In Flutter, how to use Null Safety in Future?

Question

I'm trying to migrate to Null Safety in Flutter. The following code snippet to convert ImageProvider to ByteData works fine without Null Safety.

static Future<ByteData> imgProvider2ByteData(ImageProvider img) async {
Completer<ByteData> completer = new Completer<ByteData>();
img
    .resolve(new ImageConfiguration())
    .addListener(ImageStreamListener((ImageInfo info, bool _) {
      completer.complete(info.image.toByteData(format: ui.ImageByteFormat.png));
}));
return completer.future;

}

///pass ImageProvider to above function 

ImageProvider provider=....

imgProvider2ByteData(provider).then((val) {
        var data = val.buffer.asUint8List();
 });

After enabling Null Safety, the function toByteData return Future<ByteData?> so I made the following changes (adding ? on type and ! on Future return value):

static Future<ByteData?> imgProvider2ByteData(ImageProvider img) async {
Completer<ByteData?> completer = new Completer<ByteData>();
img
    .resolve(new ImageConfiguration())
    .addListener(ImageStreamListener((ImageInfo info, bool _) {
      completer.complete(info.image.toByteData(format: ui.ImageByteFormat.png));
}));
return completer.future;

}

    /*pass ImageProvider to above function */

ImageProvider provider=....

imgProvider2ByteData(provider).then((val) {
        var data = val!.buffer.asUint8List();
 });

When I run it, I got the following error:

Exception caught by image resource service type 'Future<ByteData?>' is not a subtype of type 'FutureOr?' of 'value'

I must have done something wrong but can't figure how to use Null Safety on Future return value. Appreciate any help.

score 1 · Answer 1 · answered May 24 '21 at 06:52

Since toByteData() returns a Future<ByteData?>, you should also return the same from your method. Try this:

Future<ByteData?> imgProvider2ByteData(ImageProvider img) async {
  var completer = Completer<ByteData?>();
  img.resolve(ImageConfiguration()).addListener(ImageStreamListener((ImageInfo info, bool _) {
    completer.complete(info.image.toByteData());
  }));
  return completer.future;
}

In Flutter, how to use Null Safety in Future?

1 Answers1