Recently, the handler has been playing. I suddenly want to have a global handler. I meditate for 3 hours and finally get it done!

1. First, create the Handler and the interface for callback in the Application, and print the address value of this interface (for later testing)

   //Application.class
   private static Handler mHandler = new Handler(Looper.getMainLooper()) {
      @Override
      public void handleMessage(Message msg) {
          super.handleMessage(msg);
          mListener.heandleMessage(msg);
          System.out.println("mListener Address value==== " + mListener);
      }
   };
   
   private static HandlerListener mListener;
   public static void setOnHandlerListener(HandlerListener listener) {
      mListener = listener;
   }
   public  static HandlerListener getListener(){
      return mListener;
   }
   
   public  interface HandlerListener {
      void heandleMessage(Message msg);
   }

2. In the Application, it is simulated to create a sub thread to send a message, which is usually sent after a network request to inform the Activity to modify the UI

   //Application.class
   @Override
   public void onCreate() {
       super.onCreate();
       mContext = this;
       new Thread() {
           @Override
           public void run() {
               super.run();
               for (int i = 0; i < 15; i++) {
                   try {
                       sleep(500);
                   } catch (InterruptedException e) {
                       e.printStackTrace();
                   }
                   mHandler.sendEmptyMessage(i);
               }
           }
       }.start();
   }

3. Set this listening in MainActivity

   //MainActivity.class
       @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
       MyApplication.setOnHandlerListener(new MyApplication.HandlerListener() {
           @Override
           public void heandleMessage(Message msg) {
               System.out.println("MainActivity_msg==== " + msg.what);
           }
       });
   ｝
   
   //Add a jump button
   mButton = (Button) findViewById(R.id.button);
      mButton.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
          //Jump to HandlerActivity
              Intent intent = new Intent(MainActivity.this, HandlerActivity.class);
              startActivity(intent);
          }
      });

4. Loop print the message that starts out, and the address value of the current mListener, the good play is about to start!

   System.out: MainActivity_msg==== 0
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$2@22078df
   System.out: MainActivity_msg==== 1
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$2@22078df
   System.out: MainActivity_msg==== 2
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$2@22078df
   System.out: MainActivity_msg==== 3
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$2@22078df
   System.out: MainActivity_msg==== 4
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$2@22078df
   System.out: MainActivity_msg==== 5

5. Create another HandlerActivity and jump to HandlerActivity when the message is sent. What's the result?

   //HandlerActivity.class
   //HandlerActivity also sets a message listener
    mHandlerListener = new MyApplication.HandlerListener() {
           @Override
           public void heandleMessage(Message msg) {
               System.out.println("HandlerActivity_msg==== " + msg.what);
           }
       };

6. It's obvious that the first monitor was covered, in fact
   Omit some log s, the address changes, and the Acticviy of the received message changes

   System.out: MainActivity_msg==== 4
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$1@54a87ea
   System.out: MainActivity_msg==== 5
   System.out: mListener Address value==== com.hdjz.mytest.activity.MainActivity$1@54a87ea
   System.out: HandlerActivity_msg==== 6
   System.out: mListener Address value==== com.hdjz.mytest.activity.HandlerActivity$2@aa3a08b
   System.out: HandlerActivity_msg==== 7
   System.out: mListener Address value==== com.hdjz.mytest.activity.HandlerActivity$2@aa3a08b
   System.out: HandlerActivity_msg==== 8
   System.out: mListener Address value==== com.hdjz.mytest.activity.HandlerActivity$2@aa3a08b
   System.out: HandlerActivity_msg==== 9
   System.out: mListener Address value==== com.hdjz.mytest.activity.HandlerActivity$2@aa3a08b

7. When we return to the previous activity, the log continues to print the current activity data. The previous monitoring has been replaced. To this end, I can only say that if we lose it, we will lose it forever and never come back. Love!

8. Note: the only handler in the whole project is message.what. Different message types have different functions. It's the only way to avoid the embarrassment of creating a handler for every activity