implement LazyBindingSubscriber

Author: thomasnield

src/main/java/rx/subscribers/JavaFxSubscriber.java

@@ -23,19 +23,34 @@
 public enum JavaFxSubscriber {
     ;//no instances
     /**
-     * Turns an Observable into a JavaFX Binding. Calling the Binding's dispose() method will handle the unsubscription. 
+     * Turns an Observable into an eager JavaFX Binding that subscribes immediately to the Observable. Calling the Binding's dispose() method will handle the unsubscription.
      */
     public static <T> Binding<T> toBinding(Observable<T> obs) {
         BindingSubscriber<T> bindingSubscriber = new BindingSubscriber<>(e -> {});
         obs.subscribe(bindingSubscriber);
         return bindingSubscriber;
     }
     /**
-     * Turns an Observable into a JavaFX Binding. Calling the Binding's dispose() method will handle the unsubscription. 
+     * Turns an Observable into an eager JavaFX Binding that subscribes immediately to the Observable. Calling the Binding's dispose() method will handle the unsubscription.
      */
     public static <T> Binding<T> toBinding(Observable<T> obs, Action1<Throwable> onErrorAction ) {
         BindingSubscriber<T> bindingSubscriber = new BindingSubscriber<>(onErrorAction);
         obs.subscribe(bindingSubscriber);
         return bindingSubscriber;
     }
+
+    /**
+     * Turns an Observable into a lazy JavaFX Binding that subscribes to the Observable the first time the Binding's getValue() method is called. Calling the Binding's dispose() method will handle the unsubscription.
+     */
+    public static <T> Binding<T> toLazyBinding(Observable<T> obs) {
+        return toLazyBinding(obs, e -> {});
+    }
+
+    /**
+     * Turns an Observable into a lazy JavaFX Binding that subscribes to the Observable the first time the Binding's getValue() method is called. Calling the Binding's dispose() method will handle the unsubscription.
+     */
+    public static <T> Binding<T> toLazyBinding(Observable<T> obs, Action1<Throwable> onErrorAction ) {
+        BindingSubscriber<T> bindingSubscriber = new BindingSubscriber<>(onErrorAction);
+        return new LazyBindingSubscriber<>(obs, bindingSubscriber);
+    }
 }

src/main/java/rx/subscribers/LazyBindingSubscriber.java

@@ -0,0 +1,111 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package rx.subscribers;
+
+import com.sun.javafx.binding.ExpressionHelper;
+import javafx.beans.InvalidationListener;
+import javafx.beans.binding.Binding;
+import javafx.beans.value.ChangeListener;
+import javafx.beans.value.ObservableValue;
+import javafx.collections.ObservableList;
+import rx.Observable;
+import rx.Subscription;
+
+
+final class LazyBindingSubscriber<T> implements ObservableValue<T>, Binding<T> {
+
+    private final Observable<T> observable;
+    private final BindingSubscriber<T> binding;
+    private ExpressionHelper<T> helper;
+    private Subscription subscription;
+
+    LazyBindingSubscriber(Observable<T> observable, BindingSubscriber<T> binding) {
+        this.observable = observable;
+        this.binding = binding;
+    }
+    @Override
+    public T getValue() {
+        if (subscription == null) {
+            subscription = observable.subscribe(binding);
+        }
+        return binding.getValue();
+    }
+    @Override
+    public boolean isValid() {
+        return true;
+    }
+
+    @Override
+    public void invalidate() {
+        //does nothing
+    }
+
+    @Override
+    public ObservableList<?> getDependencies() {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void dispose() {
+        if (subscription != null) {
+            binding.dispose();
+        }
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void addListener(InvalidationListener listener) {
+        helper = ExpressionHelper.addListener(helper, this, listener);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void addListener(ChangeListener<? super T> listener) {
+        helper = ExpressionHelper.addListener(helper, this, listener);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void removeListener(InvalidationListener listener) {
+        helper = ExpressionHelper.removeListener(helper, listener);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void removeListener(ChangeListener<? super T> listener) {
+        helper = ExpressionHelper.removeListener(helper, listener);
+    }
+
+    /**
+     * 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() {
+        ExpressionHelper.fireValueChangedEvent(helper);
+    }
+}