Android 消息机制,Looper、Handler、Message 解析
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首先要知道为毛会有这样的一个机制?很多人都知道因为Android不允许在非主线程(UI线程)去更新UI的,那又为啥不允许,你想想,如果多线程去并发访问UI,会使得UI出现混乱的情况。那不是给线程加锁就可以了。我说加你煤,你考虑到加锁会造成线程阻塞么?然而会使得UI的访问效率大大降低。所以就引入了Handler的机制了。当然,这并不是Handler的全部作用。
使用方法就不用说了吧!!!
关键就是需要理解Looper、Handler、Message,Threadlocal这四个东东。啊哈哈哈。
下面看看Looper、Handler、Message的关联运作。
Looper:
Looper的作用相当于一个消息泵,当异步消息启动后,这个东东就不断的循环从MessageQueue当中去读取消息,而Handler就是发消息的。
Looper主要是有两个方法prepare()和loop()。因为在UI线程启动之后,looper已经初始化好了,无需再调用这两个方法。如果你在非主线程创建Handler,那就必须初始化Looper。
下面看看prepare()方法:
这个方法就是把一个Looper实例放到/** Initialize the current thread as a looper. * This gives you a chance to create handlers that then reference * this looper, before actually starting the loop. Be sure to call * {@link #loop()} after calling this method, and end it by calling * {@link #quit()}. */public static void prepare() { prepare(true);}private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed));}
ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();里面,ThreadLocal是一个可以在线程储存数据的类,并且确保线程初始化Looper的时候只能够调用一次prepare方法。Looper的构造方法:private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread();}噢,就是实例化mQueue对象,拿到当前线程。现在就知道了原来Looer会创建Messagequeue了。
loop()方法:/** * 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(); //拿到当前sThreadLocal存储的looper实例 if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue;//拿到当前mqueue实例 // 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();//对Binder不是好熟,打面。。。。 for (;;) {//然后就进入一个无限的循环去获取消息。 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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(msg);//这个target点进去是个Handler额,然后几经波折最后是调用到了public void handleMessage(Message msg) { },啊哈哈哈,这TM不是我们要重写的方法吗? 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();//回收一个使用过的msg。 }}日,这个内容有点多。。。。自己看注解。
接下来是Messagequeue 消息队列。它是一个单链表的数据结构来维护消息的额。这个有enqueueMessage和next两个主要方法。enqueueMessage是在消息列表中插入一条消息,next方法是从消息列表中提取一条消息,提取完后就直接把消息移出队列,555. 这个源码有兴趣的自己去看,这里就不贴了。
最后就是Handler了首先看看构造方法:
厄噢,里面拿到mLooper和mQueue实例了。再看看经常用到的sendMessage()的方法,TM点到最后就是实际调用了enqueueMessage(queue, msg, uptimeMillis);这个方法,上面提到的消息队列里的方法,插入一条消息。哈哈,终于有点恍然大悟了。。/** * Use the {@link Looper} for the current thread with the specified callback interface * and set whether the handler should be asynchronous. * * Handlers are synchronous by default unless this constructor is used to make * one that is strictly asynchronous. * * Asynchronous messages represent interrupts or events that do not require global ordering * with respect to synchronous messages. Asynchronous messages are not subject to * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}. * */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()); } } 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;}
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