Android 源码分析（四） Handler 异步消息机制
一.Handler 使用方法:
Handler handler = new Handler() { @Override public void handleMessage(Message msg) { super.handleMessage(msg); switch (msg.what) { case 1: Log.i(TAG,"UI thread sendEmptyMessage...1"); break; case 2: Log.i(TAG,"thread sendEmptyMessage...2"); break; case 3: Log.i(TAG,"thread sendMessage...3"); break; case 4: Log.i(TAG,"UI thread sendMessage...4"); break; } } }; handler.sendEmptyMessage(1); handler.postDelayed(new Runnable() { @Override public void run() { handler.sendEmptyMessage(2); Message msg = new Message(); msg.what = 3; handler.sendMessage(msg); } }, 1000);
Message msg2 = new Message(); msg2.what = 4; handler.sendMessage(msg2);
输出结果:
二.源码分析
从handler.sendMessage(msg)进去看看源码怎么调用的.
//Handler.java //Looper.prepare()方法初始化了一个Looper对象并关联在一个MessageQueue对象，并且一个线程中只有一个Looper对象，只有一个MessageQueue对象。 //Handler的构造方法则在Handler内部维护了当前线程的Looper对象 public Handler(Callback callback, boolean async) { ...... mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } ...... } // if we can get rid of this method, the handler need not remember its loop // we could instead export a getMessageQueue() method... public final Looper getLooper() { return mLooper; } /** * Pushes a message onto the end of the message queue after all pending messages * before the current time. It will be received in {@link #handleMessage}, * in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendMessage(Message msg) { //参数 0 代表,无延迟. return sendMessageDelayed(msg, 0); } /** * Enqueue a message into the message queue after all pending messages * before (current time + delayMillis). You will receive it in * {@link #handleMessage}, in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } /** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) <var>uptimeMillis</var>. * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b> * Time spent in deep sleep will add an additional delay to execution. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. * * @param uptimeMillis The absolute time at which the message should be * delivered, using the * {@link android.os.SystemClock#uptimeMillis} time-base. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public boolean sendMessageAtTime(Message msg, long uptimeMillis) { MessageQueue queue = mQueue; if (queue == null) { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); return false; } return enqueueMessage(queue, msg, uptimeMillis); } private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); } //msg.target就是Handler对象本身；而这里的queue对象就是我们的Handler内部维护的Looper对象关联的MessageQueue对象.
//MessageQueue.java boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } synchronized (this) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); msg.when = when; Message p = mMessages; boolean needWake; if (p == null || when == 0 || when < p.when) { // New head, wake up the event queue if blocked. msg.next = p; mMessages = msg; needWake = mBlocked; } else { // Inserted within the middle of the queue. Usually we don't have to wake // up the event queue unless there is a barrier at the head of the queue // and the message is the earliest asynchronous message in the queue. needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { prev = p; p = p.next; if (p == null || when < p.when) { break; } if (needWake && p.isAsynchronous()) { needWake = false; } } msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }
//Looper.java /** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); //Looper.loop()方法里起了一个死循环，不断的判断MessageQueue中的消息是否为空，如果为空则直接return掉，然后执行queue.next()方法： for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; final long traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); final long end; try { //msg.target就是Handler对象本身 msg.target.dispatchMessage(msg); end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (slowDispatchThresholdMs > 0) { final long time = end - start; if (time > slowDispatchThresholdMs) { Slog.w(TAG, "Dispatch took " + time + "ms on " + Thread.currentThread().getName() + ", h=" + msg.target + " cb=" + msg.callback + " msg=" + msg.what); } } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked(); } }
又回到Handler.java
//Handle.java /** * Handle system messages here. */ public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } private static void handleCallback(Message message) { message.callback.run(); }
补充: UI线程 调用 Loop.
//ActivityThread.java public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); //主线程启动就调用了loop(). Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }
三.总结:
一.Handler,Looper,MessageQue三者关系
1.Looper：相当于消息的载体
1) 它的内部有一个消息队列，也就是MessageQueue，Handler发送的所有消息都会走向这个消息队里。
2) 它的Looper.loop方法是一个死循环，不断的从消息队列MessageQueue中取出消息。如果有消息存在就处理该消息，否则就阻塞。
2.MessageQue：就是一个消息队列，可以向其中添加消息并处理消息。
3.Handler其实就是发送消息处理消息的封装。它与Looper相关联，也就是说在Handler的内部可以找到Looper，找到了Looper就找到了相应的消息队列。因此Handler发送的消息都会走向MessageQueue。
二.为什么不能在子线程中更新UI
如果在子线程更新UI，那么就会出现这样子的一种况：其中一个子线程更新界面还没有完成，另外一个子线程就又去更新UI了，这样子会造成子UI界面错乱。那么你可能会说，我们可以实行加锁机制啊，让更新UI的代码不能并发执行。如果每一个子线程都加锁，那么 毫无疑问程序的性能将会大大下降。因此主要的原因总结起来就是两点：
（1）为了防止界面错乱
（2）为了防止程序性能下降
因此android不允许在子线程中更新UI。为了解决这个这个问题，android就设计出这样子的一套消息处理机制，让我们尽管在子线程中发送更新UI的消息，而不用去关心多线程问题。在主线程的消息队里中采取轮询更新处理。
