本文介绍了ObservableValue接口，它是用于封装值并允许观察值变化的实体。通常不直接实现此接口，而是实现其子接口（如ObservableBooleanValue等）。文章还讨论了支持懒评估的实现方式及其如何生成变更和失效事件。
public interface ObservableValue<T>
The value of the ObservableValue can be requested with getValue().
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.
An 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(分别) becomes important, if the ObservableValue supports 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（努力） 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.
Two types of listeners can be attached to an ObservableValue: InvalidationListener to listen to invalidation events and ChangeListener to listen to change events.
Important note: attaching a ChangeListener enforces eager computation even if the implementation of the ObservableValue supports lazy evaluation.
See Also:
ObservableBooleanValue, ObservableDoubleValue, ObservableFloatValue, ObservableIntegerValue, ObservableLongValue, ObservableNumberValue, ObservableObjectValue, ObservableStringValue
extends Observable
The value of the ObservableValue can be requested with getValue().
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.
An 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(分别) becomes important, if the ObservableValue supports 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（努力） 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.
Two types of listeners can be attached to an ObservableValue: InvalidationListener to listen to invalidation events and ChangeListener to listen to change events.
Important note: attaching a ChangeListener enforces eager computation even if the implementation of the ObservableValue supports lazy evaluation.
See Also:
ObservableBooleanValue, ObservableDoubleValue, ObservableFloatValue, ObservableIntegerValue, ObservableLongValue, ObservableNumberValue, ObservableObjectValue, ObservableStringValue
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