Flutter - Essential animation concepts and classes

Flutter - Essential animation concepts and classes

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What’s the point?

The animation system in Flutter is based on typed Animation objects. Widgets can either incorporate these animations in their build functions directly by reading their current value and listening to their state changes or they can use the animations as the basis of more elaborate animations that they pass along to other widgets.

Animation <double>

In Flutter, an Animation object knows nothing about what is onscreen. An Animation is an abstract class that understands its current value and its state (completed or dismissed). One of the more commonly used animation types is Animation<double> .

An Animation object sequentially generates interpolated numbers between two values over a certain duration. The output of an Animation object might be linear, a curve, a step function, or any other mapping you can devise. Depending on how the Animation object is controlled, it could run in reverse, or even switch directions in the middle.

Animations can also interpolate types other than double, such as Animation<Color> or Animation<Size>.

An Animation object has state. Its current value is always available in the .value member.

An Animation object knows nothing about rendering or build() functions.


A CurvedAnimation defines the animation’s progress as a non-linear curve.

animation = CurvedAnimation(parent: controller, curve: Curves.easeIn);

Note: The Curves class defines many commonly used curves, or you can create your own. For example:

import 'dart:math';

class ShakeCurve extends Curve {
  double transform(double t) => sin(t * pi * 2);

Browse the Curves documentation for a complete listing (with visual previews) of the Curves constants that ship with Flutter.

CurvedAnimation and AnimationController (described in the next section) are both of type Animation<double>, so you can pass them interchangeably. The CurvedAnimation wraps the object it’s modifying—you don’t subclass AnimationController to implement a curve.


AnimationController is a special Animation object that generates a new value whenever the hardware is ready for a new frame. By default, an AnimationController linearly produces the numbers from 0.0 to 1.0 during a given duration. For example, this code creates an Animation object, but does not start it running:

controller =
    AnimationController(duration: const Duration(seconds: 2), vsync: this);

AnimationController derives from Animation<double> , so it can be used wherever an Animation object is needed. However, the AnimationController has additional methods to control the animation. For example, you start an animation with the .forward() method. The generation of numbers is tied to the screen refresh, so typically 60 numbers are generated per second. After each number is generated, each Animation object calls the attached Listener objects. To create a custom display list for each child, see RepaintBoundary.

When creating an AnimationController, you pass it a vsync argument. The presence of vsync prevents offscreen animations from consuming unnecessary resources. You can use your stateful object as the vsync by adding SingleTickerProviderStateMixin to the class definition. You can see an example of this in animate1 on GitHub.

Note: In some cases, a position might exceed the AnimationController’s 0.0-1.0 range. For example, the fling() function allows you to provide velocity, force, and position (via the Force object). The position can be anything and so can be outside of the 0.0 to 1.0 range. A CurvedAnimation can also exceed the 0.0 to 1.0 range, even if the AnimationController doesn’t. Depending on the curve selected, the output of the CurvedAnimation can have a wider range than the input. For example, elastic curves such as Curves.elasticIn significantly overshoots or undershoots the default range.


By default, the AnimationController object ranges from 0.0 to 1.0. If you need a different range or a different data type, you can use a Tween to configure an animation to interpolate to a different range or data type. For example, the following Tween goes from -200.0 to 0.0:

tween = Tween<double>(begin: -200, end: 0);

A Tween is a stateless object that takes only begin and end. The sole job of a Tween is to define a mapping from an input range to an output range. The input range is commonly 0.0 to 1.0, but that’s not a requirement.

A Tween inherits from Animatable<T>, not from Animation<T>. An Animatable, like Animation, doesn’t have to output double. For example, ColorTween specifies a progression between two colors.

colorTween = ColorTween(begin: Colors.transparent, end: Colors.black54);

A Tween object does not store any state. Instead, it provides the evaluate(Animation<double> animation) method that applies the mapping function to the current value of the animation. The current value of the Animation object can be found in the .value method. The evaluate function also performs some housekeeping, such as ensuring that begin and end are returned when the animation values are 0.0 and 1.0, respectively.

#### Tween.animate

To use a Tween object, call animate() on the Tween, passing in the controller object. For example, the following code generates the integer values from 0 to 255 over the course of 500 ms.

AnimationController controller = AnimationController(
    duration: const Duration(milliseconds: 500), vsync: this);
Animation<int> alpha = IntTween(begin: 0, end: 255).animate(controller);

Note: The animate() method returns an Animation, not an Animatable.

The following example shows a controller, a curve, and a Tween:

AnimationController controller = AnimationController(
    duration: const Duration(milliseconds: 500), vsync: this);
final Animation<double> curve =
    CurvedAnimation(parent: controller, curve: Curves.easeOut);
Animation<int> alpha = IntTween(begin: 0, end: 255).animate(curve);

### Animation notifications

An Animation object can have Listeners and StatusListeners, defined with addListener() and addStatusListener().

A Listener is called whenever the value of the animation changes. The most common behavior of a Listener is to call setState() to cause a rebuild.

A StatusListener is called when an animation begins, ends, moves forward, or moves reverse, as defined by AnimationStatus. The next section has an example of the addListener() method, and Monitoring the progress of the animation shows an example of addStatusListener().


Last update on 24 Jan 2022