WidgetManagerBase: Improve create_view return type

[martinRenou]

Given a video widget model VideoModel which has an associated view VideoView. The following code does not compile:

createVideo(videoModel: VideoModel) {
    this.create_child_view(videoModel).then((videoView: VideoView) => {
        // Do something
    });
}

With the following error:

Argument of type '(videoView: VideoView) => VideoView' is not assignable to parameter of type '(value: DOMWidgetView) => VideoView | PromiseLike<VideoView>'.
  Types of parameters 'videoView' and 'value' are incompatible.
    Type 'DOMWidgetView' is not assignable to type 'VideoView'.

This PR allows doing:

createVideo(videoModel: VideoModel) {
    this.create_child_view<VideoView>(videoModel).then((videoView: VideoView) => {
        // Do something
    });
}

[pbugnion]

Is this still relevant given the discussion on removing createView this morning? Is there still value in merging this in the interim?

[jasongrout]

We're removing display_model/display_view in favor of keeping and using create_view.

[martinRenou]

@jasongrout would it be fine to merge this PR?

[jasongrout]

I think once you have the overrides, you won't need to specify the type parameter here, as it should select the right override.

[martinRenou]

Yes! Thanks you

[martinRenou]

Actually it does not seem to compile without it:

src/widget.ts:696:9 - error TS2322: Type 'Promise<DOMWidgetView>' is not assignable to type 'Promise<VT>'.
  Type 'DOMWidgetView' is not assignable to type 'VT'.
    'DOMWidgetView' is assignable to the constraint of type 'VT', but 'VT' could be instantiated with a different subtype of constraint 'WidgetView'.

696         return this.model.widget_manager.create_view(child_model, options)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
697             .catch(utils.reject('Could not create child view', true));

I feel like this compilation error is not legit though, as DOMWidgetView is a subclass of WidgetView, what do you think?