Android消息机制 空闲消息处理器
16lz
2021-01-23
之前我们分析过Android的消息机制，但是有一个空闲消息处理器我们没有分析过，这篇博客我们来看下。
一、MessageQueue空闲处理器
之前在分析MessageQueue的next函数，这个函数就是获取队列的下个Message，我们再来看下。
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);//native函数，底层就是epoll机制 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);//下个Message的时间 } 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;//返回message } } else { // No more messages. nextPollTimeoutMillis = -1;//message为空，底层永久阻塞 } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } //当消息为空，或者消息时间没到 （这个时候代表线程空闲） 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);//把mIdleHandler的数据放入mPendingIdelHandlers } // 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();//调用IdleHandler的queueIdle函数 } 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; } }上面这个函数是一个无线循环直到找到一个Message，先调用nativePollOnce，这个函数之前分析过是在c层调用epoll机制。然后继续分析，当有消息时，消息的时间没有，我们就计算下个消息的时间，下次传入nativePollOnce。当有消息时间到了，我们就返回该消息。如果没有消息naxtPollTimeoutMillis就为-1（永久阻塞）。
下面我们再来看看关于空闲消息处理器，首先只有第一次才会赋值给pendingIdleHandlerCount。其次一定要Message为空，或者Message还没有到时间（这样代表线程空闲）。这样才会赋值给pendingIdleHandlerCount， 然后把mIdleHandlers 给mPendingIdleHandlers。
最后遍历mPendingIdleHandlers，调用每一个IdleHandler 的queueIdle方法，调用完之后就去除这个IdleHandler。
下面我们再来看下IdleHandler 这个接口。是MessageQueue的内部接口。
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(); }二、举例（ActivityThread的Idler内部类）
下面我们通过一个例子，我们先来看ActivityThread的Idler类，这个类就是在Activity的线程空闲时，调用AMS的activityIdle函数。
private class Idler implements MessageQueue.IdleHandler { @Override public final boolean queueIdle() { ActivityClientRecord a = mNewActivities; boolean stopProfiling = false; if (mBoundApplication != null && mProfiler.profileFd != null && mProfiler.autoStopProfiler) { stopProfiling = true; } if (a != null) { mNewActivities = null; IActivityManager am = ActivityManagerNative.getDefault(); ActivityClientRecord prev; do { if (localLOGV) Slog.v( TAG, "Reporting idle of " + a + " finished=" + (a.activity != null && a.activity.mFinished)); if (a.activity != null && !a.activity.mFinished) { try { am.activityIdle(a.token, a.createdConfig, stopProfiling); a.createdConfig = null; } catch (RemoteException ex) { // Ignore } } prev = a; a = a.nextIdle; prev.nextIdle = null; } while (a != null); } if (stopProfiling) { mProfiler.stopProfiling(); } ensureJitEnabled(); return false; } }...... if (!r.onlyLocalRequest) { r.nextIdle = mNewActivities; mNewActivities = r; if (localLOGV) Slog.v( TAG, "Scheduling idle handler for " + r); Looper.myQueue().addIdleHandler(new Idler()); }......addIdleHandler就是往mIdleHandlers成员变量加入给IdleHandler。
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