Widget Previews are stable: iterate on UI without booting the whole app

Booting a full app to tweak a button is slow. Widget Preview renders individual widgets in isolation. After experimental cycles, it is stable in Flutter 3.47.

What stable means here

• Faster startup via a local .widget_preview/ cache.
• PreviewThemeData for sequential theme layering (matrix tests across seeds/contrast).
• Automatic web/ asset sync when previewing web widgets.

Workflow that pays off

1. Extract the widget you are changing into a previewable class.
2. Add previews for default / dark / large-text / empty-data.
3. Iterate until the preview is boring — then wire it into the app.

This is also the fastest loop for design-system work on material_ui / custom catalogs.

Pitfalls

• Previews that hit dart:io or platform channels will not run — isolate pure UI.
• Do not put business logic in previews; put fixtures there.