Animations overview
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
The primary building block of the animation system is the Animation
class. An animation represents a value of a specific type that can change over the lifetime of the animation. Most widgets that perform an animation receive an Animation
object as a parameter, from which they read the current value of the animation and to which they listen for changes to that value.
addListener
Whenever the animation’s value changes, the animation notifies all the listeners added with addListener
. Typically, a State
object that listens to an animation will call setState
on itself in its listener callback to notify the widget system that it needs to rebuild with the new value of the animation.
This pattern is so common that there are two widgets that help widgets rebuild when animations change value: AnimatedWidget
and AnimatedBuilder
. The first, AnimatedWidget
, is most useful for stateless animated widgets. To use AnimatedWidget
, simply subclass it and implement the build
function. The second, AnimatedBuilder
, is useful for more complex widgets that wish to include an animation as part of a larger build function. To use AnimatedBuilder
, simply construct the widget and pass it a builder
function.
addStatusListener
Animations also provide an AnimationStatus
, which indicates how the animation will evolve over time. Whenever the animation’s status changes, the animation notifies all the listeners added with addStatusListener
. Typically, animations start out in the dismissed
status, which means they’re at the beginning of their range. For example, animations that progress from 0.0 to 1.0 will be dismissed
when their value is 0.0. An animation might then run forward
(e.g., from 0.0 to 1.0) or perhaps in reverse
(e.g., from 1.0 to 0.0). Eventually, if the animation reaches the end of its range (e.g., 1.0), the animation reaches the completed
status.
AnimationController
To create an animation, first create an AnimationController
. As well as being an animation itself, an AnimationController
lets you control the animation. For example, you can tell the controller to play the animation forward
or stop
the animation. You can also fling
animations, which uses a physical simulation, such as a spring, to drive the animation.
Once you’ve created an animation controller, you can start building other animations based on it. For example, you can create a ReverseAnimation
that mirrors the original animation but runs in the opposite direction (e.g., from 1.0 to 0.0). Similarly, you can create a CurvedAnimation
whose value is adjusted by a curve.
Tweens
To animate beyond the 0.0 to 1.0 interval, you can use a Tween<T>
, which interpolates between its begin
and end
values. Many types have specific Tween
subclasses that provide type-specific interpolation. For example, ColorTween
interpolates between colors and RectTween
interpolates between rects. You can define your own interpolations by creating your own subclass of Tween
and overriding its lerp
function.
By itself, a tween just defines how to interpolate between two values. To get a concrete value for the current frame of an animation, you also need an animation to determine the current state. There are two ways to combine a tween with an animation to get a concrete value:
You can
evaluate
the tween at the current value of an animation. This approach is most useful for widgets that are already listening to the animation and hence rebuilding whenever the animation changes value.You can
animate
the tween based on the animation. Rather than returning a single value, the animate function returns a newAnimation
that incorporates the tween. This approach is most useful when you want to give the newly created animation to another widget, which can then read the current value that incorporates the tween as well as listen for changes to the value.
Architecture
Animations are actually built from a number of core building blocks.
Scheduler
The SchedulerBinding
is a singleton class that exposes the Flutter scheduling primitives.
For this discussion, the key primitive is the frame callbacks. Each time a frame needs to be shown on the screen, Flutter’s engine triggers a “begin frame” callback which the scheduler multiplexes to all the listeners registered using scheduleFrameCallback()
. All these callbacks are given the official time stamp of the frame, in the form of a Duration
from some arbitrary epoch. Since all the callbacks have the same time, any animations triggered from these callbacks will appear to be exactly synchronised even if they take a few milliseconds to be executed.
Tickers
The Ticker
class hooks into the scheduler’s scheduleFrameCallback()
mechanism to invoke a callback every tick.
A Ticker
can be started and stopped. When started, it returns a Future
that will resolve when it is stopped.
Each tick, the Ticker
provides the callback with the duration since the first tick after it was started.
Because tickers always give their elapsed time relative to the first tick after they were started, tickers are all synchronised. If you start three ticks at different times between two frames, they will all nonetheless be synchronised with the same starting time, and will subsequently tick in lockstep.
Simulations
The Simulation
abstract class maps a relative time value (an elapsed time) to a double value, and has a notion of completion.
In principle simulations are stateless but in practice some simulations (for example, BouncingScrollSimulation
and ClampingScrollSimulation
) change state irreversibly when queried.
There are various concrete implementations of the Simulation
class for different effects.
Animatables
The Animatable
abstract class maps a double to a value of a particular type.
Animatable
classes are stateless and immutable.
Tweens
The Tween
abstract class maps a double value nominally in the range 0.0-1.0 to a typed value (e.g. a Color
, or another double). It is an Animatable
.
It has a notion of an output type (T
), a begin
value and an end
value of that type, and a way to interpolate (lerp
) between the begin and end values for a given input value (the double nominally in the range 0.0-1.0).
Tween
classes are stateless and immutable.
Composing animatables
Passing an Animatable<double>
(the parent) to an Animatable
’s chain()
method creates a new Animatable
subclass that applies the parent’s mapping then the child’s mapping.
Curves
The Curve
abstract class maps doubles nominally in the range 0.0-1.0 to doubles nominally in the range 0.0-1.0.
Curve
classes are stateless and immutable.
Animations
The Animation
abstract class provides a value of a given type, a concept of animation direction and animation status, and a listener interface to register callbacks that get invoked when the value or status change.
Some subclasses of Animation
have values that never change (kAlwaysCompleteAnimation
, kAlwaysDismissedAnimation
, AlwaysStoppedAnimation
); registering callbacks on these has no effect as the callbacks are never called.
The Animation<double>
variant is special because it can be used to represent a double nominally in the range 0.0-1.0, which is the input expected by Curve
and Tween
classes, as well as some further subclasses of Animation
.
Some Animation
subclasses are stateless, merely forwarding listeners to their parents. Some are very stateful.
Composable animations
Most Animation
subclasses take an explicit “parent” Animation<double>
. They are driven by that parent.
The CurvedAnimation
subclass takes an Animation<double>
class (the parent) and a couple of Curve
classes (the forward and reverse curves) as input, and uses the value of the parent as input to the curves to determine its output. CurvedAnimation
is immutable and stateless.
The ReverseAnimation
subclass takes an Animation<double>
class as its parent and reverses all the values of the animation. It assumes the parent is using a value nominally in the range 0.0-1.0 and returns a value in the range 1.0-0.0. The status and direction of the parent animation are also reversed. ReverseAnimation
is immutable and stateless.
The ProxyAnimation
subclass takes an Animation<double>
class as its parent and merely forwards the current state of that parent. However, the parent is mutable.
The TrainHoppingAnimation
subclass takes two parents, and switches between them when their values cross.
Animation Controllers
The AnimationController
is a stateful Animation<double>
that uses a Ticker
to give itself life. It can be started and stopped. Each tick, it takes the time elapsed since it was started and passes it to a Simulation
to obtain a value. That is then the value it reports. If the Simulation
reports that at that time it has ended, then the controller stops itself.
The animation controller can be given a lower and upper bound to animate between, and a duration.
In the simple case (using forward()
, reverse()
, play()
, or resume()
), the animation controller simply does a linear interpolation from the lower bound to the upper bound (or vice versa, for the reverse direction) over the given duration.
When using repeat()
, the animation controller uses a linear interpolation between the given bounds over the given duration, but does not stop.
When using animateTo()
, the animation controller does a linear interpolation over the given duration from the current value to the given target. If no duration is given to the method, the default duration of the controller and the range described by the controller’s lower bound and upper bound is used to determine the velocity of the animation.
When using fling()
, a Force
is used to create a specific simulation which is then used to drive the controller.
When using animateWith()
, the given simulation is used to drive the controller.
These methods all return the future that the Ticker
provides and which will resolve when the controller next stops or changes simulation.
Attaching animatables to animations
Passing an Animation<double>
(the new parent) to an Animatable
’s animate()
method creates a new Animation
subclass that acts like the Animatable
but is driven from the given parent.