invokeMethod method

Future invokeMethod (String method, [ dynamic arguments ])

Invokes a method on this channel with the specified arguments.

The static type of arguments is dynamic, but only values supported by the codec of this channel can be used. The same applies to the returned result. The values supported by the default codec and their platform-specific counterparts are documented with StandardMessageCodec.

Returns a Future which completes to one of the following:

  • a result (possibly null), on successful invocation;
  • a PlatformException, if the invocation failed in the platform plugin;
  • a MissingPluginException, if the method has not been implemented by a platform plugin.

Sample code

The following code snippets demonstrate how to invoke platform methods in Dart using a MethodChannel and how to implement those methods in Java (for Android) and Objective-C (for iOS). The code might be packaged up as a musical plugin, see flutter.io/developing-packages/:

class Music {
  static const MethodChannel _channel = const MethodChannel('music');

  static Future<bool> isLicensed() async {
    // invokeMethod returns a Future<dynamic>, and we cannot pass that for
    // a Future<bool>, hence the indirection.
    final bool result = await _channel.invokeMethod('isLicensed');
    return result;
  }

  static Future<List<Song>> songs() async {
    // invokeMethod here returns a Future<dynamic> that completes to a
    // List<dynamic> with Map<dynamic, dynamic> entries. Post-processing
    // code thus cannot assume e.g. List<Map<String, String>> even though
    // the actual values involved would support such a typed container.
    final List<dynamic> songs = await _channel.invokeMethod('getSongs');
    return songs.map(Song.fromJson).toList();
  }

  static Future<void> play(Song song, double volume) async {
    // Errors occurring on the platform side cause invokeMethod to throw
    // PlatformExceptions.
    try {
      await _channel.invokeMethod('play', <String, dynamic>{
        'song': song.id,
        'volume': volume,
      });
    } on PlatformException catch (e) {
      throw 'Unable to play ${song.title}: ${e.message}';
    }
  }
}

class Song {
  Song(this.id, this.title, this.artist);

  final String id;
  final String title;
  final String artist;

  static Song fromJson(dynamic json) {
    return new Song(json['id'], json['title'], json['artist']);
  }
}
// Assumes existence of an Android MusicApi.
public class MusicPlugin implements MethodCallHandler {
  @Override
  public void onMethodCall(MethodCall call, Result result) {
    switch (call.method) {
      case "isLicensed":
        result.success(MusicApi.checkLicense());
        break;
      case "getSongs":
        final List<MusicApi.Track> tracks = MusicApi.getTracks();
        final List<Object> json = new ArrayList<>(tracks.size());
        for (MusicApi.Track track : tracks) {
          json.add(track.toJson()); // Map<String, Object> entries
        }
        result.success(json);
        break;
      case "play":
        final String song = call.argument("song");
        final double volume = call.argument("volume");
        try {
          MusicApi.playSongAtVolume(song, volume);
          result.success(null);
        } catch (MusicalException e) {
          result.error("playError", e.getMessage(), null);
        }
        break;
      default:
        result.notImplemented();
    }
  }
  // Other methods elided.
}
@interface MusicPlugin : NSObject<FlutterPlugin>
@end

// Assumes existence of an iOS Broadway Play Api.
@implementation MusicPlugin
- (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
  if ([@"isLicensed" isEqualToString:call.method]) {
    result([NSNumber numberWithBool:[BWPlayApi isLicensed]]);
  } else if ([@"getSongs" isEqualToString:call.method]) {
    NSArray* items = [BWPlayApi items];
    NSMutableArray* json = [NSMutableArray arrayWithCapacity:items.count];
    for (BWPlayItem* item in items) {
      [json addObject:@{@"id":item.itemId, @"title":item.name, @"artist":item.artist}];
    }
    result(json);
  } else if ([@"play" isEqualToString:call.method]) {
    NSString* itemId = call.arguments[@"song"];
    NSNumber* volume = call.arguments[@"volume"];
    NSError* error = nil;
    BOOL success = [BWPlayApi playItem:itemId volume:volume.doubleValue error:&error];
    if (success) {
      result(nil);
    } else {
      result([FlutterError errorWithCode:[NSString stringWithFormat:@"Error %ld", error.code]
                                 message:error.domain
                                 details:error.localizedDescription]);
    }
  } else {
    result(FlutterMethodNotImplemented);
  }
}
// Other methods elided.
@end

See also:

Implementation

Future<dynamic> invokeMethod(String method, [dynamic arguments]) async {
  assert(method != null);
  final dynamic result = await BinaryMessages.send(
    name,
    codec.encodeMethodCall(new MethodCall(method, arguments)),
  );
  if (result == null)
    throw new MissingPluginException('No implementation found for method $method on channel $name');
  return codec.decodeEnvelope(result);
}