Flutter: Animated Gradients with Custom Hooks

When faced with the task of coming up with a re-usable "Animated Gradient" widget, I faced one problem, a gradient isn't actually a widget - I can't go and make an AnimatedWidget that I can use inside a BoxDecoration in my UI somewhere.

What I wanted was an actual gradient that simply took care of its own state management... not a widget, stateful or otherwise.

Enter hooks

Since discovering hooks for flutter, I've become a huge fan, I'm making heavy use of hooks in my architecture, as shown in this post, and I find them particularly useful for use-cases such the one I'm about to outline below.

A hook is a new kind of object that encapsulates behaviour, keeping the state management out of your widget hierarchy. Hooks must be used within the build method of a Hook Widget, or within another hook. Hooks; combined with HookWidget, are a drop-in replacement for StatefulWidget and either enhance or replace other forms of state management you might be using.

For animation, hooks are particularly useful because they replace a huge swath of boilerplate and the need to create multiple classes - with just a couple of lines of code.

For this example we are going to make our own custom hook that itself contains a bunch of other hooks. We will be able to use our hook anywhere we might use a gradient.

Tutorial time!

Full source code is available at: https://github.com/paddo/flutter_animated_gradient

Fire up a terminal.

flutter create animated_gradient
code animated_gradient

Open up pubspec.yaml, and add the flutter_hooks dependency, as below:

name: animated_gradient
description: A new Flutter project.
version: 1.0.0+1
environment:
  sdk: ">=2.1.0 <3.0.0"
dependencies:
  flutter:
    sdk: flutter
  flutter_hooks: ^0.5.0
dev_dependencies:
  flutter_test:
    sdk: flutter
flutter:
  uses-material-design: true

Note, we are building against flutter - stable channel - v1.5.4-hotfix.2. If you are building against a different version of flutter, you may need to use a different version of flutter_hooks, and target a different SDK version.

Let's make a custom tween, and a custom hook

Since we are animating a gradient, we'll need to make a tween for it.

Custom hooks are pretty easy to make. You simply need to create a Hook class and a corresponding HookState. Our hook, when used, should return a gradient, so it will be a Hook<Gradient>.

Create a new file: lib/animated_gradient.dart

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

Gradient useAnimatedGradient({
  Duration duration = const Duration(seconds: 5),
  List<Gradient> gradients,
  Curve curve = Curves.linear,
}) {
  return Hook.use(_AnimatedGradientHook(
    duration: duration,
    gradients: gradients,
    curve: curve,
  ));
}

class GradientTween extends Tween<Gradient> {
  GradientTween({
    Gradient begin,
    Gradient end,
  }) : super(begin: begin, end: end);

  @override
  Gradient lerp(double t) => Gradient.lerp(begin, end, t);
}

class _AnimatedGradientHook extends Hook<Gradient> {
  final List<Gradient> gradients;
  final Duration duration;
  final Curve curve;

  _AnimatedGradientHook({this.duration, this.gradients, this.curve});

  @override
  HookState<Gradient, Hook<Gradient>> createState() => _AnimatedGradientHookState();
}

class _AnimatedGradientHookState extends HookState<Gradient, _AnimatedGradientHook> {
  @override
  Gradient build(BuildContext context) {
    final controller = useAnimationController(duration: hook.duration);
    final index = useValueNotifier(0);

    useEffect(() {
      controller.repeat();
      final listener = () {
        final newIndex = (controller.value * hook.gradients.length).floor() % hook.gradients.length;
        if (newIndex != index.value) index.value = newIndex;
      };
      controller.addListener(listener);
      return () => controller.removeListener(listener);
    }, [hook.gradients, hook.duration, hook.curve]);

    return useAnimation(GradientTween(
            begin: hook.gradients[index.value], end: hook.gradients[(index.value + 1) % hook.gradients.length])
        .animate(CurvedAnimation(
            curve: Interval(
              index.value / hook.gradients.length,
              (index.value + 1) / hook.gradients.length,
              curve: hook.curve,
            ),
            parent: controller)));
  }
}

Explainer time:

Put simply, we have an animation on repeat, and based on where we are in that animation, we interpolate between 2 gradients, all of these interpolations join together (and cycle back to the start ), to form one smooth sequence.
Our hook implementation lives inside a private class, and has to be used via the "useAnimatedGradient" function, as is the convention with the built-in hooks. These private classes should look pretty similar to StatefulWidget boilerplate.
Our initialisation and tear-down code is in the useEffect hook. Here we attach a listener, which is responsible for selecting the pair of gradients that we are presently animating between, and the corresponding listener tear-down code. See useEffect docs.
The useAnimationController hook takes care of all the boilerplate required for animations - the Ticker Mix-ins, initialisation code, disposal, etc.
Since we do need some state information that survives between rebuilds (namely, the present position in the array of gradients), we use a useValueNotifier hook. We could've used useState (which forces a rebuild), but in our use-case, we don't have to.

Using the hook

If you want to go and use it in your own project, you can use this hook with a list of gradients to cycle between. Use it anywhere you might use a gradient. Good luck!

For the sake of completeness though, and to bring this short sample to a conclusion, I'm going to make a widget to house our animation, so that we can encapsulate the state (and rebuilds) into as small a surface area as possible within the widget hierarchy.

Make a new file: lib/animated_gradient_box

import 'package:animated_gradient/animated_gradient.dart';
import 'package:flutter/material.dart';
import 'package:flutter_hooks/flutter_hooks.dart';

class AnimatedGradientBox extends HookWidget {
  final List<Gradient> gradients;
  final Curve curve;

  AnimatedGradientBox(this.gradients, [this.curve = Curves.linear]);

  @override
  Widget build(BuildContext context) {
    return Container(decoration: BoxDecoration(gradient: useAnimatedGradient(gradients: gradients, curve: curve)));
  }
}

Now, layout a bunch of these boxes, and let the psychedelic adventures commence.

Make lib/main.dart look like this:

import 'package:animated_gradient/animated_gradient_box.dart';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  final List<Gradient> _linearGradients = [
    LinearGradient(
        colors: [Colors.red, Colors.purple, Colors.blue, Colors.green, Colors.yellow],
        begin: Alignment.centerLeft,
        end: Alignment.centerRight),
    LinearGradient(
        colors: [Colors.red, Colors.purple, Colors.blue, Colors.green, Colors.yellow],
        begin: Alignment.topCenter,
        end: Alignment.bottomCenter),
    LinearGradient(
        colors: [Colors.red, Colors.purple, Colors.blue, Colors.green, Colors.yellow],
        begin: Alignment.centerRight,
        end: Alignment.centerLeft),
    LinearGradient(
        colors: [Colors.red, Colors.purple, Colors.blue, Colors.green, Colors.yellow],
        begin: Alignment.bottomCenter,
        end: Alignment.topCenter),
  ];

  final List<Gradient> _sweepGradients = [
    SweepGradient(colors: [Colors.red, Colors.yellow, Colors.green, Colors.red]),
    SweepGradient(colors: [Colors.lightBlue, Colors.purple, Colors.pink, Colors.lightBlue]),
  ];

  final List<Gradient> _radialGradients = [
    RadialGradient(colors: [Colors.red, Colors.blue], radius: 1.0, center: Alignment.topCenter),
    RadialGradient(colors: [Colors.lightBlue, Colors.purple, Colors.pink], radius: 2.0, center: Alignment.bottomCenter),
  ];

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            Expanded(child: AnimatedGradientBox(_linearGradients)),
            Expanded(child: AnimatedGradientBox(_sweepGradients)),
            Expanded(child: AnimatedGradientBox(_radialGradients, Curves.easeInOut)),
            Expanded(
                child: AnimatedGradientBox(
                    []..addAll(_linearGradients)..addAll(_radialGradients)..addAll(_sweepGradients))),
          ],
        ),
      ),
    );
  }
}

Behold the amazing result!

Source code is available at https://github.com/paddo/flutter_animated_gradient

I'm also working on a flutter architecture series. See Part 1.

Flutter: Animated Gradients via Custom Hooks