Flutter Animate

A performant library that makes it simple to add almost any kind of animated effect in Flutter.

Pre-built effects, like fade, scale, slide, align, flip, blur, shake, shimmer, shadows, crossfades, follow path, and color effects (saturation, color, and tint)
Easy custom effects and simplified animated builders
Synchronize animations to scroll, notifiers, or anything
Integrated events

All via a simple, unified API without fussing with AnimationController and StatefulWidget.

Above: The included example app.

Duration extensions

Extension methods for num, to make specifying durations easier. For example: 2.seconds, 0.1.minutes, or 300.ms.

AnimatedController extensions

A loop extension method for AnimatedController which is identical to repeat, but adds a count parameter to specifiy how many times to play.

Basics

Syntax

To apply effects, wrap the target widget in Animate, and specify a list of effects:

Animate(
  effects: [FadeEffect(), ScaleEffect()],
  child: Text("Hello World!"),
)

It also adds an .animate() extension method to all widgets, which wraps the widget in Animate(). Each effect also adds a chainable extension method to Animate to enable a shorthand syntax:

Text("Hello World!").animate().fade().scale()

NOTE: The shortform style is used in this README, but all functionality is available in either format.

Delay, duration, curve

Effects have optional delay, duration, and curve parameters. Effects run in parallel, but you can use a delay to run them sequentially:

Text("Hello").animate()
  .fade(duration: 500.ms)
  .scale(delay: 500.ms) // runs after fade.

Note that effects are "active" for the duration of the full animation, so for example, two fade effects on the same target can have unexpected results (SwapEffect detailed below, can help address this).

If not specified (or null), these values are inherited from the previous effect, or from Animate.defaultDuration and Animate.defaultCurve if it is the first effect:

Text("Hello World!").animate()
  .fadeIn() // uses `Animate.defaultDuration`
  .scale() // inherits duration from fadeIn
  .move(delay: 300.ms, duration: 600.ms) // runs after the above w/new duration
  .blurXY() // inherits the delay & duration from move

Animate has its own delay parameter, which defines a delay before the animation begins playing. Unlike the delay on an Effect, it is only applied once if the animation repeats.

Text("Hello").animate(
    delay: 1000.ms, // this delay only happens once at the very start
    onPlay: (controller) => controller.repeat(), // loop
  ).fadeIn(delay: 500.ms) // this delay happens at the start of each loop

Other Effect Parameters

Most effects include begin and end parameters, which specify the start/end values. These are usually "smart" in the sense that if only one is specified then the other will default to a "neutral" value (ie. no visual effect). If both are unspecified the effect should use visually pleasing defaults.

// an opacity of 1 is "neutral"
Text("Hello").animate().fade() // begin=0, end=1
Text("Hello").animate().fade(begin: 0.5) // end=1
Text("Hello").animate().fade(end: 0.5) // begin=1

Many effects have additional parameters that influence their behavior. These should also use pleasant defaults if unspecified.

Text('Hello').animate().tint(color: Colors.purple)

Sequencing with ThenEffect

ThenEffect is a special convenience "effect" that makes it easier to sequence effects. It does this by establishing a new baseline time equal to the previous effect's end time and its own optional delay. All subsequent effect delays are relative to this new baseline.

In the following example, the slide would run 200ms after the fade ended.

Text("Hello").animate()
  .fadeIn(duration: 600.ms)
  .then(delay: 200.ms) // baseline=800ms
  .slide()

Animating lists

The AnimateList class offers similar functionality for lists of widgets, with the option to offset each child's animation by a specified interval:

Column(children: AnimateList(
  interval: 400.ms,
  effects: [FadeEffect(duration: 300.ms)],
  children: [Text("Hello"), Text("World"),  Text("Goodbye")],
))

// or shorthand:
Column(
  children: [Text("Hello"), Text("World"),  Text("Goodbye")]
    .animate(interval: 400.ms).fade(duration: 300.ms),
)

Shared effects

Because Effect instances are immutable, they can be reused. This makes it easy to create a global collection of effects that are used throughout your app and updated in one place. This is also useful for design systems.

MyGlobalEffects.transitionIn = <Effect>[
  FadeEffect(duration: 100.ms, curve: Curves.easeOut),
  ScaleEffect(begin: 0.8, curve: Curves.easeIn)
]

// then:
Text('Hello').animate(effects: MyGlobalEffects.transitionIn)

Custom effects & builders

It is easy to write new resuable effects by extending Effect, but you can also easily create one-off custom effects by using CustomEffect, ToggleEffect, and SwapEffect.

CustomEffect

CustomEffect lets you build custom animated effects. Simply specify a builder function that accepts a context, value, and child. The child is the target of the animation (which may already have been wrapped in other effects).

For example, this would add a background behind the text and fade it from red to blue:

Text("Hello World").animate().custom(
  duration: 300.ms,
  builder: (context, value, child) => Container(
    color: Color.lerp(Colors.red, Colors.blue, value),
    padding: EdgeInsets.all(8),
    child: child, // child is the Text widget being animated
  )
)

By default it provides a value from 0-1 (though some curves can generate values outside this range), based on the current time, duration, and curve. You can also specify begin and end values as demonstrated in the example below.

Animate can be created without a child, so you use CustomEffect as a simplified builder. For example, this would build text counting down from 10, and fading out:

Animate().custom(
  duration: 10.seconds,
  begin: 10,
  end: 0,
  builder: (_, value, __) => Text(value.round()),
).fadeOut()

ToggleEffect

ToggleEffect also provides builder functionality, but instead of a double, it provides a boolean value equal to true before the end of the effect and false after (ie. after its duration).

Animate().toggle(
  duration: 2.seconds,
  builder: (_, value, __) => Text(value ? "Before" : "After"),
)

This can also be used to activate "Animated" widgets, like AnimatedContainer, by toggling their values with a minimal delay:

Animate().toggle(
  duration: 1.ms,
  builder: (_, value, __) => AnimatedContainer(
    duration: 1.seconds,
    color: value ? Colors.red : Colors.green,
  ),
)

SwapEffect

SwapEffect lets you swap out the whole target widget at a specified time:

Text("Before").animate()
  .swap(duration: 900.ms, builder: (_, __) => Text("After"))

This can also be useful for creating sequential effects, by swapping the target widget back in, effectively wiping all previous effects:

text.animate().fadeOut(300.ms) // fade out & then...
  // swap in original widget & fade back in via a new Animate:
  .swap(builder: (_, child) => child.animate().fadeIn())

Events & callbacks

Animate includes the following callbacks:

onInit: the internal AnimationController has been initialized
onPlay: the animation has started playing after any Animate.delay
onComplete: the animation has finished

These callbacks return the AnimationController, which can be used to manipulate the animation (ex. repeat, reverse, etc).

Text("Horrible Pulsing Text")
  .animate(onPlay: (controller) => controller.repeat(reverse: true))
  .fadeOut(curve: Curves.easeInOut)

For more nuanced callbacks, use CallbackEffect or ListenEffect.

CallbackEffect

CallbackEffect lets you add a callback to an arbitrary postion in your animations. For example, adding a callback halfway through a fade:

Text("Hello").animate().fadeIn(duration: 600.ms)
  .callback(duration: 300.ms, callback: (_) => print('halfway'))

As with other effects, it will inherit the delay and duration of prior effects:

Text("Hello").animate().scale(delay: 200.ms, duration: 400.ms)
  .callback(callback: (_) => print('scale is done'))

ListenEffect

ListenEffect lets you register a callback to receive the animation value (as a double) for a given delay, duration, curve, begin, and end.

Text("Hello").animate().fadeIn(curve: Curves.easeOutExpo)
  .listen(callback: (value) => print('current opacity: $value'))

The above example works, because the listen effect inherits duration and curve from the fade, and both use begin=0, end=1 by default.

Adapters and Controllers

By default, all animations are driven by an internal AnimationController, and update based on elapsed time. For more control, you can specify your own external controller, or use an adapter. You can also set autoPlay=false if you want to start the animation manually.

Adapters synchronize the AnimationController to an external source. For example, the ScrollAdapter updates an animation based on a ScrollController so you can run complex animations based on scroll interactions.

You still define animations using durations, but the external source must provide a 0-1 value.

Flutter Animate ships with a collection of useful adapters. Check them out for more information.

Reacting to State Changes

Animate can react to state changes similar to "Animated" widgets (ex. AnimatedOpacity). Simply set up your animation normally, but set a target value. When the value of target changes, it will automatically animate to the new target position (where 0 is the beginning and 1 is the end).

For example, combined with logic that toggles _over via setState, this will fade and scale the button on roll over:

MyButton().animate(target: _over ? 1 : 0)
  .fade(end: 0.8).scaleXY(end: 1.1)

Testing Animations

When testing animations, you can set Animate.restartOnHotReload=true which will cause all animations to automatically restart every time you hot reload your app.

Installation

Grab it from pub.dev.

gskinner/flutter_animate

gskinner

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