【Android 异步操作】Handler 机制 ( MessageQueue 空闲任务 IdleHandler 机制 )

韩曙亮

发布于 2023-03-28 18:49:44

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发布于 2023-03-28 18:49:44

文章被收录于专栏：韩曙亮的移动开发专栏韩曙亮的移动开发专栏韩曙亮的移动开发专栏

文章目录

一、MessageQueue 空闲任务 IdleHandler 机制

在消息队列 MessageQueue 的 next 获取下一条消息的方法中 ,

在循环获取消息的最后有如下代码 :

public final class MessageQueue {
    @UnsupportedAppUsage
    Message next() {
    	// ...
                // 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)) {
                    // 如果取出的 消息为空 , 就会执行 IdleHandler
                    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);
        // ...
    }
}

开始判定 pendingIdleHandlerCount 大小 , 如果小于

则执行后续与 IdleHandler 相关操作 ;

IdleHandler 是一个接口 , 代码如下 :

public final class MessageQueue {
    /**
     * 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();
    }
}

在上述如果取出的 mMessage 消息为空 mMessages == null , 则执行 IdleHandler ;

IdleHandler 使用时 , 调用 addIdleHandler 函数 , 该函数会将 IdleHandler 加入到 mIdleHandlers 集合中 ,

MessageQueue 消息队列 addIdleHandler 函数如下 :

public final class MessageQueue {
    /**
     * 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}.
     *
     * <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);
        }
    }
}

在 MessageQueue 的 next 方法中继续向下执行 , 开始遍历 mIdleHandlers 集合 , 先将集合 mIdleHandlers 转为 mPendingIdleHandlers 数组 , 然后遍历该数组 ,

遍历时 , 调用 IdleHandler 对象的 queueIdle 方法 ,

如果上面的 queueIdle 方法返回值是 false , 就会从 mIdleHandlers 集合中移除该元素 ,

如果上面的 queueIdle 方法返回值是 true , 那么下一次再次执行到这里时 , 还会继续回调该函数 ;

上述操作对应的 MessageQueue 的 next 方法内的部分源码 :

public final class MessageQueue {
    @UnsupportedAppUsage
    Message next() {
			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;
    }
}

IdleHandler 的作用是 , 当消息队列 MessageQueue 中没有消息处理时 , 处于闲置状态时 , 此时就会回调一次用户注册的 IdleHandler ,

如果该 IdleHandler 的 queueIdle 方法返回 false , 那么该 IdleHandler 只会执行一次 ,

如果该 IdleHandler 的 queueIdle 方法返回 true , 则每次空闲 , 都要执行一次该 IdleHandler ;

二、MessageQueue 中空闲任务 IdleHandler 相关源码

public final class MessageQueue {

    @UnsupportedAppUsage
    Message mMessages;
    @UnsupportedAppUsage
    private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
    private IdleHandler[] mPendingIdleHandlers;

    @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;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
             	// 获取当前的时间 , 需要判定是否有需要延迟发送的消息 
                // 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)) {
                    // 如果取出的 消息为空 , 就会执行 IdleHandler
                    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;
        }
    }

    /**
     * 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();
    }

}

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原始发表：2020-11-05，如有侵权请联系 cloudcommunity@tencent.com 删除

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