Flutter code recipe for AnimatedOpacity widget

AnimatedOpacity widget is used to achieve a smooth transition from one UI element to another.

Recipe: Achieve a smooth animation when transitioning from one UI element to another.

Focus Widget: AnimatedOpacity

Goal: Fade in and out a blue rectangle on click of button. Interface is super simple and will look like this:

Blue Rectangle animates when button is pressed:

Checkout AnimatedOpacity animation

Lets's go!

We need three things to accomplish our goal of animating a blue rectangle.

1. Widget to animate: A blue rectangle

    Container(
        width: 100,
        height: 150,
        color: Colors.blue,
    );

2. StatefulWidget: A way to hold visibility state of rectangle.StatefulWidget has State object that can hold and update some data for app. State object's setState method helps to rebuild widget reflecting the updated state of the app data.

class AnimatedOpacityRecipe extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _AnimatedOpacityRecipeState();
}

The State object will have the flag about the rectangle widget's visibility state say _visible._visible to true means that rectangle is visible or vice versa.

class _AnimatedOpacityRecipeState extends State<AnimatedOpacityRecipe> {
  bool _visible = true;
  
} 

Method animatedOpacityWidget returns the AnimatedOpacity widget.duration property is to control the speed of animation. In this example, it takes 900 milliseconds to fade out the rectangle. This is important to show a smooth transition.

Widget animatedOpacityWidget() {
    return AnimatedOpacity(
      duration: Duration(milliseconds: 900),
      opacity: _visible ? 1 : 0,
      child: _blueRectangle(),
    );
  }

4. OutlineButton widget: A button to toggle fade in/out rectangle Button labeled 'Animate' to fade in/out rectangle.setState() method toggles the visibility flag for the rectangle widget. It forces Flutter to rebuilt AnimatedOpacity widget in accordance with new value of _visible flag.

...
OutlineButton(
  shape: new RoundedRectangleBorder(
    borderRadius: new BorderRadius.circular(8.0),
  ),
  child: Text("Press to Animate"),
  onPressed: () {
    setState(() => _visible = !_visible);
  },
)
...

Complete example code

import 'package:flutter/material.dart';

//starting the app
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Cookbook',
      theme: ThemeData(
        // This is the theme of your application.
        primarySwatch: Colors.green,
      ),
      home: AnimatedOpacityRecipe(title: 'AnimatedOpacity Demo'),
    );
  }
}

class AnimatedOpacityRecipe extends StatefulWidget {
  final String title;

  AnimatedOpacityRecipe({Key key, this.title}) : super(key: key);
  @override
  State<StatefulWidget> createState() => _AnimatedOpacityRecipeState();
}

class _AnimatedOpacityRecipeState extends State<AnimatedOpacityRecipe> {
  bool _visible = true;

  Widget _blueRectangle() {
    return Container(
      width: 100,
      height: 150,
      color: Colors.blue,
    );
  }

  Widget animatedOpacityWidget() {
    return AnimatedOpacity(
      duration: Duration(milliseconds: 900),
      opacity: _visible ? 1 : 0,
      child: _blueRectangle(),
    );
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title:Text(widget.title),
      ),
      body: buildDemoComponent(context),
    );
  }

  Widget buildDemoComponent(BuildContext context) {
    return Material(
      child: Padding(
        padding: EdgeInsets.all(8.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Padding(
              child: Text(
                  'Press button to see blue rectangle fade out and in'
              ),
              padding: EdgeInsets.all(20.0),
            ),
            animatedOpacityWidget(),
            OutlineButton(
              shape: new RoundedRectangleBorder(
                borderRadius: new BorderRadius.circular(8.0),
              ),
              child: Text("Press to Animate"),
              onPressed: () {
                //Toggles visibility of the rectangle
                setState(() => _visible = !_visible);
              },
            )
          ],
        ),
      ),
    );
  }
}

Source code repo: Recipe source code is available here

References:

1. https://flutter.io/docs/cookbook/animation/opacity-animation.html
2. https://docs.flutter.io/flutter/widgets/Opacity-class.html

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