Flutter Animate

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

1. Pre-built effects, like blur, fade, scale, and slide
2. Easy custom effects
3. Simplified animated builders
4. Synchronized events

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

NOTE: This library is currently in prerelease. Some aspects of the API
will change as it is refined. Your feedback is welcome via Github issues.

Duration extensions

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

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
  .blur(end: 8.0) // inherits the delay & duration from move

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),
)

Custom effects & builders

It is easy to write new resuable effects by extending Effect, but you can also
create one-off custom effects easily 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 value 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 display 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 (ie.
after its duration) and false after.

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.second,
    color: value ? Colors.red : Colors.green,
  ),
)

SwapEffect

SwapEffect lets you swap out the 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:

Widget text = Text("Hello World!");

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

Events & callbacks

There are onInit and onComplete callbacks on Animate that trigger when the
whole animation starts or ends. Use the provided the AnimationController to
manipulate the animation (ex. repeat, reverse, etc).

Text("Pulsing Text")
  .animate(onComplete: (controller) => controller.repeat(reverse: true))
  .fadeOut(delay: 300.ms, duration: 300.ms, curve: Curves.easeIn)

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, and curve.

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.