Handler机制

概述

一个Thread里有一个Looper，一个Looper里有一个MessageQueue，这个MessageQueue里维护一个消息队列有存取消息的功能，而Handle是用来发消息和处理消息的

在一个线程中开启一个死循环Loop，然后Loop不停地从MessageQueue中取得消息，把取得的消息交给Handler去处理，当然Handler首先要往消息队里发消息

Looper

1.初始化一个消息队列 2.将looper对象和当前线程一一对应，线程安全，通过ThreadLocal 3.开启循环,运行消息队列，不断里从消息队列里取得消息

1.looper.prepare()

1.初始化一个消息队列 2.将looper对象和当前线程一一对应，线程安全

``` private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } //通过ThreadLoacal，将当前线程和该loop绑定子一起（为了线程安全，确保一个线程内只有一个looper） sThreadLocal.set(new Looper(quitAllowed)); } //这里的构造方法是一个私有的Looper，外界要通过Looper.myLooper()得到looper的实例，而myLooper()方法，得到的是一个与所在线程绑定的一个Looper private Looper(boolean quitAllowed) { //在looper内部建立一个消息队列 mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); } public static @Nullable Looper myLooper() { return sThreadLocal.get(); } ------------------------- public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) return (T)e.value; } return setInitialValue(); } ------------------------- public void set(T value) { Thread t = Thread.currentThread(); //ThreadLocalMap 类似于一个HashMap ThreadLocalMap map = getMap(t); if (map != null) //以自己(ThreadLoca属于一个Thread，所以ThreadLocal代表了Thread)为key，传入的looper为value，绑定线程与looper的关系 map.set(this, value); else createMap(t, value); }

looper.loop()

1.开启循环,运行消息队列，不断里从消息队列里取得消息

/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ public static void loop() { //这一步本质是sThreadLocal.get()，如果没有进行prepare，（prepare本质是 sThreadLocal.set(looper);），这里取到的Looper 是null final Looper me = myLooper(); if (me == null) { //确保在loop开启死循环时，进行了prepare操作 throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } //得到looper里的消息队列 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(); //开启了死循环 for (;;) { //从消息队列里去消息 Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. //没消息，消息队列终止 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 traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } try { //msg.target指的就是发送给msg的那个handler，下面就是调用了handler.dispatchMessage方法 msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } 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); } //消息回收，因为Message维护一个消息池，用完的消息要回收，以便复用 msg.recycleUnchecked(); } }

Handler

处理消息发送消息

1. 处理消息handler.dispatchMessage(msg)

/** * Handle system messages here. */ public void dispatchMessage(Message msg) { //你handler.post（runnable），相当于把这个runnable赋值给了msg.callBack if (msg.callback != null) { //此时msg.callBack不是null，执行下面方法，导致 ★handleMessage(msg)方法不执行 handleCallback(msg); } else { if (mCallback != null) { //mCallback是Handler的成员变量，是一个接口，你可以给这个接口赋值（new Handler(callback)），并重写handleMessage执行其内部方法，并设置返回值true，表示不执行★handleMessage(msg)方法；返回false，表示继续执行★handleMessage(msg)方法 if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg);//★这个就是我们一般在new handler重写的handleMessage方法，我们自己去定义怎样去处理消息。但是这个方法不是一定会执行 } }

2. 发送消息

post(Runnable), postAtTime(Runnable, long), postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message), sendMessageAtTime(Message, long), and sendMessageDelayed(Message, long)

基本分为两类post->runnable和send->message(发送延迟和定点消息就不说了，可用于实现定时任务和u循环任务)

post(Runnable r)

//★★★如果你的Handler在主线程，那么post(Runnable r)中的runnable的run方法也在主线程中，不会开启子线程的 public final boolean post(Runnable r) { //本质是发送了一个message，只不过通关过getPostMessage(r),把r复制给了msg.callback,注意此方法的副作用，导致在dispatchMessage时，★handleMessage(msg)不执行 //或者说sendMessage是发送不含runnable的msg，而post是发送带有runnable的msg return sendMessageDelayed(getPostMessage(r), 0); } private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }

sendMessageDelayed(Message msg, long delayMillis)

public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } 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; } //发送消息，本质上就是将消息把消息排序到消息队列中 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); }

MessageQueue

用来存放线程放入的消息。

enqueueMessage(Message msg, long when)把消息按照时间顺序压入队列中，消息可以插队哦

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; }

next()取消息

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; } }

handler—thread—looper—-messageQueue四个之间如何产生的对应关系

handler在初始化时拿到了他所处线程的mLooper 线程与looper之前通过threadLocal建立关系），而这个mLooper 里有MessageQueue，这样handler—thread—looper—-messageQueue四个之间产生了关系

public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) { final Class<? extends Handler> klass = getClass(); if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && (klass.getModifiers() & Modifier.STATIC) == 0) { Log.w(TAG, "The following Handler class should be static or leaks might occur: " + klass.getCanonicalName()); } } //looper拿到了 mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }