On Thu, 26 Feb 2026 01:39:14 GMT, John Hendrikx <[email protected]> wrote:
>> This provides and uses a new implementation of `ExpressionHelper`, called
>> `ListenerManager` with improved semantics.
>>
>> See also #837 for a previous attempt which instead of triggering nested
>> emissions immediately (like this PR and `ExpressionHelper`) would wait until
>> the current emission finishes and then start a new (non-nested) emission.
>>
>> # Behavior
>>
>> |Listener...|ExpressionHelper|ListenerManager|
>> |---|---|---|
>> |Invocation Order|In order they were registered, invalidation listeners
>> always before change listeners|(unchanged)|
>> |Removal during Notification|All listeners present when notification started
>> are notified, but excluded for any nested changes|Listeners are removed
>> immediately regardless of nesting|
>> |Addition during Notification|Only listeners present when notification
>> started are notified, but included for any nested changes|New listeners are
>> never called during the current notification regardless of nesting|
>>
>> ## Nested notifications:
>>
>> | |ExpressionHelper|ListenerManager|
>> |---|---|---|
>> |Type|Depth first (call stack increases for each nested level)|(same)|
>> |# of Calls|Listeners * Depth (using incorrect old values)|Collapses nested
>> changes, skipping non-changes|
>> |Vetoing Possible?|No|Yes|
>> |Old Value correctness|Only for listeners called before listeners making
>> nested changes|Always|
>>
>> # Performance
>>
>> |Listener|ExpressionHelper|ListenerManager|
>> |---|---|---|
>> |Addition|Array based, append in empty slot, resize as needed|(same)|
>> |Removal|Array based, shift array, resize as needed|(same)|
>> |Addition during notification|Array is copied, removing collected
>> WeakListeners in the process|Appended when notification finishes|
>> |Removal during notification|As above|Entry is `null`ed (to avoid moving
>> elements in array that is being iterated)|
>> |Notification completion with changes|-|Null entries (and collected
>> WeakListeners) are removed|
>> |Notifying Invalidation Listeners|1 ns each|(same)|
>> |Notifying Change Listeners|1 ns each (*)|2-3 ns each|
>>
>> (*) a simple for loop is close to optimal, but unfortunately does not
>> provide correct old values
>>
>> # Memory Use
>>
>> Does not include alignment, and assumes a 32-bit VM or one that is using
>> compressed oops.
>>
>> |Listener|ExpressionHelper|ListenerManager|OldValueCaching ListenerManager|
>> |---|---|---|---|
>> |No Listeners|none|none|none|
>> |Single InvalidationListener|16 bytes overhead|none|none|
>> |Single ChangeListener|20 bytes overhead|none|16 bytes overhe...
>
> John Hendrikx has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Remove unreliable unnecessary extra check
We should probably update the specification in some places. Here are some
suggestions:
public interface ChangeListener<T> {
/**
* Called when the value of an {@link ObservableValue} changes.
* <p>
- * In general, it is considered bad practice to modify the observed value in
- * this method.
+ * When this method is invoked, {@code newValue} represents the current
value of the {@code observable};
+ * that is, it is equal to {@code observable.getValue()} at the time of
invocation. The {@code oldValue}
+ * is equivalent to the {@code newValue} of the previous notification
delivered to that listener.
+ * <p>
+ * If a change listener modifies the observable value in its callback,
other registered change listeners
+ * that have not been notified at that point will receive the
newly-modified value. This ensures that a
+ * change listener will always observe the effective change from its
last-observed {@code oldValue} to
+ * the current value of the {@code observable} at the time the listener is
invoked.
+ * <p>
+ * However, it is usually considered bad practice to modify the observable
value from a listener callback
+ * because the order of listener registrations is often not strictly
enforceable. This gives earlier
+ * listeners an order-dependent veto over the values that later listeners
will observe.
+ *
- * @param observable
- * The {@code ObservableValue} which value changed
- * @param oldValue
- * The old value
- * @param newValue
- * The new value
+ * @param observable the changed {@code ObservableValue}
+ * @param oldValue the last value that was observed by this listener
+ * @param newValue the current value of the {@code observable}
*/
void changed(ObservableValue<? extends T> observable, T oldValue, T
newValue);
}
public interface ObservableValue<T> extends Observable {
/**
* ...
+ * <p>
+ * If this method is called from a listener callback while a change
notification
+ * is in progress, the added listener will not be notified as part of the
ongoing
+ * notification.
*/
void addListener(ChangeListener<? super T> listener);
/**
* ...
+ * <p>
+ * If this method is called from a listener callback while a change
notification
+ * is in progress, the removal takes effect immediately and the removed
listener
+ * will not be notified as part of the ongoing notification if it has not
been
+ * notified so far.
*/
void removeListener(ChangeListener<? super T> listener);
...
}
The following spec seems to contradict the semantics of this PR:
public abstract class ObservableValueBase<T> implements ObservableValue<T> {
/**
* Notify the currently registered observers of a value change.
- *
- * This implementation will ignore all adds and removes of observers that
- * are done while a notification is processed. The changes take effect in
- * the following call to fireValueChangedEvent.
*/
protected void fireValueChangedEvent();
...
}
-------------
PR Comment: https://git.openjdk.org/jfx/pull/1081#issuecomment-3968231835
