Add Observable.lazy() as a companion for Observable.defer()

src/main/java/rx/Observable.java

@@ -1069,6 +1069,29 @@ public final static <T> Observable<T> defer(Func0<Observable<T>> observableFacto
         return create(new OnSubscribeDefer<T>(observableFactory));
     }
 
+    /**
+     * Returns an Observable that invokes passed function and emits its result for each new Observer that subscribes.
+     * <p>
+     * Allows you to defer execution of passed function until Observer subscribes to the Observable.
+     * It makes passed function "lazy".
+     * Result of the function invocation will be emitted by the Observable.
+     * <dl>
+     *   <dt><b>Scheduler:</b></dt>
+     *   <dd>{@code deferJust} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @param func
+     *         function which execution should be deferred, it will be invoked when Observer will subscribe to the Observable
+     * @param <T>
+     *         the type of the item emitted by the Observable
+     * @return an Observable whose {@link Observer}s' subscriptions trigger an invocation of the given function
+     * @see #defer(Func0)
+     */
+    @Experimental
+    public static <T> Observable<T> deferJust(Callable<? extends T> func) {
+        return create(new OnSubscribeDeferJust<T>(func));
+    }
+
     /** Lazy initialized Holder for an empty observable which just emits onCompleted to any subscriber. */
     private static final class EmptyHolder {
         final static Observable<Object> INSTANCE = create(new OnSubscribe<Object>() {

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

@@ -0,0 +1,38 @@
+package rx.internal.operators;
+
+import rx.Observable;
+import rx.Subscriber;
+import rx.exceptions.Exceptions;
+import rx.internal.producers.SingleDelayedProducer;
+
+import java.util.concurrent.Callable;
+
+/**
+ * Do not invoke the function until an Observer subscribes; Invokes function on each
+ * subscription.
+ * <p>
+ * Pass {@code deferJust} a function, and {@code deferJust} will call this function to emit result of invocation
+ * afresh each time a new Observer subscribes.
+ */
+public final class OnSubscribeDeferJust<T> implements Observable.OnSubscribe<T> {
+
+    private final Callable<? extends T> resultFactory;
+
+    public OnSubscribeDeferJust(Callable<? extends T> resultFactory) {
+        this.resultFactory = resultFactory;
+    }
+
+    @Override
+    public void call(Subscriber<? super T> subscriber) {
+        final SingleDelayedProducer<T> singleDelayedProducer = new SingleDelayedProducer<T>(subscriber);
+
+        subscriber.setProducer(singleDelayedProducer);
+
+        try {
+            singleDelayedProducer.setValue(resultFactory.call());
+        } catch (Throwable t) {
+            Exceptions.throwIfFatal(t);
+            subscriber.onError(t);
+        }
+    }
+}

src/test/java/rx/internal/operators/OnSubscribeDeferJustTest.java

@@ -0,0 +1,140 @@
+package rx.internal.operators;
+
+import org.junit.Test;
+import org.mockito.invocation.InvocationOnMock;
+import org.mockito.stubbing.Answer;
+import rx.Observable;
+import rx.Observer;
+import rx.Subscription;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.CountDownLatch;
+
+import static org.mockito.Mockito.*;
+import static rx.schedulers.Schedulers.computation;
+
+public class OnSubscribeDeferJustTest {
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void shouldNotInvokeFuncUntilSubscription() throws Exception {
+        Callable<Object> func = mock(Callable.class);
+
+        when(func.call()).thenReturn(new Object());
+
+        Observable<Object> deferJustObservable = Observable.deferJust(func);
+
+        verifyZeroInteractions(func);
+
+        deferJustObservable.subscribe();
+
+        verify(func).call();
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void shouldCallOnNextAndOnCompleted() throws Exception {
+        Callable<String> func = mock(Callable.class);
+
+        when(func.call()).thenReturn("test_value");
+
+        Observable<String> deferJustObservable = Observable.deferJust(func);
+
+        Observer<String> observer = mock(Observer.class);
+
+        deferJustObservable.subscribe(observer);
+
+        verify(observer).onNext("test_value");
+        verify(observer).onCompleted();
+        verify(observer, never()).onError(any(Throwable.class));
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void shouldCallOnError() throws Exception {
+        Callable<Object> func = mock(Callable.class);
+
+        Throwable throwable = new IllegalStateException("Test exception");
+        when(func.call()).thenThrow(throwable);
+
+        Observable<Object> deferJustObservable = Observable.deferJust(func);
+
+        Observer<Object> observer = mock(Observer.class);
+
+        deferJustObservable.subscribe(observer);
+
+        verify(observer, never()).onNext(anyObject());
+        verify(observer, never()).onCompleted();
+        verify(observer).onError(throwable);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void shouldNotDeliverResultIfSubscriberUnsubscribedBeforeEmission() throws Exception {
+        Callable<String> func = mock(Callable.class);
+
+        final CountDownLatch funcLatch = new CountDownLatch(1);
+        final CountDownLatch observerLatch = new CountDownLatch(1);
+
+        when(func.call()).thenAnswer(new Answer<String>() {
+            @Override
+            public String answer(InvocationOnMock invocation) throws Throwable {
+                observerLatch.countDown();
+
+                try {
+                    funcLatch.await();
+                } catch (InterruptedException e) {
+                    // It's okay, unsubscription causes Thread interruption
+
+                    // Restoring interruption status of the Thread
+                    Thread.currentThread().interrupt();
+                }
+
+                return "should_not_be_delivered";
+            }
+        });
+
+        Observable<String> deferJustObservable = Observable.deferJust(func);
+
+        Observer<String> observer = mock(Observer.class);
+
+        Subscription subscription = deferJustObservable
+                .subscribeOn(computation())
+                .subscribe(observer);
+
+        // Wait until func will be invoked
+        observerLatch.await();
+
+        // Unsubscribing before emission
+        subscription.unsubscribe();
+
+        // Emitting result
+        funcLatch.countDown();
+
+        // func must be invoked
+        verify(func).call();
+
+        // Observer must not be notified at all
+        verifyZeroInteractions(observer);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void shouldAllowToThrowCheckedException() {
+        final Exception checkedException = new Exception("test exception");
+
+        Observable<Object> deferJustObservable = Observable.deferJust(new Callable<Object>() {
+            @Override
+            public Object call() throws Exception {
+                throw checkedException;
+            }
+        });
+
+        Observer<Object> observer = mock(Observer.class);
+
+        deferJustObservable.subscribe(observer);
+
+        verify(observer).onError(checkedException);
+        verifyNoMoreInteractions(observer);
+    }
+}