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.
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
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.
Whenever the animation’s value changes,
the animation notifies all the listeners added with
addListener. Typically, a
object that listens to an animation calls
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, is most useful for
stateless animated widgets. To use
simply subclass it and implement the
AnimatedBuilder, is useful for more complex widgets
that wish to include an animation as part of a larger build function.
AnimatedBuilder, simply construct the widget
and pass it a
Animations also provide an
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
dismissed when their value is 0.0.
An animation might then run
forward (from 0.0 to 1.0)
or perhaps in
reverse (from 1.0 to 0.0).
Eventually, if the animation reaches the end of its range
(1.0), the animation reaches the
To create an animation, first create an
As well as being an animation itself, an
lets you control the animation. For example,
you can tell the controller to play the animation
stop the animation.
You can also
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
that mirrors the original animation but runs in the
opposite direction (from 1.0 to 0.0).
Similarly, you can create a
whose value is adjusted by a
To animate beyond the 0.0 to 1.0 interval, you can use a
Tween<T>, which interpolates between its
end values. Many types have specific
Tween subclasses that provide type-specific interpolation.
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
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:
evaluatethe 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.
animatethe tween based on the animation. Rather than returning a single value, the animate function returns a new
Animationthat 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.
Animations are actually built from a number of core building blocks.
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 that
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.
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 tickers at different times between two ticks, they will all nonetheless be synchronised with the same starting time, and will subsequently tick in lockstep. Like people at a bus-stop, all the tickers will wait for a regularly occuring event (the tick) to begin moving (counting time).
Simulation abstract class maps a
relative time value (an elapsed time) to a
double value, and has a notion of completion.
There are various concrete implementations
Simulation class for different effects.
Animatable abstract class maps a
double to a value of a particular type.
Animatable classes are stateless and immutable.
Tween<T> abstract class maps a double
value nominally in the range 0.0-1.0 to a typed value
(for example, a
Color, or another double).
It is an
It has a notion of an output type (
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.
Animatable<double> (the parent) to an
chain() method creates a new
Animatable subclass that applies the
parent’s mapping then the child’s mapping.
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.
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
AlwaysStoppedAnimation); registering callbacks on
these has no effect as the callbacks are never called.
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
Tween classes, as well as some further
Animation subclasses are stateless,
merely forwarding listeners to their parents.
Some are very stateful.
Animation subclasses take an explicit “parent”
Animation<double>. They are driven by that parent.
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
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
ProxyAnimation subclass takes an
Animation<double> class as
its parent and merely forwards the current state of that parent.
However, the parent is mutable.
TrainHoppingAnimation subclass takes two parents,
and switches between them when their values cross.
AnimationController is a stateful
Animation<double> that uses a
Ticker to give itself life.
It can be started and stopped. At 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
reports that at that time it has ended, then the controller stops
The animation controller can be given a lower and upper bound to animate between, and a duration.
In the simple case (using
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.
repeat(), the animation controller uses a linear
interpolation between the given bounds over the given duration, but
does not stop.
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
Force is used to create a specific
simulation which is then used to drive the controller.
animateWith(), the given simulation is used to drive the
These methods all return the future that the
Ticker provides and
which will resolve when the controller next stops or changes
Attaching animatables to animations
Animation<double> (the new parent) to an
animate() method creates a new
Animation subclass that acts like
Animatable but is driven from the given parent.