IdleHandler類在android中的使用

介紹

在android的MessageQueue中有一個static的接口IdleHandler，這個接口用於在MessageQueue中沒有可處理的Message的時候回調，這樣就能夠在UI線程中處理完全部的view事務以後，回調一些額外的操做而不會block UI線程。

public static interface IdleHandler {
        /**
         * Called when the message queue has run out of messages and will now
         * wait for more.  Return true to keep your idle handler active, false
         * to have it removed.  This may be called if there are still messages
         * pending in the queue, but they are all scheduled to be dispatched
         * after the current time.
         */
        boolean queueIdle();
    }

使用

接口很簡單，只有一個queueIdle方法，用於在進入Idle的時候執行額外的操做，返回值是ture的話，執行完queueIdle方法以後會保留這個IdleHandler，反之則刪除這個IdleHandler。

使用起來也很簡單。

mHandler.getLooper().getQueue().addIdleHandler(new MessageQueue.IdleHandler() {
            @Override
            public boolean queueIdle() {
                Log.e(TAG, "----I am idle");
                return false;
            }
        });

源碼淺析

MessageQueue的Message都是在next方法中執行的，直接看next方法

Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        
        //-------------------第一部分---------------------------
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            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) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }
                //-----------第二部分-----------------------------------
                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                
               //第一次進入，而且msg爲null或者是當前的msg schedule到後面執行
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    //獲取當前IdleHandler的數量
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                 //沒有須要處理的idleHandler，退出
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                //用將全部的idleHandler存入mPendingIdleHandlers
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            //迭代處理
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }
                //根據返回值，選擇是否remove這個idleHandler
                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }

這段代碼能夠分爲兩部分來看，第一部分用於從MessageQueue中取出可用的message，第二部分用於處理IdleHandler。咱們直接從第二部分來看，首先會對當前的msg做一個判斷，若是message爲null或者是schedule到未來執行，那麼就開始準備執行IdleHandler。全部的idleHandler放在mIdleHandlers中，將這些idleHandler轉存到mPendingIdleHandlers,接下來就是迭代處理這些IdleHandler，若是返回值是false，就把這個idleHandler從mIdleHandlers中remove掉。