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different range or data type. For example, the
following Tween goes from -200.0 to 0.0:
<code_start>tween = Tween<double>(begin: -200, end: 0);<code_end>
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.
<code_start>colorTween = ColorTween(begin: Colors.transparent, end: Colors.black54);<code_end>
A Tween object doesn’t store any state. Instead, it provides the
evaluate(Animation<double> animation) method that uses the
transform function to map the current value of the animation
(between 0.0 and 1.0), to the actual animation value.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.<topic_end>
<topic_start>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.
<code_start>AnimationController controller = AnimationController(
duration: const Duration(milliseconds: 500), vsync: this);
Animation<int> alpha = IntTween(begin: 0, end: 255).animate(controller);<code_end>
info Note
The animate() method returns an Animation,
not an Animatable.The following example shows a controller, a curve, and a Tween:
<code_start>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);<code_end>
<topic_end>
<topic_start>
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().<topic_end>
<topic_start>
Animation examples
This section walks you through 5 animation examples.
Each section provides a link to the source code for that example.<topic_end>
<topic_start>
Rendering animations
<topic_end>
<topic_start>What's the point?
So far you’ve learned how to generate a sequence of numbers over time.
Nothing has been rendered to the screen. To render with an
Animation object, store the Animation object as a
member of your widget, then use its value to decide how to draw.Consider the following app that draws the Flutter logo without animation:
<code_start>import 'package:flutter/material.dart';
void main() => runApp(const LogoApp());
class LogoApp extends StatefulWidget {
const LogoApp({super.key});
@override
State<LogoApp> createState() => _LogoAppState();
}
class _LogoAppState extends State<LogoApp> {
@override
Widget build(BuildContext context) {
return Center(
child: Container(
margin: const EdgeInsets.symmetric(vertical: 10),
height: 300,
width: 300,
child: const FlutterLogo(),
),
);
}
}<code_end>
App source: animate0The following shows the same code modified to animate the
logo to grow from nothing to full size.
When defining an AnimationController, you must pass in a
vsync object. The vsync parameter is described in the
AnimationController section.The changes from the non-animated example are highlighted:App source: animate1The addListener() function calls setState(),
so every time the Animation generates a new number,
the current frame is marked dirty, which forces
build() to be called again. In build(),
the container changes size because its height and
width now use animation.value instead of a hardcoded value.
Dispose of the controller when the State object is
discarded to prevent memory leaks.With these few changes,
you’ve created your first animation in Flutter!Dart language tricks:
You might not be familiar with Dart’s cascade notation—the two
dots in ..addListener(). This syntax means that the addListener()
method is called with the return value from animate().
Consider the following example:This code is equivalent to:To learn more about cascades,
check out Cascade notation
in the Dart language documentation.<topic_end>
<topic_start>
Simplifying with Animated­Widget
<topic_end>
<topic_start>What's the point?