ObservableValue is an entity that wraps a value and allows to
observe the value for changes. In general this interface should not be
implemented directly but one of its sub-interfaces
The value of the
ObservableValue can be requested with
An implementation of
ObservableValue may support lazy evaluation,
which means that the value is not immediately recomputed after changes, but
lazily the next time the value is requested. All bindings and properties in
this library support lazy evaluation.
ObservableValue generates two types of events: change events and
invalidation events. A change event indicates that the value has changed. An
invalidation event is generated, if the current value is not valid anymore.
This distinction becomes important, if the
lazy evaluation, because for a lazily evaluated value one does not know if an
invalid value really has changed until it is recomputed. For this reason,
generating change events requires eager evaluation while invalidation events
can be generated for eager and lazy implementations.
Implementations of this class should strive to generate as few events as possible to avoid wasting too much time in event handlers. Implementations in this library mark themselves as invalid when the first invalidation event occurs. They do not generate anymore invalidation events until their value is recomputed and valid again.
Important note: attaching a
ChangeListener enforces eager computation
even if the implementation of the
ObservableValue supports lazy
|The type of the wrapped value.|