Android MessageQueue详解

一、MessageQueue介绍

 

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 * 低级类,持有一个由Looper分配发送的Message集合。
  Message不是直接添加到MessageQueue的,而是通过与Looper相关联的Handler对象来添加。
* <p>You can retrieve the MessageQueue for the current thread with * {
@link Looper#myQueue() Looper.myQueue()}.
  你可以用Looper.myQueue()方法检索到当前线程的MessageQueue。
*/

 

 二、MessageQueue源码

1.IdleHandler

在阅读源码时候看到个有意思的接口:IdleHandler。

这个idleHandler被添加后会在message处理结束后回调,返回false时,说明looper中的message暂时全部处理完,同时IdleHandler会被移除,当返回true时,说明队列中仍有未处理的message,并且在下次message处理完成后继续回掉直到全部message处理完成。

/**
     * Callback interface for discovering when a thread is going to block
     * waiting for more messages.
一个回调接口,用于发现线程何时因等待更多消息而阻塞。
*/ 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.
     当消息队列运行完所有消息并等待更多时调用。
    
返回 true 以保持空闲handle处于活动状态,返回 false 以将其删除。
    如果队列中仍有待处理的消息,则可能会调用此方法,但它们都被安排在当前时间之后调度。
*/ boolean queueIdle(); /** * Returns true if the looper has no pending messages which are due to be processed.
     * 如果 Looper 没有待处理的待处理消息,则返回 true。
     * <p>This method is safe to call from any thread.
    该方法可以在任意线程安全的调用。 * *
@return True if the looper is idle. */ public boolean isIdle() { synchronized (this) { final long now = SystemClock.uptimeMillis(); return mMessages == null || now < mMessages.when; } } /** * Add a new {@link IdleHandler} to this message queue. This may be * removed automatically for you by returning false from * {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is * invoked, or explicitly removing it with {@link #removeIdleHandler}. * 在消息队列里添加一个新的IdleHandler。这会在queueIdle()方法调用时返回false自动删除,
    或者在removeIdleHandler中明确删除。
* <p>This method is safe to call from any thread. * *
@param handler The IdleHandler to be added. */ public void addIdleHandler(@NonNull IdleHandler handler) { if (handler == null) { throw new NullPointerException("Can't add a null IdleHandler"); } synchronized (this) { mIdleHandlers.add(handler); } } /** * Remove an {@link IdleHandler} from the queue that was previously added * with {@link #addIdleHandler}. If the given object is not currently * in the idle list, nothing is done. * * <p>This method is safe to call from any thread. * * @param handler The IdleHandler to be removed. */ public void removeIdleHandler(@NonNull IdleHandler handler) { synchronized (this) { mIdleHandlers.remove(handler); } }

 看下调用地方的代码:

 

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;
        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;
                }

                // 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.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                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)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            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);
                    }
                }
            }

            // 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;
        }
    }

 

posted @ 2021-02-05 16:27  灰熊Grizzly  阅读(502)  评论(0编辑  收藏  举报