add Observable.switchMapSingle (#5161)

src/main/java/io/reactivex/Observable.java

@@ -10865,6 +10865,67 @@ public final <R> Observable<R> switchMap(Function<? super T, ? extends Observabl
         return RxJavaPlugins.onAssembly(new ObservableSwitchMap<T, R>(this, mapper, bufferSize, false));
     }
 
+    /**
+     * Returns a new ObservableSource by applying a function that you supply to each item emitted by the source
+     * ObservableSource that returns a SingleSource, and then emitting the item emitted by the most recently emitted
+     * of these SingleSources.
+     * <p>
+     * The resulting ObservableSource completes if both the upstream ObservableSource and the last inner SingleSource, if any, complete.
+     * If the upstream ObservableSource signals an onError, the inner SingleSource is unsubscribed and the error delivered in-sequence.
+     * <p>
+     * <img width="640" height="350" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/switchMap.png" alt="">
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code switchMapSingle} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @param <R> the element type of the inner SingleSources and the output
+     * @param mapper
+     *            a function that, when applied to an item emitted by the source ObservableSource, returns a
+     *            SingleSource
+     * @return an Observable that emits the item emitted by the SingleSource returned from applying {@code func} to the most recently emitted item emitted by the source ObservableSource
+     * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
+     * @since 2.0.8
+     */
+    @Experimental
+    @CheckReturnValue
+    @SchedulerSupport(SchedulerSupport.NONE)
+    @NonNull
+    public final <R> Observable<R> switchMapSingle(@NonNull Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        return ObservableInternalHelper.switchMapSingle(this, mapper);
+    }
+
+    /**
+     * Returns a new ObservableSource by applying a function that you supply to each item emitted by the source
+     * ObservableSource that returns a SingleSource, and then emitting the item emitted by the most recently emitted
+     * of these SingleSources and delays any error until all SingleSources terminate.
+     * <p>
+     * The resulting ObservableSource completes if both the upstream ObservableSource and the last inner SingleSource, if any, complete.
+     * If the upstream ObservableSource signals an onError, the termination of the last inner SingleSource will emit that error as is
+     * or wrapped into a CompositeException along with the other possible errors the former inner SingleSources signalled.
+     * <p>
+     * <img width="640" height="350" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/switchMap.png" alt="">
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code switchMapSingleDelayError} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @param <R> the element type of the inner SingleSources and the output
+     * @param mapper
+     *            a function that, when applied to an item emitted by the source ObservableSource, returns a
+     *            SingleSource
+     * @return an Observable that emits the item emitted by the SingleSource returned from applying {@code func} to the most recently emitted item emitted by the source ObservableSource
+     * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
+     * @since 2.0.8
+     */
+    @Experimental
+    @CheckReturnValue
+    @SchedulerSupport(SchedulerSupport.NONE)
+    @NonNull
+    public final <R> Observable<R> switchMapSingleDelayError(@NonNull Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        return ObservableInternalHelper.switchMapSingleDelayError(this, mapper);
+    }
+
     /**
      * Returns a new ObservableSource by applying a function that you supply to each item emitted by the source
      * ObservableSource that returns an ObservableSource, and then emitting the items emitted by the most recently emitted

src/main/java/io/reactivex/internal/operators/observable/ObservableInternalHelper.java

@@ -18,7 +18,10 @@
 import io.reactivex.*;
 import io.reactivex.functions.*;
 import io.reactivex.internal.functions.Functions;
+import io.reactivex.internal.functions.ObjectHelper;
+import io.reactivex.internal.operators.single.SingleToObservable;
 import io.reactivex.observables.ConnectableObservable;
+import io.reactivex.plugins.RxJavaPlugins;
 
 /**
  * Helper utility class to support Observable with inner classes.
@@ -315,4 +318,35 @@ public static <T, R> Function<List<ObservableSource<? extends T>>, ObservableSou
         return new ZipIterableFunction<T, R>(zipper);
     }
 
+    public static <T,R> Observable<R> switchMapSingle(Observable<T> source, final Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        return source.switchMap(convertSingleMapperToObservableMapper(mapper), 1);
+    }
+
+    public static <T,R> Observable<R> switchMapSingleDelayError(Observable<T> source,
+            Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        return source.switchMapDelayError(convertSingleMapperToObservableMapper(mapper), 1);
+    }
+
+    private static <T, R> Function<T, Observable<R>> convertSingleMapperToObservableMapper(
+            final Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        ObjectHelper.requireNonNull(mapper, "mapper is null");
+        return new ObservableMapper<T,R>(mapper);
+    }
+
+    static final class ObservableMapper<T,R> implements Function<T,Observable<R>> {
+
+        final Function<? super T, ? extends SingleSource<? extends R>> mapper;
+
+        ObservableMapper(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+            this.mapper = mapper;
+        }
+
+        @Override
+        public Observable<R> apply(T t) throws Exception {
+            return RxJavaPlugins.onAssembly(new SingleToObservable<R>(
+                ObjectHelper.requireNonNull(mapper.apply(t), "The mapper returned a null value")));
+        }
+
+    }
+
 }

src/test/java/io/reactivex/internal/operators/observable/ObservableSwitchTest.java

@@ -26,6 +26,7 @@
 import io.reactivex.*;
 import io.reactivex.disposables.*;
 import io.reactivex.exceptions.*;
+import io.reactivex.functions.Consumer;
 import io.reactivex.functions.Function;
 import io.reactivex.internal.functions.Functions;
 import io.reactivex.internal.util.ExceptionHelper;
@@ -579,7 +580,6 @@ public ObservableSource<Integer> apply(Object v) throws Exception {
         }, 16)
         .test()
         .assertResult(1);
-
     }
 
     @Test
@@ -622,7 +622,83 @@ public void switchMapInnerCancelled() {
 
         assertFalse(pp.hasObservers());
     }
+
+    @Test
+    public void switchMapSingleJustSource() {
+        Observable.just(0)
+        .switchMapSingle(new Function<Object, SingleSource<Integer>>() {
+            @Override
+            public SingleSource<Integer> apply(Object v) throws Exception {
+                return Single.just(1);
+            }
+        })
+        .test()
+        .assertResult(1);
+    }
+
+    @Test
+    public void switchMapSingleMapperReturnsNull() {
+        Observable.just(0)
+        .switchMapSingle(new Function<Object, SingleSource<Integer>>() {
+            @Override
+            public SingleSource<Integer> apply(Object v) throws Exception {
+                return null;
+            }
+        })
+        .test()
+        .assertError(NullPointerException.class);
+    }
+
+    @Test(expected=NullPointerException.class)
+    public void switchMapSingleMapperIsNull() {
+        Observable.just(0)
+        .switchMapSingle(null);
+    }
+
+    @Test
+    public void switchMapSingleFunctionDoesntReturnSingle() {
+        Observable.just(0)
+        .switchMapSingle(new Function<Object, SingleSource<Integer>>() {
+            @Override
+            public SingleSource<Integer> apply(Object v) throws Exception {
+                return new SingleSource<Integer>() {
+                    @Override
+                    public void subscribe(SingleObserver<? super Integer> s) {
+                        s.onSubscribe(Disposables.empty());
+                        s.onSuccess(1);
+                    }
+                };
+            }
+        })
+        .test()
+        .assertResult(1);
+    }
 
+    @Test
+    public void switchMapSingleDelayErrorJustSource() {
+        final AtomicBoolean completed = new AtomicBoolean();
+        Observable.just(0, 1)
+        .switchMapSingleDelayError(new Function<Integer, SingleSource<Integer>>() {
+            @Override
+            public SingleSource<Integer> apply(Integer v) throws Exception {
+                if (v == 0) {
+                    return Single.error(new RuntimeException());
+                } else {
+                    return Single.just(1).doOnSuccess(new Consumer<Integer>() {
+
+                        @Override
+                        public void accept(Integer n) throws Exception {
+                            completed.set(true);
+                        }});
+                }
+            }
+        })
+        .test()
+        .assertValue(1)
+        .assertError(RuntimeException.class);
+        assertTrue(completed.get());
+    }
+
     @Test
     public void scalarMap() {
         Observable.switchOnNext(Observable.just(Observable.just(1)))