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About Android 4 Handler

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About Android 4 Handler

  • Handler
  • Message
  • MessageQueue
  • Looper
  • HandlerThread

Handler

handler 的使用方式:

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//1 post postDelay
 Handler.post / postDelay (new Runnable() {
    @Override
    void run() {
      //todo
    }
  })

//2 sendMessage

创建handler
实现handlerMessage接口

handler.sendMessage(msg)

Activity finish时handler未执行完会引发Activity内存泄漏？

优化

  • 通过静态内部类+弱引用方式实现handler
  • 等等

HandlerThread

HandlerThread继承Thread.

run方法内进行Looper的初始化相关操作

Looper

  • Looper.prepare
    • 从TheadLocal中取得Looper对象，执行多次会抛出异常
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static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
//...
public static void prepare() {
  prepare(true);
}

private static void prepare(boolean quitAllowed) {
  if (sThreadLocal.get() != null) {
    throw new RuntimeException("Only on lopper may be created per thread");
  }
  sThreadLocal.set(new Looper(quitAllowed));
}
  • Looper.loop()
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public static void loop() {
  final Looper me = myLooper();
  //..
  final MessageQueue queue = me.mQueue;
  //..
  for(;;) {
    Message msg = queue.next();
    //..
    msg.target.dispatchMessage(msg)
    //ms.target 是一个handler对象
    //Handler 在 enqueueMessage() 内为msg添加target
  }
  //..
  msg.recycleUnchecked()
}