Widget Previews in Flutter 3.35: Designing UI Faster

Introduction

Flutter 3.35 introduces Widget Previews, an experimental yet powerful feature that transforms how developers iterate on UI components. Instead of launching the full app to test a small tweak, you can now instantly preview widgets in isolation—speeding up design validation and refining workflows.

Launching the Widget Previewer

In your project's root directory, run:

flutter widget-preview start

This spins up a local server and opens a preview environment in Chrome. As you save changes, previews update live, letting you iterate without the app compilation cycle.

Annotating Widgets for Preview

Use the @Preview annotation from package:flutter/widget_previews.dart on:

• Top-level functions returning Widget or WidgetBuilder

• Static methods within classes returning a Widget

• Public widget constructors/factories without required arguments

import 'package:flutter/widget_previews.dart';
import 'package:flutter/material.dart';

@Preview(name: 'Greeting Text')
Widget greeting() => const Text('Hello, Preview!');

Customizing Your Previews

The @Preview annotation supports customization:

• name: Descriptive label for the preview

• size: Define width/height constraints via a Size object

• textScaleFactor: Adjust font scaling

• wrapper: Inject a wrapper widget (e.g., theming or inherited state)

• theme: Provide Material or Cupertino theming data

• brightness: Set initial light or dark mode

• localizations: Apply locale configurations

These options help you test UI variations quickly—think different screen sizes, dark mode, or localized layouts.

Interactive Controls in Preview

Once active in Chrome, the previewer offers controls to aid exploration:

• Zoom in/out/reset: Adjust view scale

• Light / Dark toggle: Switch UI appearance

• Hot-restart individual previews: Apply updates quickly without restarting the whole previewer

• Global hot-restart: Reset all previews when underlying state changes

Use Cases That Shine

• Design system components: Preview buttons, cards, or icons in isolation

• Theme testing: Rapidly see how widgets look under dark/light schemes

• Localization checks: Validate visual integrity for RTL languages or longer strings

• State injection: Use wrapper to simulate inherited state—no full app launch needed

Limitations to Know

• Still experimental; APIs may evolve

• Only supports Chrome—other browsers or IDE integrations aren’t supported yet

• Requires public, constant parameters in @Preview; this may relax in future

• Native plugins, dart:io, or asset paths using local file references won’t work in this environment

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Conclusion

Widget Previews in Flutter 3.35 let you develop faster and smarter by isolating UI components for instant feedback. Whether tweaking themes, localization, or layout, this tool cuts down iteration time and makes refining visuals a breeze. Expect improvements and expanded support in upcoming versions—this is just the beginning.