Cast error when awaiting enumerateDevices()

[davidjenkins]

Dart version 2.7.0 (build 2.7.0-dev.2.1 e4344a5) -- included with Flutter (Win 10 x64)

This issue also occurs in current dev branch of Flutter, which uses Dart 2.7.0 (not sure if "2.7.0" alone means "2.7.0-dev0.0").

If I try this:

import 'dart:html';
// ...
await window.navigator.mediaDevices.enumerateDevices();

Then I get this (in Chrome):

Type 'List<dynamic>' should be 'List<MediaDeviceInfo>' to implement expected type 'FutureOr<List<MediaDeviceInfo>>'.

Full stack:

errors.dart:147 Uncaught (in promise) Error: Type 'List<dynamic>' should be 'List<MediaDeviceInfo>' to implement expected type 'FutureOr<List<MediaDeviceInfo>>'.
    at Object.throw_ [as throw] (errors.dart:196)
    at Object.castError (errors.dart:45)
    at Object.cast [as as] (operations.dart:426)
    at Function.check_FutureOr [as _check] (future.dart:44)
    at js_util_dart2js.dart:145

[sigmundch]

@terrylucas - this appears like an error in the generated dart:html code.

I believe we'll hit this issue in other APIs as well, basically anywhere we have promiseToFuture<List<X>> today, I expect the underlying list to be a List<dynamic> (or List<JSAny> once we add support for it)

[rakudrama]

Can we make JavaScript Arrays support JSArray<any> instead of these changes that make the API harder to use?

[sigmundch]

is any exposed in any way that we can declare it today (for both dart2js and ddc)?

[srujzs]

I'm going to mark this as closed since the original issue has been addressed. Please feel free to reopen or open a separate issue if this can be cleaned up.

[mnordine]

I see this is closed and it was noted that the original issue has been addressed. Does that mean that the code above now returns the expected types?

e.g. Can I do:

import 'dart:html';

main()
{
  final devices = await window.navigator.mediaDevices.enumerateDevices();
  print(devices.first.runtimeType); // MediaDeviceInfo
  print(devices.first.deviceId);
}

[sigmundch]

@mnordine - I haven't verified this, but yes, I expect that the individual elements will contain the expected type.

final devices = await window.navigator.mediaDevices.enumerateDevices();
print(decices.runtimeType); // List<dynamic>
print(devices.first.runtimeType); // MediaDeviceInfo

because the list type is dynamic and not List<Object>, unfortuantely this means that you wont get a static error if you accidentally do:

devices.first.nonExistingProperty

You can however use the static type to avoid this problem:

MediaDeviceInfo device = devices.first;
print(device.deviceId);

Note that the fix landed in b84d6ba, which will be included in the next dev channel release of the sdk.

[mnordine]

Sorry if I'm misunderstanding, but was the solution here to make all web apis that return lists return them as List<dynamic>, therefore losing the typing?

If so, is this temporary?

Also, I don't understand how, if devices.runtimeType is List<dynamic>, a member of that list could be anything other than dynamic?