Operator: WithLatestFrom

src/main/java/rx/Observable.java

@@ -8769,6 +8769,30 @@ public final Observable<T> unsubscribeOn(Scheduler scheduler) {
         return lift(new OperatorUnsubscribeOn<T>(scheduler));
     }
 
+    /**
+     * Merges the specified observable sequence into this Observable sequence by using the resultSelector 
+     * function only when the source observable sequence (this instance) produces an element.
+     * <code><pre>
+     * ----A-------B------C----->  o1
+     *
+     * --0----1-2----3-4-------->  o2
+     *
+     *     |       |      |
+     *     V       V      V
+     *
+     *   (A,0)   (B,2)  (C,4)
+     * </pre></code>
+     * @param other the other observable sequence
+     * @param resultSelector the function to call when this Observable emits an element and the other
+     *            observable sequence has already emitted a value.
+     * @return an Observable that merges the specified observable sequence into this Observable sequence 
+     *            by using the resultSelector function only when the source observable sequence 
+     *            (this instance) produces an element
+     */
+    public final <U, R> Observable<R> withLatestFrom(Observable<? extends U> other, Func2<? super T, ? super U, ? extends R> resultSelector) {
+        return lift(new OperatorWithLatestFrom<T, U, R>(other, resultSelector));
+    }
+
     /**
      * Returns an Observable that emits windows of items it collects from the source Observable. The resulting
      * Observable emits connected, non-overlapping windows. It emits the current window and opens a new one

src/main/java/rx/internal/operators/OperatorWithLatestFrom.java

@@ -0,0 +1,103 @@
+/**
+ * Copyright 2014 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.internal.operators;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import rx.*;
+import rx.Observable.Operator;
+import rx.functions.Func2;
+import rx.observers.SerializedSubscriber;
+
+/**
+ * Combines values from two sources only when the main source emits.
+ * @param <T> the element type of the main observable
+ * @param <U> the element type of the other observable that is merged into the main
+ * @param <R> the result element type
+ */
+public final class OperatorWithLatestFrom<T, U, R> implements Operator<R, T>  {
+    final Func2<? super T, ? super U, ? extends R> resultSelector;
+    final Observable<? extends U> other;
+    /** Indicates the other has not yet emitted a value. */
+    static final Object EMPTY = new Object();
+
+    public OperatorWithLatestFrom(Observable<? extends U> other, Func2<? super T, ? super U, ? extends R> resultSelector) {
+        this.other = other;
+        this.resultSelector = resultSelector;
+    }
+    @Override
+    public Subscriber<? super T> call(Subscriber<? super R> child) {
+        // onError and onCompleted may happen either from the main or from other.
+        final SerializedSubscriber<R> s = new SerializedSubscriber<R>(child, false);
+        child.add(s);
+
+        final AtomicReference<Object> current = new AtomicReference<Object>(EMPTY);
+
+        final Subscriber<T> subscriber = new Subscriber<T>(s, true) {
+            @Override
+            public void onNext(T t) {
+                Object o = current.get();
+                if (o != EMPTY) {
+                    try {
+                        @SuppressWarnings("unchecked")
+                        U u = (U)o;
+                        R result = resultSelector.call(t, u);
+
+                        s.onNext(result);
+                    } catch (Throwable e) {
+                        onError(e);
+                        return;
+                    }
+                }
+            }
+            @Override
+            public void onError(Throwable e) {
+                s.onError(e);
+                s.unsubscribe();
+            }
+            @Override
+            public void onCompleted() {
+                s.onCompleted();
+                s.unsubscribe();
+            }
+        };
+
+        Subscriber<U> otherSubscriber = new Subscriber<U>() {
+            @Override
+            public void onNext(U t) {
+                current.set(t);
+            }
+            @Override
+            public void onError(Throwable e) {
+                s.onError(e);
+                s.unsubscribe();
+            }
+            @Override
+            public void onCompleted() {
+                if (current.get() == EMPTY) {
+                    s.onCompleted();
+                    s.unsubscribe();
+                }
+            }
+        };
+        s.add(subscriber);
+        s.add(otherSubscriber);
+
+        other.unsafeSubscribe(otherSubscriber);
+
+        return subscriber;
+    }
+}

src/main/java/rx/observers/SerializedSubscriber.java

@@ -37,7 +37,17 @@ public class SerializedSubscriber<T> extends Subscriber<T> {
     private final Observer<T> s;
 
     public SerializedSubscriber(Subscriber<? super T> s) {
-        super(s);
+        this(s, true);
+    }
+    /**
+     * Constructor for wrapping and serializing a subscriber optionally sharing the same underlying subscription
+     * list.
+     * @param s the subscriber to wrap and serialize
+     * @param shareSubscriptions if {@code true}, the same subscription list is shared between this
+     * subscriber and {@code s}.
+     */
+    public SerializedSubscriber(Subscriber<? super T> s, boolean shareSubscriptions) {
+        super(s, shareSubscriptions);
         this.s = new SerializedObserver<T>(s);
     }