void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
            boolean fullInvalidate) {
            // Propagate the damage rectangle to the parent view.
            final AttachInfo ai = mAttachInfo;
            final ViewParent p = mParent;
            if (p != null && ai != null && l < r && t < b) {
                final Rect damage = ai.mTmpInvalRect;
                damage.set(l, t, r, b);
                p.invalidateChild(this, damage);
            }

ViewRootImpl.java
void invalidate() {
    mDirty.set(0, 0, mWidth, mHeight);
    if (!mWillDrawSoon) {
        scheduleTraversals();
    }
}

ViewRootImpl.java
 // 將UI繪制的mTraversalRunnable加入到下次垂直同步信號(hào)到來(lái)的等待callback中去
 // mTraversalScheduled用來(lái)保證本次Traversals未執(zhí)行前，不會(huì)要求遍歷兩邊，浪費(fèi)16ms內(nèi)，不需要繪制兩次
void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        // 防止同步柵欄，同步柵欄的意思就是攔截同步消息
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        // postCallback的時(shí)候，順便請(qǐng)求vnsc垂直同步信號(hào)scheduleVsyncLocked
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
         <!--添加一個(gè)處理觸摸事件的回調(diào)，防止中間有Touch事件過(guò)來(lái)-->
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

Choreographer.java
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) {
        <!--申請(qǐng)VSYNC同步信號(hào)-->
            scheduleFrameLocked(now);
        } 
    }
}

// mFrameScheduled保證16ms內(nèi)，只會(huì)申請(qǐng)一次垂直同步信號(hào)
// scheduleFrameLocked可以被調(diào)用多次，但是mFrameScheduled保證下一個(gè)vsync到來(lái)之前，不會(huì)有新的請(qǐng)求發(fā)出
// 多余的scheduleFrameLocked調(diào)用被無(wú)效化
private void scheduleFrameLocked(long now) {
    if (!mFrameScheduled) {
        mFrameScheduled = true;
        if (USE_VSYNC) {
            if (isRunningOnLooperThreadLocked()) {
                scheduleVsyncLocked();
            } else {
                // 因?yàn)閕nvalid已經(jīng)有了同步柵欄，所以必須mFrameScheduled，消息才能被UI線程執(zhí)行
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                msg.setAsynchronous(true);
                mHandler.sendMessageAtFrontOfQueue(msg);
            }
        }  
    }
}

  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) {
            long now = System.nanoTime();
            if (timestampNanos > now) {
            <!--正常情況，timestampNanos不應(yīng)該大于now，一般是上傳vsync的機(jī)制出了問(wèn)題-->
                timestampNanos = now;
            }
            <!--如果上一個(gè)vsync同步信號(hào)沒(méi)執(zhí)行，那就不應(yīng)該相應(yīng)下一個(gè)（可能是其他線程通過(guò)某種方式請(qǐng)求的）-->
              if (mHavePendingVsync) {
                Log.w(TAG, "Already have a pending vsync event.  There should only be "
                        + "one at a time.");
            } else {
                mHavePendingVsync = true;
            }
            <!--timestampNanos其實(shí)是本次vsync產(chǎn)生的時(shí)間，從服務(wù)端發(fā)過(guò)來(lái)-->
            mTimestampNanos = timestampNanos;
            mFrame = frame;
            Message msg = Message.obtain(mHandler, this);
            <!--由于已經(jīng)存在同步柵欄，所以VSYNC到來(lái)的Message需要作為異步消息發(fā)送過(guò)去-->
            msg.setAsynchronous(true);
            mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
        }
        @Override
        public void run() {
            mHavePendingVsync = false;
            <!--這里的mTimestampNanos其實(shí)就是本次Vynsc同步信號(hào)到來(lái)的時(shí)候，但是執(zhí)行這個(gè)消息的時(shí)候，可能延遲了-->
            doFrame(mTimestampNanos, mFrame);
        }
    }

void doFrame(long frameTimeNanos, int frame) {
    final long startNanos;
    synchronized (mLock) {
    <!--做了很多東西，都是為了保證一次16ms有一次垂直同步信號(hào)，有一次input 、刷新、重繪-->
        if (!mFrameScheduled) {
            return; // no work to do
        }
       long intendedFrameTimeNanos = frameTimeNanos;
        startNanos = System.nanoTime();
        final long jitterNanos = startNanos - frameTimeNanos;
        <!--檢查是否因?yàn)檠舆t執(zhí)行掉幀，每大于16ms，就多掉一幀-->
        if (jitterNanos >= mFrameIntervalNanos) {
            final long skippedFrames = jitterNanos / mFrameIntervalNanos;
            <!--跳幀，其實(shí)就是上一次請(qǐng)求刷新被延遲的時(shí)間，但是這里skippedFrames為0不代表沒(méi)有掉幀-->
            if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
            <!--skippedFrames很大一定掉幀，但是為 0，去并非沒(méi)掉幀-->
                Log.i(TAG, "Skipped " + skippedFrames + " frames!  "
                        + "The application may be doing too much work on its main thread.");
            }
            final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
                <!--開始doFrame的真正有效時(shí)間戳-->
            frameTimeNanos = startNanos - lastFrameOffset;
        }
        if (frameTimeNanos < mLastFrameTimeNanos) {
            <!--這種情況一般是生成vsync的機(jī)制出現(xiàn)了問(wèn)題，那就再申請(qǐng)一次-->
            scheduleVsyncLocked();
            return;
        }
          <!--intendedFrameTimeNanos是本來(lái)要繪制的時(shí)間戳，frameTimeNanos是真正的，可以在渲染工具中標(biāo)識(shí)延遲VSYNC多少-->
        mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
        <!--移除mFrameScheduled判斷，說(shuō)明處理開始了，-->
        mFrameScheduled = false;
        <!--更新mLastFrameTimeNanos-->
        mLastFrameTimeNanos = frameTimeNanos;
    }
    try {
         <!--真正開始處理業(yè)務(wù)-->
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
        <!--處理打包的move事件-->
        mFrameInfo.markInputHandlingStart();
        doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
        <!--處理動(dòng)畫-->
        mFrameInfo.markAnimationsStart();
        doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
        <!--處理重繪-->
        mFrameInfo.markPerformTraversalsStart();
        doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
        <!--提交->
        doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

  final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

// 這里是真正執(zhí)行了，
void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        <!--移除同步柵欄，只有重繪才設(shè)置了柵欄，說(shuō)明重繪的優(yōu)先級(jí)還是挺高的，所有的同步消息必須讓步-->
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        performTraversals();
    }
}

View.java
    public RenderNode updateDisplayListIfDirty() {
        final RenderNode renderNode = mRenderNode;
          ...
        if ((mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == 0
                || !renderNode.isValid()
                || (mRecreateDisplayList)) {
           <!--失效了，需要重繪-->
        } else {
        <!--依舊有效，無(wú)需重繪-->
            mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        }
        return renderNode;
    }