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--- lwc:programming:mobile:android:threads:handlers [2022/02/23 14:11] – created John Harrison
+++ lwc:programming:mobile:android:threads:handlers [2022/02/23 15:37] (current) – John Harrison
@@ Line 46: / Line 46: @@
 }
 </code>
+==== Pass message then receive back from the other class ====
+<code Java>
+// first we create the class and implement the callback interface
+public class GetMessageClass implements Handler.Callback {
+    Handler handler; // get the mainUI handler for messaging back
+    public GetMessageClass(Handler h) {
+        handler = h;
+    }
+    @Override
+    public boolean handleMessage(@NonNull Message msg) {
+        // parse the message using the message key
+        Bundle bundle = msg.getData();
+        String string = bundle.getString("mykey");
+        Log.v("javademo3","got the message: " + string);
+        // boilerplate to send a message back
+        Message msg2 = handler.obtainMessage(); // ?
+        Bundle bundle2 = new Bundle();
+        bundle2.putString("mykey2", "pong");
+        msg2.setData(bundle2);
+        handler.sendMessage(msg2);
+        return false;
+    }
+}
+</code>
+and in the main activity:
+<code Java>
+protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        setContentView(R.layout.activity_main);
+        // after we have a class with a callback, we create a thread for the handler.
+        // every handler has a looper which actually calls the handler:
+        HandlerThread handlerThread = new HandlerThread("myHandlerThread");
+        handlerThread.start();
+        Looper looper = handlerThread.getLooper();
+        // if you want to send a message back to the main activity in the main thread
+        // create a handler for it in the main thread and put the function inside the handler:
+        Handler mainThreadHandler = new Handler(getMainLooper()) {
+            @Override
+            public void handleMessage(Message msg) {
+                Bundle bundle = msg.getData();
+                Log.v("javademo3", "Got this message back: " + bundle.getString("mykey2"));
+            }
+        };
+        // create an instance of the class and pass the main thread handler to it for messaging back
+        GetMessageClass getMessageClass = new GetMessageClass(mainThreadHandler);
+        // create a new handler and assign it to the looper in the handler thread
+        Handler handler = new Handler(looper, getMessageClass);
+        // boilerplate to send a message
+        Message msg = handler.obtainMessage(); // ?
+        Bundle bundle = new Bundle();
+        bundle.putString("mykey", "ping");
+        msg.setData(bundle);
+        handler.sendMessage(msg);
+    }
+    </code>
+There are examples as yet unexplored of handlers posting runnables. For example, this sort of thing should work according to my brain and not tested by a living breathing compiler:
+<code Java>
+handler.post(new Runnable() {
+                        @Override
+                        public void run() {
+                            Log.v("someCrazyCode","I got here");
+                        }
+                    });
+</code>
+===== Resources =====
+ * https://medium.com/mindorks/mastering-android-handler-4f710296bdc6