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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.
<topic_end>
<topic_start>
animation controllers
the 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 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() or reverse()), 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.
<topic_end>
<topic_start>
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.
<topic_end>
<topic_start>
accessibility
ensuring apps are accessible to a broad range of users is an essential
part of building a high-quality app. applications that are poorly
designed create barriers to people of all ages. the UN convention on
the rights of persons with disabilities states the moral and legal
imperative to ensure universal access to information systems; countries
around the world enforce accessibility as a requirement; and companies
recognize the business advantages of maximizing access to their services.
we strongly encourage you to include an accessibility checklist
as a key criteria before shipping your app. flutter is committed to
supporting developers in making their apps more accessible, and includes
first-class framework support for accessibility in addition to that
provided by the underlying operating system, including:
details of these features are discussed below.
<topic_end>
<topic_start>
inspecting accessibility support
in addition to testing for these specific topics,
we recommend using automated accessibility scanners:
<topic_end>
<topic_start>
large fonts
both android and iOS contain system settings to configure the desired font
sizes used by apps. flutter text widgets respect this OS setting when
determining font sizes.
font sizes are calculated automatically by flutter based on the OS setting.
however, as a developer you should make sure your layout has enough room to
render all its contents when the font sizes are increased.
for example, you can test all parts of your app on a small-screen
device configured to use the largest font setting.
<topic_end>
<topic_start>
example
the following two screenshots show the standard flutter app
template rendered with the default iOS font setting,
and with the largest font setting selected in iOS accessibility settings.
<topic_end>
<topic_start>
screen readers
for mobile, screen readers (talkback, VoiceOver)
enable visually impaired users to get spoken feedback about
the contents of the screen and interact with the UI by using