groupBy with element selector

rxjava-core/src/main/java/rx/Observable.java

@@ -4931,6 +4931,44 @@ public final void forEach(final Action1<? super T> onNext, final Action1<Throwab
         subscribe(onNext, onError, onComplete);
     }
 
+    /**
+     * Groups the items emitted by an {@code Observable} according to a specified criterion, and emits these
+     * grouped items as {@link GroupedObservable}s, one {@code GroupedObservable} per group.
+     * <p>
+     * <img width="640" height="360" src="https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/groupBy.png" alt="">
+     * <p>
+     * <em>Note:</em> A {@link GroupedObservable} will cache the items it is to emit until such time as it
+     * is subscribed to. For this reason, in order to avoid memory leaks, you should not simply ignore those
+     * {@code GroupedObservable}s that do not concern you. Instead, you can signal to them that they may
+     * discard their buffers by applying an operator like {@link #take}{@code (0)} to them.
+     * <dl>
+     *  <dt><b>Backpressure Support:</b></dt>
+     *  <dd>This operator does not support backpressure as splitting a stream effectively turns it into a "hot
+     *      observable" and blocking any one group would block the entire parent stream. If you need
+     *      backpressure on individual groups then you should use operators such as {@link #onBackpressureDrop}
+     *      or {@link #onBackpressureBuffer}.</dd>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code groupBy} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     * 
+     * @param keySelector
+     *            a function that extracts the key for each item
+     * @param elementSelector
+     *            a function that extracts the return element for each item
+     * @param <K>
+     *            the key type
+     * @param <R>
+     *            the element type
+     * @return an {@code Observable} that emits {@link GroupedObservable}s, each of which corresponds to a
+     *         unique key value and each of which emits those items from the source Observable that share that
+     *         key value
+     * @see <a href="https://github.com/Netflix/RxJava/wiki/Transforming-Observables#groupby-and-groupbyuntil">RxJava wiki: groupBy</a>
+     * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.groupby.aspx">MSDN: Observable.GroupBy</a>
+     */
+    public final <K, R> Observable<GroupedObservable<K, R>> groupBy(final Func1<? super T, ? extends K> keySelector, final Func1<? super T, ? extends R> elementSelector) {
+        return lift(new OperatorGroupBy<T, K, R>(keySelector, elementSelector));
+    }
+
     /**
      * Groups the items emitted by an {@code Observable} according to a specified criterion, and emits these
      * grouped items as {@link GroupedObservable}s, one {@code GroupedObservable} per group.
@@ -4962,7 +5000,7 @@ public final void forEach(final Action1<? super T> onNext, final Action1<Throwab
      * @see <a href="http://msdn.microsoft.com/en-us/library/system.reactive.linq.observable.groupby.aspx">MSDN: Observable.GroupBy</a>
      */
     public final <K> Observable<GroupedObservable<K, T>> groupBy(final Func1<? super T, ? extends K> keySelector) {
-        return lift(new OperatorGroupBy<K, T>(keySelector));
+        return lift(new OperatorGroupBy<T, K, T>(keySelector));
     }
 
     /**

rxjava-core/src/main/java/rx/internal/operators/OperatorGroupBy.java

@@ -37,26 +37,35 @@
  * @param <K> the key type
  * @param <T> the source and group value type
  */
-public final class OperatorGroupBy<K, T> implements Operator<GroupedObservable<K, T>, T> {
+public final class OperatorGroupBy<T, K, R> implements Operator<GroupedObservable<K, R>, T> {
 
     final Func1<? super T, ? extends K> keySelector;
+    final Func1<? super T, ? extends R> elementSelector;
 
+    @SuppressWarnings("unchecked")
     public OperatorGroupBy(final Func1<? super T, ? extends K> keySelector) {
+        this(keySelector, (Func1<T, R>)IDENTITY);
+    }
+
+    public OperatorGroupBy(Func1<? super T, ? extends K> keySelector, Func1<? super T, ? extends R> elementSelector) {
         this.keySelector = keySelector;
+        this.elementSelector = elementSelector;
     }
 
     @Override
-    public Subscriber<? super T> call(final Subscriber<? super GroupedObservable<K, T>> child) {
-        return new GroupBySubscriber<K, T>(keySelector, child);
+    public Subscriber<? super T> call(final Subscriber<? super GroupedObservable<K, R>> child) {
+        return new GroupBySubscriber<K, T, R>(keySelector, elementSelector, child);
     }
-    static final class GroupBySubscriber<K, T> extends Subscriber<T> {
+    static final class GroupBySubscriber<K, T, R> extends Subscriber<T> {
         final Func1<? super T, ? extends K> keySelector;
-        final Subscriber<? super GroupedObservable<K, T>> child;
-        public GroupBySubscriber(Func1<? super T, ? extends K> keySelector, Subscriber<? super GroupedObservable<K, T>> child) {
+        final Func1<? super T, ? extends R> elementSelector;
+        final Subscriber<? super GroupedObservable<K, R>> child;
+        public GroupBySubscriber(Func1<? super T, ? extends K> keySelector, Func1<? super T, ? extends R> elementSelector, Subscriber<? super GroupedObservable<K, R>> child) {
             // a new CompositeSubscription to decouple the subscription as the inner subscriptions need a separate lifecycle
             // and will unsubscribe on this parent if they are all unsubscribed
             super();
             this.keySelector = keySelector;
+            this.elementSelector = elementSelector;
             this.child = child;
         }
         private final Map<K, BufferUntilSubscriber<T>> groups = new HashMap<K, BufferUntilSubscriber<T>>();
@@ -124,10 +133,10 @@ public void onNext(T t) {
                     group = BufferUntilSubscriber.create();
                     final BufferUntilSubscriber<T> _group = group;
 
-                    GroupedObservable<K, T> go = new GroupedObservable<K, T>(key, new OnSubscribe<T>() {
+                    GroupedObservable<K, R> go = new GroupedObservable<K, R>(key, new OnSubscribe<R>() {
 
                         @Override
-                        public void call(final Subscriber<? super T> o) {
+                        public void call(final Subscriber<? super R> o) {
                             // number of children we have running
                             COUNTER_UPDATER.incrementAndGet(GroupBySubscriber.this);
                             o.add(Subscriptions.create(new Action0() {
@@ -153,7 +162,7 @@ public void onError(Throwable e) {
 
                                 @Override
                                 public void onNext(T t) {
-                                    o.onNext(t);
+                                    o.onNext(elementSelector.call(t));
                                 }
 
                             });
@@ -185,4 +194,13 @@ private void completeInner() {
         }
 
     }
+
+    private final static Func1<Object, Object> IDENTITY = new Func1<Object, Object>() {
+
+        @Override
+        public Object call(Object t) {
+            return t;
+        }
+
+    };
 }

rxjava-core/src/test/java/rx/internal/operators/OperatorGroupByTest.java

@@ -59,11 +59,11 @@ public Integer call(String s) {
             return s.length();
         }
     };
-
+    
     @Test
     public void testGroupBy() {
         Observable<String> source = Observable.from("one", "two", "three", "four", "five", "six");
-        Observable<GroupedObservable<Integer, String>> grouped = source.lift(new OperatorGroupBy<Integer, String>(length));
+        Observable<GroupedObservable<Integer, String>> grouped = source.lift(new OperatorGroupBy<String, Integer, String>(length));
 
         Map<Integer, Collection<String>> map = toMap(grouped);
 
@@ -72,11 +72,37 @@ public void testGroupBy() {
         assertArrayEquals(Arrays.asList("four", "five").toArray(), map.get(4).toArray());
         assertArrayEquals(Arrays.asList("three").toArray(), map.get(5).toArray());
     }
+
+    @Test
+    public void testGroupByWithElementSelector() {
+        Observable<String> source = Observable.from("one", "two", "three", "four", "five", "six");
+        Observable<GroupedObservable<Integer, Integer>> grouped = source.lift(new OperatorGroupBy<String, Integer, Integer>(length, length));
+
+        Map<Integer, Collection<Integer>> map = toMap(grouped);
+
+        assertEquals(3, map.size());
+        assertArrayEquals(Arrays.asList(3, 3, 3).toArray(), map.get(3).toArray());
+        assertArrayEquals(Arrays.asList(4, 4).toArray(), map.get(4).toArray());
+        assertArrayEquals(Arrays.asList(5).toArray(), map.get(5).toArray());
+    }
+
+    @Test
+    public void testGroupByWithElementSelector2() {
+        Observable<String> source = Observable.from("one", "two", "three", "four", "five", "six");
+        Observable<GroupedObservable<Integer, Integer>> grouped = source.groupBy(length, length);
+
+        Map<Integer, Collection<Integer>> map = toMap(grouped);
+
+        assertEquals(3, map.size());
+        assertArrayEquals(Arrays.asList(3, 3, 3).toArray(), map.get(3).toArray());
+        assertArrayEquals(Arrays.asList(4, 4).toArray(), map.get(4).toArray());
+        assertArrayEquals(Arrays.asList(5).toArray(), map.get(5).toArray());
+    }
 
     @Test
     public void testEmpty() {
         Observable<String> source = Observable.empty();
-        Observable<GroupedObservable<Integer, String>> grouped = source.lift(new OperatorGroupBy<Integer, String>(length));
+        Observable<GroupedObservable<Integer, String>> grouped = source.lift(new OperatorGroupBy<String, Integer, String>(length));
 
         Map<Integer, Collection<String>> map = toMap(grouped);
 
@@ -89,7 +115,7 @@ public void testError() {
         Observable<String> errorSource = Observable.error(new RuntimeException("forced failure"));
         Observable<String> source = Observable.concat(sourceStrings, errorSource);
 
-        Observable<GroupedObservable<Integer, String>> grouped = source.lift(new OperatorGroupBy<Integer, String>(length));
+        Observable<GroupedObservable<Integer, String>> grouped = source.lift(new OperatorGroupBy<String, Integer, String>(length));
 
         final AtomicInteger groupCounter = new AtomicInteger();
         final AtomicInteger eventCounter = new AtomicInteger();