Simple animations

Flutter provides a rich set of animation APIs, and the simplest way to start using them is with implicit animations. "Implicit animations" refers to a group of widgets that automatically animate changes to their properties without you needing to manage any intermediate behavior.

In this lesson, you'll learn about one of the most common and versatile implicit animation widgets: AnimatedContainer. With just two additional lines of code, the background color of each Tile animates to a new color in about half a second.

Convert Container to AnimatedContainer

Currently, the Tile.build method returns a Container to display a letter. When the hitType changes, like from HitType.none to HitType.hit, the background color of the tile changes instantly. For example, from white to green in the case of HitType.none to HitType.hit.

For reference, here's the current implementation of the Tile widget:

class Tile extends StatelessWidget {
  const Tile(this.letter, this.hitType, {super.key});

  final String letter;
  final HitType hitType;

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 60,
      width: 60,
      decoration: BoxDecoration(
        border: Border.all(color: Colors.grey.shade300),
        color: switch (type) {
          HitType.hit => Colors.green,
          HitType.partial => Colors.yellow,
          HitType.miss => Colors.grey,
          _ => Colors.white,
        },
      ),
      child: Center(
        child: Text(
          letter.toUpperCase(),
          style: Theme.of(context).textTheme.titleLarge,
        ),
      ),
    );
  }
}

To make the color change animate smoothly, replace the Container widget with an AnimatedContainer.

An AnimatedContainer is like a Container, but it automatically animates changes to its properties over a specified duration. When properties such as color, height, width, decoration, or alignment change, AnimatedContainer interpolates between the old and new values, creating a smooth transition.

Modify your Tile widget as follows:

class Tile extends StatelessWidget {
  const Tile(this.letter, this.hitType, {super.key});

  final String letter;
  final HitType hitType;

  @override
  Widget build(BuildContext context) {
    return AnimatedContainer(
      duration: Duration(milliseconds: 500),
      height: 60,
      width: 60,
      decoration: BoxDecoration(
        border: Border.all(color: Colors.grey.shade300),
        color: switch (hitType) {
          HitType.hit => Colors.green,
          HitType.partial => Colors.yellow,
          HitType.miss => Colors.grey,
          _ => Colors.white,
        },
      ),
      child: Center(
        child: Text(
          letter.toUpperCase(),
          style: Theme.of(context).textTheme.titleLarge,
        ),
      ),
    );
  }
}

duration is a required property that specifies how long the animation should take. In this example, passing Duration(milliseconds: 500) means the color transition will take half a second. You can also specify seconds, minutes, and many other units of time.

Now, when the hitType changes and the Tile widget rebuilds (because setState was called in GamePage), the color of the tile smoothly animates from its old color to the new one over the specified duration.

Adjust the animation curve

To add a bit of customization to an implicit animation, you can pass it a different Curve. Different curves change the speed of the animation at different points throughout the animation.

For example, the default curve for Flutter animations is Curves.linear. This gif shows how the animation curve behaves:

Compare that to Curve.bounceIn, another common curve:

To change the Curve of this animation, update the code to the following:

class Tile extends StatelessWidget {
  const Tile(this.letter, this.hitType, {super.key});

  final String letter;
  final HitType hitType;

  @override
  Widget build(BuildContext context) {
    return AnimatedContainer(
      duration: Duration(milliseconds: 500),
      curve: Curves.bounceIn, // NEW
      height: 60,
      width: 60,
      decoration: BoxDecoration(
        border: Border.all(color: Colors.grey.shade300),
        color: switch (hitType) {
          HitType.hit => Colors.green,
          HitType.partial => Colors.yellow,
          HitType.miss => Colors.grey,
          _ => Colors.white,
        },
      ),
      child: Center(
        child: Text(
          letter.toUpperCase(),
          style: Theme.of(context).textTheme.titleLarge,
        ),
      ),
    );
  }
}

There are many different curves provided by the Flutter SDK, so feel free to try them out by passing different types to the curve parameter.

Implicit animations like AnimatedContainer are powerful because you just tell the widget what the new state should be, and it handles the "how" of the animation.

For complex, custom animations, you can write your own animated widgets. If you're curious, try it out in the animations tutorial.

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