fleet 0.1.3+1

An animation Framework for Flutter. It is state-based, declarative, extensible, composable and easy to use.

⚠️ This package is in an early state of development. If you find any bugs or have any suggestions, please open an issue.

Fleet is an animation framework for Flutter.

• State-based: Animate transitions from one state to another.
• Declarative: Describe an animation and apply it to a state change. No need to manage AnimationControllers or Tweens.
• Animatable widgets: Comes out of the box with general purpose widgets that support animating with Fleet.
  • Extensible: Any widget can be made to support animating with Fleet.
  • Flexible: Animatable widgets can be used with or without animations.
  • Composable: Widgets that build on animatable widgets are automatically animatable.
  • Animate parameters individually: Animate parameters of animatable widget individually, e.g. with different curves.
• User-friendly: Add and change animations with little refactoring.

Getting started

1. Add Fleet to your pubspec.yaml:

flutter pub add fleet

2. Add FleetBinding.ensureInitialized() to your main function:

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

void main() {
  FleetBinding.ensureInitialized();
  runApp(MyApp());
}

You need to use this binding instead of WidgetsFlutterBinding.

This step is currently necessary because of functionality that Fleet requires, but that is not available in Flutter. This step will become unnecessary if and when the required functionality becomes available in Flutter. Unfortunately, this also means that until that point, animations that use Fleet will not work in tests. The reason is that the test bindings cannot be extended in the same way that WidgetsFlutterBinding can be. You can still use Fleet in tests but animated values will immediately jump to their final value.

Now you're ready to use Fleet in your Flutter app.

Take a look a the examples or continue to the introduction to Fleet.

Introduction

Fleet has been heavily inspired by the animation framework that comes with SwiftUI. If you are familiar with it, you will find most concepts familiar.

I'm going to walk you through adding a simple animation to a widget. Below is the unanimated version of the widget:

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

class MyWidget extends StatefulWidget {
  const MyWidget({super.key});

  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  var _active = false;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        setState(() {
          _active = !_active;
        });
      },
      child: ColoredBox(
        color: _active ? Colors.blue : Colors.grey,
      ),
    );
  }
}

Now lets animate the state change of _active:

 class _MyWidgetState extends State<MyWidget> {
   var _active = false;

   @override
   Widget build(BuildContext context) {
     return GestureDetector(
       onTap: () {
-        setState(() {
+        setStateWithAnimation(Curves.ease.animation(250.ms), () {
           _active = !_active;
         });
       },
-      child: ColoredBox(
+      child: AColoredBox(
         color: _active ? Colors.blue : Colors.grey,
       ),
     );
   }
 }

All we did was replace ColoredBox with AColoredBox and use setStateWithAnimation instead of setState.

The AColoredBox widget is a drop-in replacement for ColoredBox that supports animating with Fleet. Widgets that support animating with Fleet don't have any special parameters related to animation and can be used without animation, just as well.

Fleet provides drop-in replacements for a number of generally useful Flutter framework widgets (all with the prefix A). Any widget can be made to support state-based animation through components provided by Fleet (see AnimatableStateMixin). Issues or PRs for adding support for more widgets are welcome!

Note that we did not explicitly tell setStateWithAnimation what to animate. This is because Fleet uses a state-based approach. All state changes caused by executing the provided callback will be animated. Even the state changes which are indirect, like the color parameter of AColoredBox going from Colors.grey to Colors.blue. Fleet does this by tracking the state of animatable parameters of animatable widgets from one build to the next.

setStateWithAnimation is a little bit special in that it does not immediately execute the callback, like it is the case for setState. Instead, setStateWithAnimation executes the callback as part of building the next frame. In practice this seldomly makes a difference. setStateWithAnimation is a method from an extension on State.

Curves.ease.animation(250.ms) creates an AnimationSpec that we pass to setStateWithAnimation to specify how to animate from the old to the new state.

.animate is an extension method on Curve that creates an AnimationSpec form the curve. It optionally takes a Duration. For a nicer syntax for specifying Durations, Fleet provides extensions on int, e.g 250.ms is equivalent to Duration(milliseconds: 250).

Animatable widgets

The following drop-in replacements for Flutter framework widgets are provided for animating with Fleet:

• AAlign
• AColoredBox
• AContainer
• AOpacity
• APadding
• APositioned
• APositionedDirectional
• ASizedBox
• ASliverOpacity
• ASliverPadding
• ATransform

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Gabriel Terwesten • GitHub @blaugold • Twitter @GTerwesten • Medium @gabriel.terwesten