skills/moai-lang-flutter/reference/animations.md

# Flutter Animations

## Overview

Create smooth, performant animations in Flutter using the right approach for each use case. This reference covers complete animation workflow: from choosing between implicit/explicit approaches to implementing complex effects like hero transitions and staggered animations.

## Animation Type Decision Tree

Choose the right animation type based on your requirements:

**Implicit Animations** - Use when:
- Animating a single property (color, size, position)
- Animation is triggered by state change
- No need for fine-grained control

**Explicit Animations** - Use when:
- Need full control over animation lifecycle
- Animating multiple properties simultaneously
- Need to react to animation state changes
- Creating custom animations or transitions

**Hero Animations** - Use when:
- Sharing an element between two screens
- Creating shared element transitions
- User expects element to "fly" between routes

**Staggered Animations** - Use when:
- Multiple animations should run sequentially or overlap
- Creating ripple effects or sequential reveals
- Animating list items in sequence

**Physics-Based Animations** - Use when:
- Animations should feel natural/physical
- Spring-like behavior, scrolling gestures
- Draggable interactions

## Implicit Animations

Implicit animations automatically handle the animation when properties change. No controller needed.

### Common Implicit Widgets

**AnimatedContainer** - Animates multiple properties (size, color, decoration, padding):

```dart
AnimatedContainer(
  duration: const Duration(milliseconds: 300),
  curve: Curves.easeInOut,
  width: _expanded ? 200 : 100,
  height: _expanded ? 200 : 100,
  color: _expanded ? Colors.blue : Colors.red,
  child: const FlutterLogo(),
)
```

**AnimatedOpacity** - Simple fade animation:

```dart
AnimatedOpacity(
  opacity: _visible ? 1.0 : 0.0,
  duration: const Duration(milliseconds: 300),
  child: const Text('Hello'),
)
```

**TweenAnimationBuilder** - Custom tween animation without boilerplate:

```dart
TweenAnimationBuilder<double>(
  tween: Tween<double>(begin: 0, end: 1),
  duration: const Duration(seconds: 1),
  builder: (context, value, child) {
    return Opacity(
      opacity: value,
      child: Transform.scale(
        scale: value,
        child: child,
      ),
    );
  },
  child: const FlutterLogo(),
)
```

**Other implicit widgets:**
- `AnimatedPadding` - Padding animation
- `AnimatedPositioned` - Position animation (in Stack)
- `AnimatedAlign` - Alignment animation
- `AnimatedSwitcher` - Cross-fade between widgets
- `AnimatedDefaultTextStyle` - Text style animation

### Best Practices

- Prefer implicit animations for simple cases
- Use appropriate curves for natural motion (see `Curves` class)
- Set `curve` and `duration` for predictable behavior
- Use `onEnd` callback when needed
- Avoid nested implicit animations for performance

## Explicit Animations

Explicit animations provide full control with AnimationController.

### Core Components

**AnimationController** - Drives the animation:

```dart
late AnimationController _controller;

@override
void initState() {
  super.initState();
  _controller = AnimationController(
    duration: const Duration(seconds: 2),
    vsync: this,
  );
}

@override
void dispose() {
  _controller.dispose();
  super.dispose();
}
```

**Tween** - Interpolates between begin and end values:

```dart
animation = Tween<double>(begin: 0, end: 300).animate(_controller);
```

**CurvedAnimation** - Applies a curve to the animation:

```dart
animation = CurvedAnimation(
  parent: _controller,
  curve: Curves.easeInOut,
);
```

### AnimatedWidget Pattern

Best for reusable animated widgets:

```dart
class AnimatedLogo extends AnimatedWidget {
  const AnimatedLogo({super.key, required Animation<double> animation})
    : super(listenable: animation);

  @override
  Widget build(BuildContext context) {
    final animation = listenable as Animation<double>;
    return Center(
      child: Container(
        height: animation.value,
        width: animation.value,
        child: const FlutterLogo(),
      ),
    );
  }
}
```

### AnimatedBuilder Pattern

Best for complex widgets with animations:

```dart
class GrowTransition extends StatelessWidget {
  const GrowTransition({
    required this.child,
    required this.animation,
    super.key,
  });

  final Widget child;
  final Animation<double> animation;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: AnimatedBuilder(
        animation: animation,
        builder: (context, child) {
          return SizedBox(
            height: animation.value,
            width: animation.value,
            child: child,
          );
        },
        child: child,
      ),
    );
  }
}
```

### Built-in Explicit Transitions

Flutter provides ready-to-use transitions:
- `FadeTransition` - Fade animation
- `ScaleTransition` - Scale animation
- `SlideTransition` - Slide animation
- `SizeTransition` - Size animation
- `RotationTransition` - Rotation animation
- `PositionedTransition` - Position animation (in Stack)

## Hero Animations

Hero animations create shared element transitions between screens.

### Basic Hero Animation

**Source screen:**
```dart
Hero(
  tag: 'hero-image',
  child: Image.asset('images/logo.png'),
)
```

**Destination screen:**
```dart
Hero(
  tag: 'hero-image',  // Same tag!
  child: Image.asset('images/logo.png'),
)
```

### Hero Best Practices

- Use unique, consistent tags (often the data object itself)
- Keep hero widget trees similar between routes
- Wrap images in `Material` with transparent color for "pop" effect
- Use `timeDilation` to debug transitions
- Consider `HeroMode` to disable hero animations when needed

## Staggered Animations

Run multiple animations with different timing.

### Basic Staggered Animation

All animations share one controller:

```dart
class StaggerAnimation extends StatelessWidget {
  StaggerAnimation({super.key, required this.controller})
    : opacity = Tween<double>(begin: 0.0, end: 1.0).animate(
        CurvedAnimation(
          parent: controller,
          curve: const Interval(0.0, 0.100, curve: Curves.ease),
        ),
      ),
      width = Tween<double>(begin: 50.0, end: 150.0).animate(
        CurvedAnimation(
          parent: controller,
          curve: const Interval(0.125, 0.250, curve: Curves.ease),
        ),
      );

  final AnimationController controller;
  final Animation<double> opacity;
  final Animation<double> width;

  Widget _buildAnimation(BuildContext context, Widget? child) {
    return Container(
      alignment: Alignment.bottomCenter,
      child: Opacity(
        opacity: opacity.value,
        child: Container(
          width: width.value,
          height: 150,
          color: Colors.blue,
        ),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: controller,
      builder: _buildAnimation,
    );
  }
}
```

### Interval-Based Timing

Each animation has an Interval between 0.0 and 1.0:

```dart
animation = Tween<double>(begin: 0, end: 300).animate(
  CurvedAnimation(
    parent: controller,
    curve: const Interval(
      0.25,  // Start at 25% of controller duration
      0.50,  // End at 50% of controller duration
      curve: Curves.ease,
    ),
  ),
);
```

## Physics-Based Animations

Create natural-feeling animations using physics simulations.

### Fling Animation

```dart
_controller.fling(
  velocity: 2.0,  // Units per second
);
```

### Custom Physics Simulation

```dart
_controller.animateWith(
  SpringSimulation(
    spring: const SpringDescription(
      mass: 1,
      stiffness: 100,
      damping: 10,
    ),
    start: 0.0,
    end: 1.0,
    velocity: 0.0,
  ),
);
```

## Best Practices

### DO

- Dispose AnimationController in widget disposal
- Use `AnimatedBuilder`/`AnimatedWidget` instead of `setState()` in listeners
- Choose appropriate curves for natural motion
- Use `timeDilation` for debugging animations
- Consider performance (avoid heavy widgets in animation builds)
- Test animations on various devices
- Support reverse animations for intuitive feel

### DON'T

- Forget to dispose AnimationController (memory leak)
- Use `setState()` in animation listeners when `AnimatedBuilder` suffices
- Assume animation completes instantly (handle `AnimationStatus`)
- Over-animate (animations can distract users)
- Create animations that feel "jerky" (use smooth curves)
- Ignore accessibility (respect `disableAnimations` preference)