flutter to get notifications from system activity from kotlin host

Question

I want my flutter app to get a notification every time the android system back button is pressed, and furthermore, I want to otherwise disable the back button. So this is the code I have on the kotlin side of things:

package com.example.ui

import io.flutter.embedding.android.FlutterActivity
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.MethodChannel

class MainActivity: FlutterActivity() {

    override fun onBackPressed() {
        // send message to flutter that the system back button was pressed
        val flutterEngine = getFlutterEngine()
        if (flutterEngine != null) {
            MethodChannel(flutterEngine!!.dartExecutor.binaryMessenger, "backButtonChannel").invokeMethod("backButtonPressed", null)
        }
    }
}

and here's my flutter app main function:

Future<void> backButtonPressed() async {
  // Code to run when the back button is pressed
  print('PRESSED!');
}

const platform = MethodChannel('com.example.ui/back_button');

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  platform.setMethodCallHandler((call) async {
    if (call.method == "backButtonPressed") {
      return backButtonPressed();
    }
  });
  return runApp(MyApp());
}

Unfortunately it seems to have no effect, are you able to see why?

Answer 1

You should make a call by using registered MethodChannel channel

Your kotlin code should be like

MethodChannel(flutterEngine!!.dartExecutor.binaryMessenger, "com.example.ui/back_button").invokeMethod("backButtonPressed", null)

I suggest to use WillPopScope widget to handle back press from dart code