Add WidgetPreview and WidgetPreviewerWindowConstraints widgets and @Preview() annotation

[bkonyi]

This is the initial work for the upcoming Widget Preview functionality.

The WidgetPreview widget allows for users to specify overrides and details to apply to its child Widget. Initially, this includes:

• Setting arbitrary width and height constraints to mimic changes in device resolution
• Custom text scaling factors

The previewed widget is currently wrapped in a Material Card which includes controls for zooming and panning within the zoomed in view.

The WidgetPreviewerWindowConstraints widget propagates the constraints for the preview environment to each WidgetPreview widget to be used by the WidgetPreviewWrapper widget, a SingleChildRenderObjectWidget, to force constraints onto unconstrained children of WidgetPreview. This will eventually be used in a "preview scaffold" application generated by tooling to host the widget previews for display in the widget previewer environment.

[navaronbracke]

Is there a design doc for this new feature? I wonder if the framework should contain this source code, or if this is something that is intended for Devtools / Devtools extentions?

[bkonyi]

Is there a design doc for this new feature? I wonder if the framework should contain this source code, or if this is something that is intended for Devtools / Devtools extentions?

There's currently not a publicly shared design doc, but I think I'm going to pull one together to better communicate how this feature is going to be architected.

This particular code should live in the framework since it's meant to be included in a developer's codebase. The other components of this feature will live in the flutter tool, DevTools, and the IDE plugins.

For example, this is an early prototype showing how a widget (in this case, the root of the Flutter Gallery application) can be previewed within VSCode by applying the @Preview() annotation to a function that returns instances of WidgetPreview:

[bkonyi]

A design doc can be found here: https://github.com/flutter/flutter/issues/159342

[goderbauer]

@bkonyi and I talked on Friday and we had the idea that what lives in the framework would ideally be fairly minimal. Maybe just the annotation and a WidgetPreview data class.

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[bkonyi]

@bkonyi and I talked on Friday and we had the idea that what lives in the framework would ideally be fairly minimal. Maybe just the annotation and a WidgetPreview data class.

Done, PTAL when you have a chance!

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autosubmit label was removed for flutter/flutter/159219, because - The status or check suite Linux analyze has failed. Please fix the issues identified (or deflake) before re-applying this label.

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