HandlerThread源码

/**
 * Handy class for starting a new thread that has a looper. The looper can then be 
 * used to create handler classes. Note that start() must still be called.
 */开启一个线程内部有一个looper, 这个looper可以用来创建handler. 切记一定要调用start方法.
public class HandlerThread extends Thread {
    .....
}
public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
 }

@Override
public void run() {
    mTid = Process.myTid();
    Looper.prepare();
    synchronized (this) {
        mLooper = Looper.myLooper();
        notifyAll();
    }
    Process.setThreadPriority(mPriority);
    onLooperPrepared();
    Looper.loop();
    mTid = -1;
}

其实我们初始化和启动了一个线程，既然是线程那就看 run 方法， run 方法中创建了 Looper 对象，然后设置线程等级，开启循环 loop() .