一、概述
Choreographer翻译成中文是编舞者的意思，在Android系统4.1开始加入这个类，主要来控制同步处理输入（input），动画（animation），绘制（draw），在UI显示的时候每一帧完成的只有这三种。在这个类的前面有一行注释，大概意思就是要协调控制三个UI操作的时序，这也和其编舞者名称相符合。
/**
* Coordinates the timing of animations, input and drawing.
*
* 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.
*
* Applications typically interact with the choreographer indirectly using
* higher level abstractions in the animation framework or the view hierarchy.
* Here are some examples of things you can do using the higher-level APIs.
*
*
* To post an animation to be processed on a regular time basis synchronized with
* display frame rendering, use {@link android.animation.ValueAnimator#start}.
* To post a {@link Runnable} to be invoked once at the beginning of the next display
* frame, use {@link View#postOnAnimation}.
* To post a {@link Runnable} to be invoked once at the beginning of the next display
* frame after a delay, use {@link View#postOnAnimationDelayed}.
* To post a call to {@link View#invalidate()} to occur once at the beginning of the
* next display frame, use {@link View#postInvalidateOnAnimation()} or
* {@link View#postInvalidateOnAnimation(int, int, int, int)}.
* To ensure that the contents of a {@link View} scroll smoothly and are drawn in
* sync with display frame rendering, do nothing. This already happens automatically.
* {@link View#onDraw} will be called at the appropriate time.
* However, there are a few cases where you might want to use the functions of the
* choreographer directly in your application. Here are some examples.
*
*
* If your application does its rendering in a different thread, possibly using GL,
* or does not use the animation framework or view hierarchy at all
* and you want to ensure that it is appropriately synchronized with the display, then use
* {@link Choreographer#postFrameCallback}.
* ... and that's about it.
*
*
* Each {@link Looper} thread has its own choreographer. Other threads can
* post callbacks to run on the choreographer but they will run on the {@link Looper}
* to which the choreographer belongs.
*
*/在类中有四种任务队列，当收到Vsync信号时会执行这四种任务队列里面的任务
private final CallbackQueue[] mCallbackQueues;
void doFrame(long frameTimeNanos, int frame) {
....
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);
....
}CALLBACK_INPUTL：输入
CALLBACK_ANIMATION：动画
CALLBACK_TRAVERSAL：绘制，执行measure，layout，draw
CALLBACK_COMMIT：绘制完成的提交操作，用来修正动画的启动时间。这里主要是为了解决ValueAnimator问题而引入的，因为遍历时间过长导致动画时间启动过长，时间缩短，导致跳帧，这里修改动画第一个frame开始时间延后。
// Update the frame time if necessary when committing the frame.
// We only update the frame time if we are more than 2 frames late reaching
// the commit phase. This ensures that the frame time which is observed by the
// callbacks will always increase from one frame to the next and never repeat.
// We never want the next frame's starting frame time to end up being less than
// or equal to the previous frame's commit frame time. Keep in mind that the
// next frame has most likely already been scheduled by now so we play it
// safe by ensuring the commit time is always at least one frame behind.
if (callbackType == Choreographer.CALLBACK_COMMIT) {
final long jitterNanos = now - frameTimeNanos;
Trace.traceCounter(Trace.TRACE_TAG_VIEW, "jitterNanos", (int) jitterNanos);
if (jitterNanos >= 2 * mFrameIntervalNanos) {
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos
+ mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Commit callback delayed by " + (jitterNanos * 0.000001f)
+ " ms which is more than twice the frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Setting frame time to " + (lastFrameOffset * 0.000001f)
+ " ms in the past.");
mDebugPrintNextFrameTimeDelta = true;
}
frameTimeNanos = now - lastFrameOffset;
mLastFrameTimeNanos = frameTimeNanos;
}
}
二、Choreographer启动流程
在前面介绍的view绘制原理的流程中，在WMG.addView会对ViewRootImpl进行初始化操作
2.1 创建ViewRootImpl
[->ViewRootImpl.java]
public ViewRootImpl(Context context, Display display) {
mContext = context;
mWindowSession = WindowManagerGlobal.getWindowSession();
mDisplay = display;
mBasePackageName = context.getBasePackageName();
mThread = Thread.currentThread();
mLocation = new WindowLeaked(null);
mLocation.fillInStackTrace();
mWidth = -1;
mHeight = -1;
mDirty = new Rect();
mTempRect = new Rect();
mVisRect = new Rect();
mWinFrame = new Rect();
mWindow = new W(this);
mTargetSdkVersion = context.getApplicationInfo().targetSdkVersion;
mViewVisibility = View.GONE;
mTransparentRegion = new Region();
mPreviousTransparentRegion = new Region();
mFirst = true; // true for the first time the view is added
mAdded = false;
mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this,
context);
mAccessibilityManager = AccessibilityManager.getInstance(context);
mAccessibilityManager.addAccessibilityStateChangeListener(
mAccessibilityInteractionConnectionManager, mHandler);
mHighContrastTextManager = new HighContrastTextManager();
mAccessibilityManager.addHighTextContrastStateChangeListener(
mHighContrastTextManager, mHandler);
mViewConfiguration = ViewConfiguration.get(context);
mDensity = context.getResources().getDisplayMetrics().densityDpi;
mNoncompatDensity = context.getResources().getDisplayMetrics().noncompatDensityDpi;
mFallbackEventHandler = new PhoneFallbackEventHandler(context);
//获取Choreographer
mChoreographer = Choreographer.getInstance();
mDisplayManager = (DisplayManager)context.getSystemService(Context.DISPLAY_SERVICE);
if (!sCompatibilityDone) {
sAlwaysAssignFocus = mTargetSdkVersion < Build.VERSION_CODES.P;
sCompatibilityDone = true;
}
loadSystemProperties();
}2.2 创建Choreographer
[->Choreographer.java]
public static Choreographer getInstance() {
return sThreadInstance.get();
}[->Choreographer.java]
// Thread local storage for the choreographer.
private static final ThreadLocal<Choreographer> sThreadInstance =
new ThreadLocal<Choreographer>() {
@Override
protected Choreographer initialValue() {
//获取当前线程的Looper
Looper looper = Looper.myLooper();
if (looper == null) {
throw new IllegalStateException("The current thread must have a looper!");
}
Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
//如果是主线程
if (looper == Looper.getMainLooper()) {
mMainInstance = choreographer;
}
return choreographer;
}
};当前线程所在的为主线程（UI线程），可以看到每个线程都有一个Choreographer
[->Choreographer.java]
private Choreographer(Looper looper, int vsyncSource) {
mLooper = looper;
//初始化FrameHandler
mHandler = new FrameHandler(looper);
//创建用于接收Vsync信号的对象
mDisplayEventReceiver = USE_VSYNC
? new FrameDisplayEventReceiver(looper, vsyncSource)
: null;
mLastFrameTimeNanos = Long.MIN_VALUE;
//getRefreshRate()一般为60Hz
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
//创建回调任务队列
mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
for (int i = 0; i <= CALLBACK_LAST; i++) {
mCallbackQueues[i] = new CallbackQueue();
}
// b/68769804: For low FPS experiments.
setFPSDivisor(SystemProperties.getInt(ThreadedRenderer.DEBUG_FPS_DIVISOR, 1));
}mLastFrameTimeNanos：上一次帧绘制的时间点
mFrameIntervalNanos,帧间时长一般等于16.7ms
2.3 初始化FrameHandler
[->Choreographer.java]
private final class FrameHandler extends Handler {
public FrameHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_DO_FRAME:
doFrame(System.nanoTime(), 0);
break;
case MSG_DO_SCHEDULE_VSYNC:
doScheduleVsync();
break;
case MSG_DO_SCHEDULE_CALLBACK:
doScheduleCallback(msg.arg1);
break;
}
}
}2.4 创建FrameDisplayEventReceiver
[->Choreographer.java]
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable {
....
public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
super(looper, vsyncSource);
}
....
}2.4.1 DisplayEventReceiver
[->DisplayEventReceiver.java]
public DisplayEventReceiver(Looper looper) {
this(looper, VSYNC_SOURCE_APP);
}
/**
* Creates a display event receiver.
*
* @param looper The looper to use when invoking callbacks.
* @param vsyncSource The source of the vsync tick. Must be on of the VSYNC_SOURCE_* values.
*/
public DisplayEventReceiver(Looper looper, int vsyncSource) {
if (looper == null) {
throw new IllegalArgumentException("looper must not be null");
}
//获取主线程的消息队列
mMessageQueue = looper.getQueue();
mReceiverPtr = nativeInit(new WeakReference<DisplayEventReceiver>(this), mMessageQueue,
vsyncSource);
mCloseGuard.open("dispose");
}2.4.2 nativeInit
[->/core/jni/android_view_DisplayEventReceiver.cpp]
static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
jobject messageQueueObj, jint vsyncSource) {
sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
if (messageQueue == NULL) {
jniThrowRuntimeException(env, "MessageQueue is not initialized.");
return 0;
}
sp<NativeDisplayEventReceiver> receiver = new NativeDisplayEventReceiver(env,
receiverWeak, messageQueue, vsyncSource);
status_t status = receiver->initialize();
if (status) {
String8 message;
message.appendFormat("Failed to initialize display event receiver. status=%d", status);
jniThrowRuntimeException(env, message.string());
return 0;
}
//获取DisplayEventReceiver对象的引用
receiver->incStrong(gDisplayEventReceiverClassInfo.clazz); // retain a reference for the object
return reinterpret_cast<jlong>(receiver.get());
}2.4.3 NativeDisplayEventReceiver
[->/core/jni/android_view_DisplayEventReceiver.cpp]
NativeDisplayEventReceiver::NativeDisplayEventReceiver(JNIEnv* env,
jobject receiverWeak, const sp<MessageQueue>& messageQueue, jint vsyncSource) :
DisplayEventDispatcher(messageQueue->getLooper(),
static_cast<ISurfaceComposer::VsyncSource>(vsyncSource)),
mReceiverWeakGlobal(env->NewGlobalRef(receiverWeak)),
mMessageQueue(messageQueue) {
ALOGV("receiver %p ~ Initializing display event receiver.", this);
}DisplayEventDispatcher继承于LooperCallback，mReceiverWeakGlobal记录java层DisplayEventReceiver对象的全局引用。
2.4.4 initialize
[->libs/androidfw/DisplayEventDispatcher.cpp]
status_t DisplayEventDispatcher::initialize() {
status_t result = mReceiver.initCheck();
if (result) {
ALOGW("Failed to initialize display event receiver, status=%d", result);
return result;
}
int rc = mLooper->addFd(mReceiver.getFd(), 0, Looper::EVENT_INPUT,
this, NULL);
if (rc < 0) {
return UNKNOWN_ERROR;
}
return OK;
}监听mReceiver所获取文件句柄，一旦有数据到来，则回调this,所复写LooperCallback对象的handleEvent方法
2.4.5 addFd
[->system/core/libutils/Looper.cpp]
int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ addFd - fd=%d, ident=%d, events=0x%x, callback=%p, data=%p", this, fd, ident,
events, callback.get(), data);
#endif
if (!callback.get()) {
if (! mAllowNonCallbacks) {
ALOGE("Invalid attempt to set NULL callback but not allowed for this looper.");
return -1;
}
if (ident < 0) {
ALOGE("Invalid attempt to set NULL callback with ident < 0.");
return -1;
}
} else {
ident = POLL_CALLBACK;
}
{ // acquire lock
AutoMutex _l(mLock);
Request request;
request.fd = fd;
request.ident = ident;
request.events = events;
request.seq = mNextRequestSeq++;
//将callback赋值
request.callback = callback;
request.data = data;
if (mNextRequestSeq == -1) mNextRequestSeq = 0; // reserve sequence number -1
struct epoll_event eventItem;
request.initEventItem(&eventItem);
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex < 0) {
int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d: %s", fd, strerror(errno));
return -1;
}
mRequests.add(fd, request);
} else {
int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);
if (epollResult < 0) {
if (errno == ENOENT) {
// Tolerate ENOENT because it means that an older file descriptor was
// closed before its callback was unregistered and meanwhile a new
// file descriptor with the same number has been created and is now
// being registered for the first time. This error may occur naturally
// when a callback has the side-effect of closing the file descriptor
// before returning and unregistering itself. Callback sequence number
// checks further ensure that the race is benign.
//
// Unfortunately due to kernel limitations we need to rebuild the epoll
// set from scratch because it may contain an old file handle that we are
// now unable to remove since its file descriptor is no longer valid.
// No such problem would have occurred if we were using the poll system
// call instead, but that approach carries others disadvantages.
#if DEBUG_CALLBACKS
ALOGD("%p ~ addFd - EPOLL_CTL_MOD failed due to file descriptor "
"being recycled, falling back on EPOLL_CTL_ADD: %s",
this, strerror(errno));
#endif
epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
if (epollResult < 0) {
ALOGE("Error modifying or adding epoll events for fd %d: %s",
fd, strerror(errno));
return -1;
}
scheduleEpollRebuildLocked();
} else {
ALOGE("Error modifying epoll events for fd %d: %s", fd, strerror(errno));
return -1;
}
}
mRequests.replaceValueAt(requestIndex, request);
}
} // release lock
return 1;
}2.4.6 handleEvent
[->system/core/libutils/Looper.cpp]
int Looper::pollInner(int timeoutMillis) {
...
// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
if (response.request.ident == POLL_CALLBACK) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
this, response.request.callback.get(), fd, events, data);
#endif
// Invoke the callback. Note that the file descriptor may be closed by
// the callback (and potentially even reused) before the function returns so
// we need to be a little careful when removing the file descriptor afterwards.
//调用handleEvent方法
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd, response.request.seq);
}
// Clear the callback reference in the response structure promptly because we
// will not clear the response vector itself until the next poll.
response.request.callback.clear();
result = POLL_CALLBACK;
}
}
return result;
}三、Vsync信号回调流程
当Vsync信号来时，经过层层调用后会执行handleEvent方法，详细流程会在SurfaceFlinger中介绍。
3.1 handleEvent
[->libs/androidfw/DisplayEventDispatcher.cpp]
int DisplayEventDispatcher::handleEvent(int, int events, void*) {
if (events & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP)) {
ALOGE("Display event receiver pipe was closed or an error occurred. "
"events=0x%x", events);
return 0; // remove the callback
}
if (!(events & Looper::EVENT_INPUT)) {
ALOGW("Received spurious callback for unhandled poll event. "
"events=0x%x", events);
return 1; // keep the callback
}
// Drain all pending events, keep the last vsync.
nsecs_t vsyncTimestamp;
int32_t vsyncDisplayId;
uint32_t vsyncCount;
//清除所有的pending事件，只保留最后一次Vsync
if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount)) {
ALOGV("dispatcher %p ~ Vsync pulse: timestamp=%" PRId64 ", id=%d, count=%d",
this, ns2ms(vsyncTimestamp), vsyncDisplayId, vsyncCount);
mWaitingForVsync = false;
//分发Vsync
dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount);
}
return 1; // keep the callback
}3.1.1 processPendingEvents
[->libs/androidfw/DisplayEventDispatcher.cpp]
bool DisplayEventDispatcher::processPendingEvents(
nsecs_t* outTimestamp, int32_t* outId, uint32_t* outCount) {
bool gotVsync = false;
DisplayEventReceiver::Event buf[EVENT_BUFFER_SIZE];
ssize_t n;
while ((n = mReceiver.getEvents(buf, EVENT_BUFFER_SIZE)) > 0) {
ALOGV("dispatcher %p ~ Read %d events.", this, int(n));
for (ssize_t i = 0; i < n; i++) {
const DisplayEventReceiver::Event& ev = buf[i];
switch (ev.header.type) {
case DisplayEventReceiver::DISPLAY_EVENT_VSYNC:
// Later vsync events will just overwrite the info from earlier
// ones. That's fine, we only care about the most recent.
gotVsync = true;
*outTimestamp = ev.header.timestamp;
*outId = ev.header.id;
*outCount = ev.vsync.count;
break;
case DisplayEventReceiver::DISPLAY_EVENT_HOTPLUG:
dispatchHotplug(ev.header.timestamp, ev.header.id, ev.hotplug.connected);
break;
default:
ALOGW("dispatcher %p ~ ignoring unknown event type %#x", this, ev.header.type);
break;
}
}
}
if (n < 0) {
ALOGW("Failed to get events from display event dispatcher, status=%d", status_t(n));
}
return gotVsync;
}遍历所有的事件，当有多个Vsync事件来时，则只关注最近一次的事件
3.1.2 dispatchVsync(C++)
[->core/jni/android_view_DisplayEventReceiver.cpp]
void NativeDisplayEventReceiver::dispatchVsync(nsecs_t timestamp, int32_t id, uint32_t count) {
JNIEnv* env = AndroidRuntime::getJNIEnv();
ScopedLocalRef<jobject> receiverObj(env, jniGetReferent(env, mReceiverWeakGlobal));
if (receiverObj.get()) {
ALOGV("receiver %p ~ Invoking vsync handler.", this);
//调用java层的dispatchVsync方法
env->CallVoidMethod(receiverObj.get(),
gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, id, count);
ALOGV("receiver %p ~ Returned from vsync handler.", this);
}
mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}3.2 dispatchVsync(Java)
[->DisplayEventReceiver.java]
// Called from native code.
@SuppressWarnings("unused")
@UnsupportedAppUsage
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
onVsync(timestampNanos, builtInDisplayId, frame);
}之后的流程参考View的绘制原理。在之前写的文章中有详细分析Android10从WMS角度分析应用启动过程
四、总结
这里主要介绍了Choreographer的启动流程以及其回调的流程。
1.Choreographer主要来控制同步处理输入（input），动画（animation），绘制（draw）；
2.通过监听mReceiver所获取文件句柄，来获取回调；
3.可以通过Choreographer getInstance().postFrameCallback来监听帧率。
