文章目录
一、MessageQueue 空闲任务 IdleHandler 机制
二、MessageQueue 中空闲任务 IdleHandler 相关源码
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 0 00 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(); } }
