handler机制详解

2015年1月30日 13:51:32     晴 

handler机制详解

1.     handler机制
1. 一个Activity启动的时候，先是ActivityManager先为每一个application创建一个ActivityThread，然后调用ActivityThread里面的main方法，其中有一个 代码  
   

   Looper.prepareMainLooper();

   而Looper类中  Looper.prepareMainLooper();方法调用的是

   public static final void prepare() {
   if (sThreadLocal.get() != null) {
   throw new RuntimeException("Only one Looper may be created per thread");
   }
   sThreadLocal.set(new Looper());
   }

   从线程级共享对象中设置一个looper对象，如果这个对象已经被创建过，则抛出异常
    
   接下来回到ActivityThread接着看它的main方法
   接下来有一行代码

   Looper.loop();

   Looper类中
   

   public static final void loop() {
   Looper me = myLooper();
   MessageQueue queue = me.mQueue;
   while (true) {
   Message msg = queue.next(); // might block
   //if (!me.mRun) {
   // break;
   //}
   if (msg != null) {
   if (msg.target == null) {
   // No target is a magic identifier for the quit message.
   return;
   }
   if (me.mLogging!= null) me.mLogging.println(
   ">>>>> Dispatching to " + msg.target + " "
   + msg.callback + ": " + msg.what
   );
   msg.target.dispatchMessage(msg);
   if (me.mLogging!= null) me.mLogging.println(
   "<<<<< Finished to " + msg.target + " "
   + msg.callback);
   msg.recycle();
   }
   }
   }

   说明一个Application应用中，主线程ActivityThread一直存在一个looper死循环，
   处理界面更新和activity生命周期及service等操作

    

   还是看它的第一行
   

   Looper me = myLooper();

    

   接着往下看  myLooper()代码;

   public static final Looper myLooper() {
   return (Looper)sThreadLocal.get();
   }

   从线程级共享对象中获取Looper.prepareMainLooper()

   存入的looper对象，而且因为上面的一个判断，所以每个界面取出来的looper都是相同的；
   这里实现了多个Activity取得都是同一个Looper
   
   
2. 主线程ActivityThread中还有一个点 就是它的handler是它自己自定义的
   

   private final class H extends Handler {
   public static final int LAUNCH_ACTIVITY = 100;
   public static final int PAUSE_ACTIVITY = 101;
   public static final int PAUSE_ACTIVITY_FINISHING= 102;
   public static final int STOP_ACTIVITY_SHOW = 103;
   public static final int STOP_ACTIVITY_HIDE = 104;
   public static final int SHOW_WINDOW = 105;
   public static final int HIDE_WINDOW = 106;
   public static final int RESUME_ACTIVITY = 107;
   public static final int SEND_RESULT = 108;
   public static final int DESTROY_ACTIVITY = 109;
   public static final int BIND_APPLICATION = 110;
   public static final int EXIT_APPLICATION = 111;
   public static final int NEW_INTENT = 112;
   public static final int RECEIVER = 113;
   public static final int CREATE_SERVICE = 114;
   public static final int SERVICE_ARGS = 115;
   public static final int STOP_SERVICE = 116;
   public static final int REQUEST_THUMBNAIL = 117;
   public static final int CONFIGURATION_CHANGED = 118;
   public static final int CLEAN_UP_CONTEXT = 119;
   public static final int GC_WHEN_IDLE = 120;
   public static final int BIND_SERVICE = 121;
   public static final int UNBIND_SERVICE = 122;
   public static final int DUMP_SERVICE = 123;
   public static final int LOW_MEMORY = 124;
   public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
   public static final int RELAUNCH_ACTIVITY = 126;
   public static final int PROFILER_CONTROL = 127;
   public static final int CREATE_BACKUP_AGENT = 128;
   public static final int DESTROY_BACKUP_AGENT = 129;
   public static final int SUICIDE = 130;
   public static final int REMOVE_PROVIDER = 131;
   public static final int ENABLE_JIT = 132;
   public static final int DISPATCH_PACKAGE_BROADCAST = 133;
   public static final int SCHEDULE_CRASH = 134;

   我们可以看懂很多的状态，当状态改变时发送不同Message的H消息，从而进行相应的处理
3. 最终总结
   1. 先ActivityThread被ActivityManager创建，然往ThreadLocal里面存一个创建的Looper，接下来执行Loop 一直阻塞操作更新UI和生命周期，用户点击后发消息时,调用的是sendMessageAtTime()方法,然后被放入到消息队列MessageQueue中,等待Looper去next获取,当获取到了时候,looper调用的msg.target的dispatchMessage方法,这个target指的就是与之对应的handler,最后调用的handler的handeMessage方法,实现多线程之间的通讯
   2. 子线程中可以有handler,但是必须有一个与之对应的looper,调用looper.prepare来初始化一个looper,其中在它的构造函数中已经创建了一个与之对应的消息队列MessageQueue,然后调用looper.loop在子线程中开启轮询,即可实现子线程自己的消息队列,这点有些类似于线程池