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package javafx.beans.value;
import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
An 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 (ObservableBooleanValue etc.). 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 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 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.
Type parameters: - <T> –
The type of the wrapped value.
See Also: Since: JavaFX 2.0
/**
* An {@code 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
* ({@code ObservableBooleanValue} etc.).
* <p>
* The value of the {@code ObservableValue} can be requested with
* {@link #getValue()}.
* <p>
* An implementation of {@code 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.
* <p>
* An {@code 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 {@code 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.
* <p>
* 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.
* <p>
* Two types of listeners can be attached to an {@code ObservableValue}:
* {@link InvalidationListener} to listen to invalidation events and
* {@link ChangeListener} to listen to change events.
* <p>
* Important note: attaching a {@code ChangeListener} enforces eager computation
* even if the implementation of the {@code ObservableValue} supports lazy
* evaluation.
*
* @param <T>
* The type of the wrapped value.
*
* @see ObservableBooleanValue
* @see ObservableDoubleValue
* @see ObservableFloatValue
* @see ObservableIntegerValue
* @see ObservableLongValue
* @see ObservableNumberValue
* @see ObservableObjectValue
* @see ObservableStringValue
*
*
* @since JavaFX 2.0
*/
public interface ObservableValue<T> extends Observable {
Adds a ChangeListener which will be notified whenever the value of the ObservableValue changes. If the same listener is added more than once, then it will be notified more than once. That is, no check is made to ensure uniqueness. Note that the same actual ChangeListener instance may be safely registered for different ObservableValues.
The ObservableValue stores a strong reference to the listener which will prevent the listener from being garbage collected and may result in a memory leak. It is recommended to either unregister a listener by calling
removeListener after use or to use an instance of WeakChangeListener avoid this situation.
Params: - listener –
The listener to register
Throws: - NullPointerException –
if the listener is null
See Also:
/**
* Adds a {@link ChangeListener} which will be notified whenever the value
* of the {@code ObservableValue} changes. If the same listener is added
* more than once, then it will be notified more than once. That is, no
* check is made to ensure uniqueness.
* <p>
* Note that the same actual {@code ChangeListener} instance may be safely
* registered for different {@code ObservableValues}.
* <p>
* The {@code ObservableValue} stores a strong reference to the listener
* which will prevent the listener from being garbage collected and may
* result in a memory leak. It is recommended to either unregister a
* listener by calling {@link #removeListener(ChangeListener)
* removeListener} after use or to use an instance of
* {@link WeakChangeListener} avoid this situation.
*
* @see #removeListener(ChangeListener)
*
* @param listener
* The listener to register
* @throws NullPointerException
* if the listener is null
*/
void addListener(ChangeListener<? super T> listener);
Removes the given listener from the list of listeners that are notified whenever the value of the ObservableValue changes.
If the given listener has not been previously registered (i.e. it was
never added) then this method call is a no-op. If it had been previously
added then it will be removed. If it had been added more than once, then
only the first occurrence will be removed.
Params: - listener –
The listener to remove
Throws: - NullPointerException –
if the listener is null
See Also:
/**
* Removes the given listener from the list of listeners that are notified
* whenever the value of the {@code ObservableValue} changes.
* <p>
* If the given listener has not been previously registered (i.e. it was
* never added) then this method call is a no-op. If it had been previously
* added then it will be removed. If it had been added more than once, then
* only the first occurrence will be removed.
*
* @see #addListener(ChangeListener)
*
* @param listener
* The listener to remove
* @throws NullPointerException
* if the listener is null
*/
void removeListener(ChangeListener<? super T> listener);
Returns the current value of this ObservableValue Returns: The current value
/**
* Returns the current value of this {@code ObservableValue}
*
* @return The current value
*/
T getValue();
}