Previewing Flutter Widgets

Contents

• Preview Guidelines
• Handling Limitations
• Workflows
• Examples

Preview Guidelines

• 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., themes, wrappers) 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., generating names based on dynamic values, which is impossible in a const context).

Handling Limitations

• 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 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 the previewer defaults to constraining them to approximately half the viewport.

Workflows

Creating a 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, theme, brightness, etc.) as needed.
• If applying the same configuration to multiple widgets, extract the configuration into a custom class extending Preview.

Interacting with Previews

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.

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

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: 'My Sample Text', group: 'Typography')
Widget mySampleText() {
  return const Text('Hello, World!');
}

Custom Preview with Runtime Transformation

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

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

  PreviewThemeData _themeBuilder() {
    return PreviewThemeData(
      materialLight: ThemeData.light(),
      materialDark: ThemeData.dark(),
    );
  }

  @override
  Preview transform() {
    final originalPreview = super.transform();
    final builder = originalPreview.toBuilder();
    
    builder
      ..name = 'Transformed - ${originalPreview.name}'
      ..theme = _themeBuilder;

    return builder.toPreview();
  }
}

@TransformativePreview(name: 'Custom Themed Button')
Widget myButton() => const ElevatedButton(onPressed: null, child: Text('Click'));

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')));