8290040: Provide simplified deterministic way to manage listeners

modules/javafx.base/src/main/java/com/sun/javafx/binding/ConditionalBinding.java

@@ -0,0 +1,67 @@
+package com.sun.javafx.binding;
+
+import java.util.Objects;
+
+import javafx.beans.value.ObservableValue;
+
+public class ConditionalBinding<T> extends LazyObjectBinding<T> {
+
+    private final ObservableValue<T> source;
+    private final ObservableValue<Boolean> nonNullCondition;
+
+    private Subscription subscription;
+
+    public ConditionalBinding(ObservableValue<T> source, ObservableValue<Boolean> condition) {
+        this.source = Objects.requireNonNull(source, "source cannot be null");
+        this.nonNullCondition = Objects.requireNonNull(condition, "condition cannot be null").orElse(false);
+
+        // condition is always observed and never unsubscribed
+        Subscription.subscribe(nonNullCondition, current -> {
+            invalidate();
+
+            if (!current) {
+                getValue();
+            }
+        });
+    }
+
+    /**
+     * This binding is valid whenever it is observed, or it is currently inactive.
+     * When inactive, the binding has the value of its source at the time it became
+     * inactive.
+     */
+    @Override
+    protected boolean allowValidation() {
+        return super.allowValidation() || !isActive();
+    }
+
+    @Override
+    protected T computeValue() {
+        if (isObserved() && isActive()) {
+            if (subscription == null) {
+                subscription = Subscription.subscribeInvalidations(source, this::invalidate);
+            }
+        }
+        else {
+            unsubscribe();
+        }
+
+        return source.getValue();
+    }
+
+    @Override
+    protected Subscription observeSources() {
+        return this::unsubscribe;
+    }
+
+    private boolean isActive() {
+        return nonNullCondition.getValue();
+    }
+
+    private void unsubscribe() {
+        if (subscription != null) {
+            subscription.unsubscribe();
+            subscription = null;
+        }
+    }
+}

modules/javafx.base/src/main/java/javafx/beans/value/ObservableValue.java

@@ -27,6 +27,7 @@
 
 import java.util.function.Function;
 
+import com.sun.javafx.binding.ConditionalBinding;
 import com.sun.javafx.binding.FlatMappedBinding;
 import com.sun.javafx.binding.MappedBinding;
 import com.sun.javafx.binding.OrElseBinding;
@@ -251,4 +252,55 @@ default ObservableValue<T> orElse(T constant) {
     default <U> ObservableValue<U> flatMap(Function<? super T, ? extends ObservableValue<? extends U>> mapper) {
         return new FlatMappedBinding<>(this, mapper);
     }
+
+    /**
+     * Returns an {@code ObservableValue} that holds this value and is updated only
+     * when {@code condition} holds {@code true}.
+     * <p>
+     * The returned {@code ObservableValue} only observes this value when
+     * {@code condition} holds {@code true}. This allows this {@code ObservableValue}
+     * and the conditional {@code ObservableValue} to be garbage collected if neither is
+     * otherwise strongly referenced when {@code condition} holds {@code false}.
+     * This is in contrast to the general behavior of bindings, where the binding is
+     * only eligible for garbage collection when not observed itself.
+     * <p>
+     * A {@code condition} holding {@code null} is treated as holding {@code false}.
+     * <p>
+     * For example:
+     * <pre>{@code
+     * ObservableValue<Boolean> condition = new SimpleBooleanProperty(true);
+     * ObservableValue<String> longLivedProperty = new SimpleStringProperty("A");
+     * ObservableValue<String> whenProperty = longLivedProperty.when(condition);
+     *
+     * // observe whenProperty, which will in turn observe longLivedProperty
+     * whenProperty.addListener((ov, old, current) -> System.out.println(current));
+     *
+     * longLivedProperty.setValue("B");  // "B" is printed
+     *
+     * condition.setValue(false);
+     *
+     * // After condition becomes false, whenProperty stops observing longLivedProperty; condition
+     * // and whenProperty may now be eligible for GC despite being observed by the ChangeListener
+     *
+     * longLivedProperty.setValue("C");  // nothing is printed
+     * longLivedProperty.setValue("D");  // nothing is printed
+     *
+     * condition.setValue(true);  // longLivedProperty is observed again, and "D" is printed
+     * }</pre>
+     * An example for binding a label's text to a long-lived property only when it is shown:
+     * <pre>{@code
+     * Label label = ... ;
+     * ObservableValue<String> longLivedProperty = new SimpleStringProperty("A");
+     * label.textProperty().bind(longLivedProperty.when(label::isShownProperty));
+     * }</pre>
+     *
+     * @param condition a boolean {@code ObservableValue}, cannot be {@code null}
+     * @return an {@code ObservableValue} that holds this value whenever the given
+     *     condition evaluates to {@code true}, otherwise holds the last seen value;
+     *     never returns {@code null}
+     * @since 20
+     */
+    default ObservableValue<T> when(ObservableValue<Boolean> condition) {
+        return new ConditionalBinding<>(this, condition);
+    }
 }