/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
new Thread(new PrintLog()).start();
}
private static int START_MESSAGE = 100;
private static int PRINT_MESSAGE = 101;
private static int INNER_MESSAGE = 102;
private static int SYNC_LATER_INTERVAL = 3 * 1000;
private static int count = 0;
private PrintHandler mHandler;
public class PrintHandler extends Handler{
public void handleMessage(Message msg) {
if (msg.what == START_MESSAGE) {
count++;
Log.d("PrintLog","START_MESSAGE:count :" + String.valueOf(count));
Message newmsg = obtainMessage(PRINT_MESSAGE);
sendMessageDelayed(newmsg, SYNC_LATER_INTERVAL);
}
if (msg.what == PRINT_MESSAGE) {
count++;
Message newmsg = obtainMessage(START_MESSAGE);
sendMessageDelayed(newmsg, SYNC_LATER_INTERVAL);
Log.d("PrintLog","print_message:count :" + String.valueOf(count));
}
}
};
@Override
public void run() {
Log.d("PrintLog","run() start ");
// TODO Auto-generated method stub
Looper.prepare();
mHandler = new PrintHandler();
Message msg = mHandler.obtainMessage(START_MESSAGE);
mHandler.sendMessageDelayed(msg, SYNC_LATER_INTERVAL);
//mHandler.sendMessage(msg);
Looper.loop();
Log.d("PrintLog","run() end");
}
}
(1) Looper类别用来为一个线程开启一个消息循环。默认情况下Android中新诞生的线程是没有开启消息循环的。(主线程除外,主线程系统会自动为其创建Looper对象,开启消息循环)
Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper,对应一个MessageQueue。
(2) 通常是通过Handler对象来与Looper交互的。Handler可看做是Looper的一个接口,用来向指定的Looper发送消息及定义处理方法。
默认情况下Handler会与其被定义时所在线程的Looper绑定,比如,在主线程中定义,其是与主线程的Looper绑定。
mainHandler = new Handler() 等价于new Handler(Looper.myLooper()).
Looper.myLooper():Return the Looper object associated with the current thread 获取当前进程的looper对象。
还有一个类似的 Looper.getMainLooper() 用于获取主线程的Looper对象。
(3) 在非主线程中直接new Handler() 会报如下的错误:
E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception
E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
原因是非主线程中默认没有创建Looper对象,需要先调用Looper.prepare()启用Looper。
(4) Looper.loop(); 让Looper开始工作,从消息队列里取消息,处理消息。
注意:写在Looper.loop()之后的代码不会被执行,这个函数内部应该是一个循环,当调用mHandler.getLooper().quit()后,loop才会中止,其后的代码才能得以运行。
Android Looper和Handler分析http://blog.csdn.net/Innost/article/details/6055793