错误：无法将参数类型“void Function(MidiDevice)”分配给参数类型“FutureOr”<dynamic> 函数（列表<mididevice> ？）'</mididevice></dynamic>

Question

I am new to Flutter and I am trying to get familiar with the flutter_midi_command package. I have an ElevatedButton and I want to print the list of available MIDI devices on click.

ElevatedButton(
  onPressed: () => MidiCommand().devices.then((MidiDevice midiDevice) => print(midiDevice)),
  child: const Text('Show available MIDI devices'),
)

But in the .then part, I am getting the following error as soon as I define the type of the returned value from the callback:

The argument type 'void Function(MidiDevice)' can't be assigned to the parameter type 'FutureOr<dynamic> Function(List<MidiDevice>?)'.

And but when I remove the type ( MidiDevice ), the error is removed. I believe I am missing a core concept from Dart/Flutter. My question is, how I am able to simply make this callback function to work when I have the type of the returned value defined?

Answer 1

I managed to solve my question. The type of the returned value is not MidiDevice , its List<MidiDevice>? .

The following is the correct implementation:

ElevatedButton(
  onPressed: () => MidiCommand().devices.then((List<MidiDevice>? midiDevice) => print(midiDevice)),
  child: const Text('Show available MIDI devices'),
)