为什么我们能在主线程直接使用 Handler，而不需要创建 Looper ？

//android.app.ActivityThread
public static void main(String[] args) {
  //...
  Looper.prepareMainLooper();

  ActivityThread thread = new ActivityThread();
  thread.attach(false);

  if (sMainThreadHandler == null) {
    sMainThreadHandler = thread.getHandler();
  }
  //...
  Looper.loop();

  throw new RuntimeException("Main thread loop unexpectedly exited");
}

/**
 * Initialize the current thread as a looper, marking it as an
 * application's main looper. The main looper for your application
 * is created by the Android environment, so you should never need
 * to call this function yourself.  See also: {@link #prepare()}
 */
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}