Previewing Flutter Widgets

Contents

• Preview Guidelines
• Preview Parameters
• Handling Limitations
• Workflows
• Examples

Preview Guidelines

Use the Flutter Widget Previewer to render widgets in real-time, isolated from the full application context.

• Target Elements: Apply the @Preview annotation to top-level functions, static methods within a class, or public widget constructors/factories that have no required arguments and return a Widget or WidgetBuilder.
• Imports: Always import package:flutter/widget_previews.dart to access the preview annotations.
• Custom Annotations: Extend the Preview class to create custom annotations that inject common properties (e.g., custom themes, wrappers, sizes) across multiple widgets.
• Multiple Configurations: Apply multiple @Preview annotations to a single target to generate multiple preview instances. Alternatively, extend MultiPreview to encapsulate common multi-preview configurations.
• Runtime Transformations: Override the transform() method in custom Preview or MultiPreview classes to modify preview configurations dynamically at runtime (e.g., building names dynamically based on parameters, which is impossible in a const constructor).

Preview Parameters

The @Preview annotation supports the following parameters for tailoring preview environments:

• name: A descriptive name for the preview instance.
• group: A name used to group related previews together in the previewer.
• size: Artificial constraints using a Size object.
• textScaleFactor: Custom font scale factor.
• wrapper: A function that wraps the previewed widget in a specific widget tree (e.g., to inject state or inherited widgets).
• theme: A function returning a PreviewThemeData subclass instance to provide custom theming with support for sequential theme layering.
• brightness: The initial theme Brightness (Brightness.light or Brightness.dark).
• localizations: A function applying a localization configuration.

Handling Limitations

Adhere to the following constraints when authoring previewable widgets, as the Widget Previewer runs in a web environment:

• No Native APIs: Do not use native plugins or APIs from dart:io or dart:ffi. Widgets with transitive dependencies on dart:io or dart:ffi will throw exceptions upon invocation. Use conditional imports or wrappers to mock or bypass these in preview mode.
• Asset Paths: Use package-based paths for assets loaded via dart:ui fromAsset APIs (e.g., packages/my_package_name/assets/my_image.png instead of assets/my_image.png).
• Public Callbacks: Ensure all callback arguments provided to preview annotations are public and constant to satisfy code generation requirements.
• Constraints: Apply explicit constraints using the size parameter in the @Preview annotation if your widget is unconstrained, as unconstrained widgets default to roughly half the viewport.
• Build Caching: The previewer caches builds in a .widget_preview/ folder in the project root.

Workflows

Creating a Widget Preview

Copy and track this checklist when implementing a new widget preview:

• Import package:flutter/widget_previews.dart.
• Identify a valid target (top-level function, static method, or parameter-less public constructor).
• Apply the @Preview annotation to the target.
• Configure preview parameters (name, group, size, textScaleFactor, wrapper, theme, brightness, localizations) as needed.
• If applying the same configuration to multiple widgets, extract the configuration into a custom class extending Preview or MultiPreview.

Interacting with Previews

Follow the appropriate conditional workflow to launch and interact with the Widget Previewer:

If using a supported IDE (Android Studio, IntelliJ, VS Code with Flutter 3.47+):

1. Launch the IDE. The Widget Previewer starts automatically.
2. Open the "Flutter Widget Preview" tab in the sidebar.
3. Toggle "Filter previews by selected file" at the bottom left if you want to view previews outside the currently active file.

If using the Command Line:

1. Navigate to the Flutter project's root directory.
2. Run flutter widget-preview start.
3. View the automatically opened Chrome environment.

Feedback Loop: Preview Iteration

1. Modify the widget code or preview configuration.
2. Observe the automatic update in the Widget Previewer.
3. If global state (e.g., static initializers) was modified: Click the global hot restart button at the bottom right.
4. If only the local widget state needs resetting: Click the individual hot restart button on the specific preview card.
5. Review errors in the IDE/CLI console -> fix -> repeat.

Examples

Basic Preview

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

@Preview(name: 'Submit Button', group: 'Form Controls')
Widget submitButtonPreview() => const ElevatedButton(
      onPressed: null,
      child: Text('Submit'),
    );

Custom Preview with PreviewThemeData and Transformation

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

final class ThemedPreview extends Preview {
  const ThemedPreview({
    super.name,
    super.group,
    super.brightness,
  });

  PreviewThemeData _themeBuilder() =>
      CustomThemeData(brightness: brightness);

  @override
  Preview transform() {
    final originalPreview = super.transform();
    final themeVariant = switch (brightness) {
      null => 'Responsive',
      final b => b.name,
    };

    final builder = originalPreview.toBuilder();
    final baseName = originalPreview.name ?? 'Preview';
    builder
      ..name = '$baseName [$themeVariant]'
      ..theme = _themeBuilder;

    return builder.build();
  }
}

final class CustomThemeData extends PreviewThemeData {
  const CustomThemeData({this.brightness});

  final Brightness? brightness;

  @override
  Widget apply(BuildContext context, Widget child) {
    return Theme(
      data: ThemeData.from(
        colorScheme: ColorScheme.fromSeed(
          seedColor: Colors.teal,
          brightness: brightness ??
              MediaQuery.maybePlatformBrightnessOf(context) ??
              Brightness.light,
        ),
      ),
      child: child,
    );
  }
}

@ThemedPreview(name: 'Themed Action Button', brightness: Brightness.dark)
Widget themedButtonPreview() => const ElevatedButton(
      onPressed: null,
      child: Text('Action'),
    );

MultiPreview Implementation

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

/// Creates light and dark mode previews automatically.
final class MultiBrightnessPreview extends MultiPreview {
  const MultiBrightnessPreview({required this.name});

  final String name;

  @override
  List<Preview> get previews => const [
        Preview(brightness: Brightness.light),
        Preview(brightness: Brightness.dark),
      ];

  @override
  List<Preview> transform() {
    final previews = super.transform();
    return previews.map((preview) {
      final builder = preview.toBuilder()
        ..group = 'Brightness'
        ..name = '$name - ${preview.brightness!.name}';
      return builder.toPreview();
    }).toList();
  }
}

@MultiBrightnessPreview(name: 'Primary Card')
Widget cardPreview() => const Card(
      child: Padding(
        padding: EdgeInsets.all(8.0),
        child: Text('Content'),
      ),
    );