QueuedWork中的sFinishers

[alwaystest]

QueuedWork中的sFinishers

标签（空格分隔）： Android

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QueuedWork.waitToFinish 方法

    /**
     * Trigger queued work to be processed immediately. The queued work is processed on a separate
     * thread asynchronous. While doing that run and process all finishers on this thread. The
     * finishers can be implemented in a way to check weather the queued work is finished.
     *
     * Is called from the Activity base class's onPause(), after BroadcastReceiver's onReceive,
     * after Service command handling, etc. (so async work is never lost)
     */
    public static void waitToFinish() {
        long startTime = System.currentTimeMillis();
        boolean hadMessages = false;

        Handler handler = getHandler();

        synchronized (sLock) {
            if (handler.hasMessages(QueuedWorkHandler.MSG_RUN)) {
                // Delayed work will be processed at processPendingWork() below
                handler.removeMessages(QueuedWorkHandler.MSG_RUN);

                if (DEBUG) {
                    hadMessages = true;
                    Log.d(LOG_TAG, "waiting");
                }
            }

            // We should not delay any work as this might delay the finishers
            sCanDelay = false;
        }

        StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskWrites();
        try {
            processPendingWork();
        } finally {
            StrictMode.setThreadPolicy(oldPolicy);
        }

        try {
            while (true) {
                Runnable finisher;

                synchronized (sLock) {
                    finisher = sFinishers.poll();
                }

                if (finisher == null) {
                    break;
                }

                finisher.run();
            }
        } finally {
            sCanDelay = true;
        }

        synchronized (sLock) {
            long waitTime = System.currentTimeMillis() - startTime;

            if (waitTime > 0 || hadMessages) {
                mWaitTimes.add(Long.valueOf(waitTime).intValue());
                mNumWaits++;

                if (DEBUG || mNumWaits % 1024 == 0 || waitTime > MAX_WAIT_TIME_MILLIS) {
                    mWaitTimes.log(LOG_TAG, "waited: ");
                }
            }
        }
    }

一开始没有看明白finisher的用途，知道是用来阻塞UI进程，保证所有的PendingWork都被处理完之后才能继续运行的，但是乍一看第32行processPendingWork已经把PendingWork都清理完了，finisher可能是个多余的操作，但是后来仔细想了一下，发现的finisher的作用。

    private static void processPendingWork() {
        long startTime = 0;

        if (DEBUG) {
            startTime = System.currentTimeMillis();
        }

        synchronized (sProcessingWork) {
            LinkedList<Runnable> work;

            synchronized (sLock) {
                work = (LinkedList<Runnable>) sWork.clone();
                sWork.clear();

                // Remove all msg-s as all work will be processed now
                getHandler().removeMessages(QueuedWorkHandler.MSG_RUN);
            }

            if (work.size() > 0) {
                for (Runnable w : work) {
                    w.run();
                }

                if (DEBUG) {
                    Log.d(LOG_TAG, "processing " + work.size() + " items took " +
                            +(System.currentTimeMillis() - startTime) + " ms");
                }
            }
        }
    }
    
    private static class QueuedWorkHandler extends Handler {
        static final int MSG_RUN = 1;

        QueuedWorkHandler(Looper looper) {
            super(looper);
        }

        public void handleMessage(Message msg) {
            if (msg.what == MSG_RUN) {
                processPendingWork();
            }
        }
    }

本来使用apply方法，IO会在单独的线程中处理，如果负责处理IO的HandlerThread正在处理上一批PendingWork，UIThread又调用了waitToFinish, 此时如果UIThread先完成所有的任务，那么finisher就起作用了，UIThread会继续等待HandlerThread操作完另外一批PendingWork才能继续运行。