How to Create Custom Animations Using Flutter's Animation API

In Flutter, the core of animation is the Animation object. The Animation object itself is decoupled from visual representation; it merely represents the state of the animation, such as the current value and whether the animation has completed.

1. Animation Controller

First, the core of custom animations is the AnimationController. It is a special type of Animation used to control the timing of the animation. It can produce a number between 0 and 1, representing the current state of the animation.

For example, to create a simple fade-in animation, we first need to initialize an AnimationController:

dart
AnimationController controller = AnimationController(
  duration: const Duration(seconds: 2),
  vsync: this,  // Requires a TickerProvider type for vsync
);

2. Tween

Next, we typically use Tween to define the range and data type of the animation. Tween manages the mapping from the input range to the output range. For example, to create an animation with transparency changes, we can set it up as follows:

dart
var opacityTween = Tween<double>(begin: 0.0, end: 1.0);

3. Animation

Then, we combine this tween with the previous controller to create an Animation object. This object is the animation we directly use to build the widget.

dart
Animation<double> animation = opacityTween.animate(controller);

4. Listening and Building

To make the animation run, we need to listen for updates to the animation at some point and call setState to rebuild the relevant widget.

dart
controller.addListener(() {
  setState(() {
    // Add animation logic here, which is called whenever the AnimationController updates
  });
});

controller.forward(); // Start the animation

5. Using AnimatedBuilder

For more efficient animation building, we typically use the AnimatedBuilder widget. AnimatedBuilder automatically listens for changes in the animation and rebuilds only the necessary parts of the widget.

dart
AnimatedBuilder(
  animation: animation,
  builder: (context, child) {
    return Opacity(
      opacity: animation.value,  // Use the current value of the Animation object
      child: child,
    );
  },
  child: Text('Hello Flutter!'),
);

This completes a simple fade-in animation. Flutter's animation API is very powerful, supporting the creation of various custom and advanced animation effects.

Conclusion

Through this example, you can see how creating animations in Flutter is straightforward and flexible. By combining different animation controllers and Tweens, we can achieve rich and colorful animation effects. In actual projects, this allows us to enhance the application's user experience, making the user interface more lively and friendly.