Listenable class Null safety

An object that maintains a list of listeners.

The listeners are typically used to notify clients that the object has been updated.

There are two variants of this interface:

• ValueListenable, an interface that augments the Listenable interface with the concept of a current value.

• Animation, an interface that augments the ValueListenable interface to add the concept of direction (forward or reverse).

Many classes in the Flutter API use or implement these interfaces. The following subclasses are especially relevant:

• ChangeNotifier, which can be subclassed or mixed in to create objects that implement the Listenable interface.

• ValueNotifier, which implements the ValueListenable interface with a mutable value that triggers the notifications when modified.

The terms "notify clients", "send notifications", "trigger notifications", and "fire notifications" are used interchangeably.

See also:

• AnimatedBuilder, a widget that uses a builder callback to rebuild whenever a given Listenable triggers its notifications. This widget is commonly used with Animation subclasses, hence its name, but is by no means limited to animations, as it can be used with any Listenable. It is a subclass of AnimatedWidget, which can be used to create widgets that are driven from a Listenable.
• ValueListenableBuilder, a widget that uses a builder callback to rebuild whenever a ValueListenable object triggers its notifications, providing the builder with the value of the object.
• InheritedNotifier, an abstract superclass for widgets that use a Listenable's notifications to trigger rebuilds in descendant widgets that declare a dependency on them, using the InheritedWidget mechanism.
• Listenable.merge, which creates a Listenable that triggers notifications whenever any of a list of other Listenables trigger their notifications.