Animations with Redux (or Streams or ChangeNotifiers, etc)

main.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_redux/flutter_redux.dart';
import 'package:redux/redux.dart';

void main() {
  runApp(
    MyApp(
      store: Store<AppState>(
        reducer,
        initialState: AppState(),
      ),
    ),
  );
}

class UpdateThemeAction {
  final bool lightTheme;

  UpdateThemeAction(this.lightTheme);
}

class AppState {
  final bool lightTheme;

  AppState({this.lightTheme = true});

  AppState copyWith({bool lightTheme}) {
    return AppState(
      lightTheme: lightTheme ?? this.lightTheme,
    );
  }
}

AppState reducer(AppState state, dynamic action) {
  if (action is UpdateThemeAction) {
    return state.copyWith(lightTheme: action.lightTheme);
  }

  return state;
}

class MyApp extends StatelessWidget {
  final Store<AppState> store;

  const MyApp({Key key, this.store}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return StoreProvider<AppState>(
      store: store,
      child: MaterialApp(
        title: 'Flutter Redux Animations',
        home: MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Redux Animations'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('Enable light theme?'),
            StoreBuilder<AppState>(
              builder: (context, store) {
                return Switch(
                  // Every time the Store emits a change event, the builder
                  // function will run, updating the value of the Switch!
                  //
                  // The Switch widget will see that a new value has been
                  // provided and then run the appropriate animation.
                  //
                  // We can do the same thing with our own Widgets!
                  value: store.state.lightTheme,
                  onChanged: (lightTheme) {
                    store.dispatch(UpdateThemeAction(lightTheme));
                  },
                );
              },
            ),
            StoreBuilder<AppState>(
              builder: (context, store) {
                return CurrentThemeDisplay(lightTheme: store.state.lightTheme);
              },
            ),
          ],
        ),
      ),
    );
  }
}

/// Our custom class that acts exactly like the Switch! The user provides
/// whether `lightTheme` is true or false, and this Widget takes care of
/// performing an animation when the provided `lightTheme` value changes.
class CurrentThemeDisplay extends StatefulWidget {
  final bool lightTheme;

  const CurrentThemeDisplay({Key key, this.lightTheme}) : super(key: key);

  @override
  _CurrentThemeDisplayState createState() => _CurrentThemeDisplayState();
}

class _CurrentThemeDisplayState extends State<CurrentThemeDisplay>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation<Color> _animation;

  @override
  void initState() {
    // Create an animation controller to drive the animation
    _controller = AnimationController(
      vsync: this,
      duration: Duration(milliseconds: 300),
    );

    // Our animation will go between a "Light Blue" and a "Dark Blue" depending
    // on whether the user has selected a light or dark theme
    _animation = ColorTween(
      begin: Colors.blue.shade100,
      end: Colors.blue.shade900,
    ).animate(_controller);

    super.initState();
  }

  @override
  void dispose() {
    // Make sure to clean up after yourself!
    _controller.dispose();
    super.dispose();
  }

  /// This is where the magic happens! You must override `didUpdateWidget` to
  /// capture when the Widget is rebuilt with new values.
  ///
  /// To create this animation, we check if the old theme is the same as the new
  /// theme. If both are the same, we do not take any action.
  ///
  /// If they are different, then we use the controller to play the animation
  /// in the correct direction (forward or reverse).
  @override
  void didUpdateWidget(CurrentThemeDisplay oldWidget) {
    if (oldWidget.lightTheme != widget.lightTheme) {
      if (widget.lightTheme) {
        _controller.reverse();
      } else {
        _controller.forward();
      }
    }

    super.didUpdateWidget(oldWidget);
  }

  @override
  Widget build(BuildContext context) {
    // Use the animation to display the appropriate color
    return AnimatedBuilder(
      animation: _animation,
      builder: (context, _) {
        return Container(
          width: 200,
          height: 200,
          color: _animation.value,
        );
      },
    );
  }
}