android在不同的版本都會優化“UI的流暢性”問題，但是直到在android 4.1版本中做了有效的優化，這就是Project Butter。
Project Butter加入了三個核心元素:VSYNC、Triple Buffer和Choreographer。其中，VSYNC是理解Project Buffer的核心。VSYNC是Vertical Synchronization的縮寫 也就是“垂直同步”
- VSYNC：產生一箇中斷訊號
- Triple Buffer：當雙Buffer不夠使用時，該系統可分配第三塊Buffer
- Choreographer：這個了用來接受一個VSYNC訊號來統一協調UI更新
檢測應用卡頓的方案，Android系統每隔16.6ms發出VSYNC訊號，來通知介面進行輸入、動畫、繪製等動作，每一次同步的週期為16.6ms，代表一幀的重新整理頻率，理論上來說兩次回撥的時間週期應該在16.6ms，如果超過了16.6ms我們則認為發生了卡頓，利用兩次回撥間的時間週期來判斷是否發生卡頓 這個方案的原理主要是通過Choreographer類設定它的FrameCallback函式，當每一幀被渲染時會觸發回撥FrameCallback， FrameCallback回撥void doFrame (long frameTimeNanos)函式。一次介面渲染會回撥doFrame方法，如果兩次doFrame之間的間隔大於16.6ms說明發生了卡頓。
監控應用的流暢度一般都是通過Choreographer類的postFrameCallback方法註冊一個VSYNC回撥事件
public static void start(final Builder builder) {
Choreographer.getInstance().postFrameCallback(new Choreographer.FrameCallback() {
long lastFrameTimeNanos = 0;
long currentFrameTimeNanos = 0;
@Override
public void doFrame(long frameTimeNanos) {
if (lastFrameTimeNanos == 0) {
lastFrameTimeNanos = frameTimeNanos;
LogMonitor.getInstance().setFrequency(builder.frame * 17 / 2);
if (builder.targetPackageName != null) {
LogMonitor.getInstance().setTargetPackageName(builder.targetPackageName);
}
LogMonitor.getInstance().setDumpListener(builder.onDumpListener);
}
currentFrameTimeNanos = frameTimeNanos;
skipFrameCount = skipFrameCount(lastFrameTimeNanos, currentFrameTimeNanos, deviceRefreshRateMs);
LogMonitor.getInstance().setFrame(skipFrameCount);
if (LogMonitor.getInstance().isMonitor()) {
LogMonitor.getInstance().removeMonitor();
}
LogMonitor.getInstance().startMonitor();
lastFrameTimeNanos = currentFrameTimeNanos;
Choreographer.getInstance().postFrameCallback(this);
}
});
}
複製程式碼postFrameCallback(FrameCallback callback)的原始碼
public void postFrameCallback(FrameCallback callback) {
postFrameCallbackDelayed(callback, 0);
}
......
public void postFrameCallbackDelayed(FrameCallback callback, long delayMillis) {
if (callback == null) {
throw new IllegalArgumentException("callback must not be null");
}
postCallbackDelayedInternal(CALLBACK_ANIMATION,
callback, FRAME_CALLBACK_TOKEN, delayMillis);
}
複製程式碼postFrameCallback最終呼叫了postCallbackDelayedInternal()方法，我們在跟蹤進去這個方法
postCallbackDelayedInternal()
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG_FRAMES) {
Log.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + 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);
}
}
}
複製程式碼postCallbackDelayedInternal方法中首選通過 mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);將我們註冊的回撥介面新增到mCallbackQueues佇列中。每種型別的callback按照設定的執行時間(dueTime)順序排序分別儲存在一個單連結串列中。
之後判斷執行時間是否為當前時間，如果是直接呼叫scheduleFrameLocked(now);否則傳送一個MSG_DO_SCHEDULE_CALLBACK一個訊息，其實傳送資訊最後也是呼叫scheduleFrameLocked(now)方法，所以我們直接看這個方法的程式碼
scheduleFrameLocked
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;
if (USE_VSYNC) {//預設為true
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.
if (isRunningOnLooperThreadLocked()) { //是否是主執行緒
scheduleVsyncLocked();
} else {//發訊息給主執行緒
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);
}
}
}
複製程式碼USE_VSYNC 預設是 true，表示預設開啟垂直同步
private static final boolean USE_VSYNC = SystemProperties.getBoolean(
"debug.choreographer.vsync", true);
複製程式碼scheduleVsyncLocked方法的程式碼
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);
}
}
複製程式碼最終通過調native方法nativeScheduleVsync(mReceiverPtr)向底層註冊我們的回撥事件。 到此我們向系統註冊“垂直同步”事件的流程已經結束。所有的流程如下：
接收VSYNS訊號
VSYNC訊號怎麼同步到我們的程式碼的呢，Java 層接收 VSYNC 的入口是 dispatchVsync()，也就是說每當系統底層產生一個VSYNC訊號，系統都會回撥這個方法。
dispatchVsync()
// Called from native code.
@SuppressWarnings("unused")
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
onVsync(timestampNanos, builtInDisplayId, frame);
}
複製程式碼 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) {
......
mTimestampNanos = timestampNanos;
mFrame = 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);
}
}
複製程式碼onVsync方法中，Message.obtain(mHandler, this) 所以 msg.callback 是 this，最後會呼叫到 msg.callback.run()，也就是 FrameDisplayEventReceiver run()，進入 doFrame() mTimestampNanos，它是來自 onVsync 的 timestampNanos 引數，代表產生 VSYNC 的時間
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
if (!mFrameScheduled) {
return; // no work to do
}
if (DEBUG_JANK && mDebugPrintNextFrameTimeDelta) {
mDebugPrintNextFrameTimeDelta = false;
Log.d(TAG, "Frame time delta: "
+ ((frameTimeNanos - mLastFrameTimeNanos) * 0.000001f) + " ms");
}
long intendedFrameTimeNanos = frameTimeNanos;
startNanos = System.nanoTime();//正真開始的時間
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
// 時間差除以每幀時間間隔，來計算丟掉了幾幀。其中mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());一般重新整理率為60，時間間隔為16.6ms
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
// 取餘數，作為幀偏移時間
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
+ "which is more than the frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Skipping " + skippedFrames + " frames and setting frame "
+ "time to " + (lastFrameOffset * 0.000001f) + " ms in the past.");
}
frameTimeNanos = startNanos - lastFrameOffset;
}
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.");
}
scheduleVsyncLocked();
return;
}
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
mFrameScheduled = false;
mLastFrameTimeNanos = frameTimeNanos;
}
//執行對應的callBack
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);
}
if (DEBUG_FRAMES) {
final long endNanos = System.nanoTime();
Log.d(TAG, "Frame " + frame + ": Finished, took "
+ (endNanos - startNanos) * 0.000001f + " ms, latency "
+ (startNanos - frameTimeNanos) * 0.000001f + " ms.");
}
}
複製程式碼如果你平時注意卡頓的日誌資訊，那麼下面這個段log就不會陌生了
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
複製程式碼SKIPPED_FRAME_WARNING_LIMIT的預設值是30，也就說當我們的程式卡頓大於30時會列印這條log資訊 doFrame()方法最後呼叫doCallbacks()來處理使用者輸入，動畫，繪製等UI操作。
doCallbacks()方法
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
// We use "now" to determine when callbacks become due because it's possible
// for earlier processing phases in a frame to post callbacks that should run
// in a following phase, such as an input event that causes an animation to start.
final long now = System.nanoTime();
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
if (callbacks == null) {
return;
}
mCallbacksRunning = true;
......
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
for (CallbackRecord c = callbacks; c != null; c = c.next) {
if (DEBUG_FRAMES) {
Log.d(TAG, "RunCallback: type=" + callbackType
+ ", action=" + c.action + ", token=" + c.token
+ ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
}
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);
}
}
複製程式碼extractDueCallbacksLocked 是取出執行時間在當前時間之前的所有 CallbackRecord，CallbackRecord 是一個連結串列，然後遍歷 callbacks 執行 run 方法
private static final class CallbackRecord {
public CallbackRecord next;
public long dueTime;
public Object action; // Runnable or FrameCallback
public Object token;
public void run(long frameTimeNanos) {
if (token == FRAME_CALLBACK_TOKEN) {
((FrameCallback)action).doFrame(frameTimeNanos);
} else {
((Runnable)action).run();
}
}
}
複製程式碼如果們通過postFrameCallback(FrameCallback)註冊回撥事件，下一次 Choreographer doFrame 時就會呼叫 FrameCallback.doFrame，還記得剛開始我們註冊FrameCallback是系統為封裝FrameCallback的型別嗎 正是FRAME_CALLBACK_TOKEN，因此這裡會走 ((FrameCallback)action).doFrame(frameTimeNanos);，為什麼我們每次都需要註冊一個下呢，這是因為每次“垂直同步”都會刪除呼叫的註冊事件。 如果這個CallbackRecord是view動畫或繪製就會呼叫((Runnable)action).run();
下面是收到VSYNC的流程圖
小結
- Choreographer是執行緒單例的，而且必須要和一個Looper繫結，因為其內部有一個Handler需要和Looper繫結。
- 首先我們通過postFrameCallback(FrameCallback callback)方法，最終通過native方法 nativeScheduleVsync(mReceiverPtr)和一個VSYNC繫結。
- 當下一次VSYNC訊號來時，回撥我們繫結的介面，然後統計兩針之間的時間，判斷是否掉幀
- 如果掉幀獲取卡頓日誌，繼續監控下個VSYNC訊號