vsync event will be handled first.
long now = System.nanoTime();
// 更正时间戳，当前纳秒
if (timestampNanos > now) {
Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f)
+ " ms in the future! Check that graphics HAL is generating vsync "
+ "timestamps using the correct timebase.");
timestampNanos = now;
}
if (mHavePendingVsync) {
Log.w(TAG, "Already have a pending vsync event. There should only be "
+ "one at a time.");
} else {
mHavePendingVsync = true;
}
mTimestampNanos = timestampNanos;
mFrame = frame;
Message msg = Message.obtain(mHandler, this); //1 callback是this，会回调run方法
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame); //2
}
}
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void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
...
long intendedFrameTimeNanos = frameTimeNanos;
startNanos = System.nanoTime();
// 1 当前时间戳减去vsync来的时间，也就是主线程的耗时时间
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
//1帧是16毫秒，计算当前跳过了多少帧，好比超时162毫秒，那么就是跳过了10帧
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
// SKIPPED_FRAME_WARNING_LIMIT 默认是30，超时了30帧以上，那么就log提示
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
// 取余，计算离上一帧多久了，一帧是16毫秒，因此lastFrameOffset 在0-15毫秒之间，这里单位是纳秒
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
+ "which is more than the 8frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Skipping " + skippedFrames + " frames and setting frame "
+ "time to " + (lastFrameOffset * 0.000001f) + " ms in the past.");
}
// 出现掉帧，把时间修正一下，对比的是上一帧时间
frameTimeNanos = startNanos - lastFrameOffset;
}
//二、时间倒退了，多是因为改了系统时间，此时就从新申请vsync信号(通常不会走这里)
if (frameTimeNanos < mLastFrameTimeNanos) {
if (DEBUG_JANK) {
Log.d(TAG, "Frame time appears to be going backwards. May be due to a "
+ "previously skipped frame. Waiting for next vsync.");
}
//这里申请下一次vsync信号，流程跟上面分析同样了。
scheduleVsyncLocked();
return;
}
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
mFrameScheduled = false;
mLastFrameTimeNanos = frameTimeNanos;
}
//3 能绘制的话，就走到下面
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);
}
}
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public static void loop() {
...
for (;;) {
...
// This must be in a local variable, incase a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
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