IdleHandler分析

/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
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();
}

@UnsupportedAppUsage
    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.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        //注释0
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            nativePollOnce(ptr, nextPollTimeoutMillis);
            /**
            *这部分代码在上一篇讲解过了
            */               
                // 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.
                //注释1
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                //注释2
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }
                //注释3
                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }
            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            // 注释4
            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);
                }
                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }
            //注释5 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;
        }
    }