flutter - How to call methods in Dart portion of the app, from the native platform using MethodChannel?

Question

I am writing a native plugin that, in some cases, has to call functions in the Flutter portion of the app, written in Dart. How it's achieved, is explained here: https://flutter.io/platform-channels/

Furthermore, an example of invoking a method from the native/platform part towards the Dart/non-native is here: https://github.com/flutter/plugins/tree/master/packages/quick_actions

Now, this example is really nice in case the platform only needs to invoke a method, i.e. that call returns nothing/void, but in case it needs to invoke a function, i.e. needs a return value from the non-native/Dart part, I could not have found an example or documentation on the internet. I believe it can be implemented though, because in the native Java part, there is a method:

public void invokeMethod(String method, Object arguments, MethodChannel.Result callback)

So, there is a callback object that could have a return value from the non-native part - or, I am mistaken here, and there is currently no way of returning a value from the non-native Dart portion of the app?

Answer 1

The signature is void setMethodCallHandler(Future<dynamic> handler(MethodCall call)), so we need to provide a function at the Dart end that returns Future<dynamic>, for example _channel.setMethodCallHandler(myUtilsHandler);

Then implement the handler. This one handles two methods foo and bar returning respectively String and double.

  Future<dynamic> myUtilsHandler(MethodCall methodCall) async {
    switch (methodCall.method) {
      case 'foo':
        return 'some string';
      case 'bar':
        return 123.0;
      default:
        throw MissingPluginException('notImplemented');
    }
  }

At the Java end the return value is passed to the success method of the Result callback.

channel.invokeMethod("foo", arguments, new Result() {
  @Override
  public void success(Object o) {
    // this will be called with o = "some string"
  }

  @Override
  public void error(String s, String s1, Object o) {}

  @Override
  public void notImplemented() {}
});

In Swift, the return value is an Any? passed to the result closure. (Not implemented is signaled by the any parameter being the const NSObject value FlutterMethodNotImplemented.)

channel.invokeMethod("foo", arguments: args, result: {(r:Any?) -> () in
  // this will be called with r = "some string" (or FlutterMethodNotImplemented)
})