3.x: Add X.fromSupplier()

src/main/java/io/reactivex/Completable.java

@@ -444,6 +444,8 @@ public static Completable fromAction(final Action run) {
      * </dl>
      * @param callable the callable instance to execute for each subscriber
      * @return the new Completable instance
+     * @see #defer(Supplier)
+     * @see #fromSupplier(Supplier)
      */
     @CheckReturnValue
     @NonNull
@@ -609,6 +611,36 @@ public static <T> Completable fromSingle(final SingleSource<T> single) {
         return RxJavaPlugins.onAssembly(new CompletableFromSingle<T>(single));
     }
 
+    /**
+     * Returns a Completable which when subscribed, executes the supplier function, ignores its
+     * normal result and emits onError or onComplete only.
+     * <p>
+     * <img width="640" height="286" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.fromCallable.png" alt="">
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code fromSupplier} does not operate by default on a particular {@link Scheduler}.</dd>
+     *  <dt><b>Error handling:</b></dt>
+     *  <dd> If the {@link Supplier} throws an exception, the respective {@link Throwable} is
+     *  delivered to the downstream via {@link CompletableObserver#onError(Throwable)},
+     *  except when the downstream has disposed this {@code Completable} source.
+     *  In this latter case, the {@code Throwable} is delivered to the global error handler via
+     *  {@link RxJavaPlugins#onError(Throwable)} as an {@link io.reactivex.exceptions.UndeliverableException UndeliverableException}.
+     *  </dd>
+     * </dl>
+     * @param supplier the Supplier instance to execute for each subscriber
+     * @return the new Completable instance
+     * @see #defer(Supplier)
+     * @see #fromCallable(Callable)
+     * @since 3.0.0
+     */
+    @CheckReturnValue
+    @NonNull
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public static Completable fromSupplier(final Supplier<?> supplier) {
+        ObjectHelper.requireNonNull(supplier, "supplier is null");
+        return RxJavaPlugins.onAssembly(new CompletableFromSupplier(supplier));
+    }
+
     /**
      * Returns a Completable instance that subscribes to all sources at once and
      * completes only when all source Completables complete or one of them emits an error.

src/main/java/io/reactivex/Flowable.java

@@ -2087,6 +2087,7 @@ public static <T> Flowable<T> fromArray(T... items) {
      *         the type of the item emitted by the Publisher
      * @return a Flowable whose {@link Subscriber}s' subscriptions trigger an invocation of the given function
      * @see #defer(Supplier)
+     * @see #fromSupplier(Supplier)
      * @since 2.0
      */
     @CheckReturnValue
@@ -2331,6 +2332,47 @@ public static <T> Flowable<T> fromPublisher(final Publisher<? extends T> source)
         return RxJavaPlugins.onAssembly(new FlowableFromPublisher<T>(source));
     }
 
+    /**
+     * Returns a Flowable that, when a Subscriber subscribes to it, invokes a supplier function you specify and then
+     * emits the value returned from that function.
+     * <p>
+     * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/fromCallable.png" alt="">
+     * <p>
+     * This allows you to defer the execution of the function you specify until a Subscriber subscribes to the
+     * Publisher. That is to say, it makes the function "lazy."
+     * <dl>
+     *   <dt><b>Backpressure:</b></dt>
+     *   <dd>The operator honors backpressure from downstream.</dd>
+     *   <dt><b>Scheduler:</b></dt>
+     *   <dd>{@code fromSupplier} does not operate by default on a particular {@link Scheduler}.</dd>
+     *   <dt><b>Error handling:</b></dt>
+     *   <dd> If the {@link Supplier} throws an exception, the respective {@link Throwable} is
+     *   delivered to the downstream via {@link Subscriber#onError(Throwable)},
+     *   except when the downstream has canceled this {@code Flowable} source.
+     *   In this latter case, the {@code Throwable} is delivered to the global error handler via
+     *   {@link RxJavaPlugins#onError(Throwable)} as an {@link io.reactivex.exceptions.UndeliverableException UndeliverableException}.
+     *   </dd>
+     * </dl>
+     *
+     * @param supplier
+     *         a function, the execution of which should be deferred; {@code fromSupplier} will invoke this
+     *         function only when a Subscriber subscribes to the Publisher that {@code fromSupplier} returns
+     * @param <T>
+     *         the type of the item emitted by the Publisher
+     * @return a Flowable whose {@link Subscriber}s' subscriptions trigger an invocation of the given function
+     * @see #defer(Supplier)
+     * @see #fromCallable(Callable)
+     * @since 3.0.0
+     */
+    @CheckReturnValue
+    @NonNull
+    @BackpressureSupport(BackpressureKind.FULL)
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public static <T> Flowable<T> fromSupplier(Supplier<? extends T> supplier) {
+        ObjectHelper.requireNonNull(supplier, "supplier is null");
+        return RxJavaPlugins.onAssembly(new FlowableFromSupplier<T>(supplier));
+    }
+
     /**
      * Returns a cold, synchronous, stateless and backpressure-aware generator of values.
      * <p>

src/main/java/io/reactivex/Maybe.java

@@ -763,6 +763,8 @@ public static <T> Maybe<T> fromSingle(SingleSource<T> singleSource) {
      * @param <T>
      *         the type of the item emitted by the {@link Maybe}.
      * @return a new Maybe instance
+     * @see #defer(Supplier)
+     * @see #fromSupplier(Supplier)
      */
     @CheckReturnValue
     @NonNull
@@ -865,6 +867,51 @@ public static <T> Maybe<T> fromRunnable(final Runnable run) {
         return RxJavaPlugins.onAssembly(new MaybeFromRunnable<T>(run));
     }
 
+    /**
+     * Returns a {@link Maybe} that invokes the given {@link Supplier} for each individual {@link MaybeObserver} that
+     * subscribes and emits the resulting non-null item via {@code onSuccess} while
+     * considering a {@code null} result from the {@code Supplier} as indication for valueless completion
+     * via {@code onComplete}.
+     * <p>
+     * This operator allows you to defer the execution of the given {@code Supplier} until a {@code MaybeObserver}
+     * subscribes to the  returned {@link Maybe}. In other terms, this source operator evaluates the given
+     * {@code Supplier} "lazily".
+     * <p>
+     * Note that the {@code null} handling of this operator differs from the similar source operators in the other
+     * {@link io.reactivex base reactive classes}. Those operators signal a {@code NullPointerException} if the value returned by their
+     * {@code Supplier} is {@code null} while this {@code fromSupplier} considers it to indicate the
+     * returned {@code Maybe} is empty.
+     * <dl>
+     *   <dt><b>Scheduler:</b></dt>
+     *   <dd>{@code fromSupplier} does not operate by default on a particular {@link Scheduler}.</dd>
+     *   <dt><b>Error handling:</b></dt>
+     *   <dd>Any non-fatal exception thrown by {@link Supplier#get()} will be forwarded to {@code onError},
+     *   except if the {@code MaybeObserver} disposed the subscription in the meantime. In this latter case,
+     *   the exception is forwarded to the global error handler via
+     *   {@link io.reactivex.plugins.RxJavaPlugins#onError(Throwable)} wrapped into a
+     *   {@link io.reactivex.exceptions.UndeliverableException UndeliverableException}.
+     *   Fatal exceptions are rethrown and usually will end up in the executing thread's
+     *   {@link java.lang.Thread.UncaughtExceptionHandler#uncaughtException(Thread, Throwable)} handler.</dd>
+     * </dl>
+     *
+     * @param supplier
+     *         a {@link Supplier} instance whose execution should be deferred and performed for each individual
+     *         {@code MaybeObserver} that subscribes to the returned {@link Maybe}.
+     * @param <T>
+     *         the type of the item emitted by the {@link Maybe}.
+     * @return a new Maybe instance
+     * @see #defer(Supplier)
+     * @see #fromCallable(Callable)
+     * @since 3.0.0
+     */
+    @CheckReturnValue
+    @NonNull
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public static <T> Maybe<T> fromSupplier(@NonNull final Supplier<? extends T> supplier) {
+        ObjectHelper.requireNonNull(supplier, "supplier is null");
+        return RxJavaPlugins.onAssembly(new MaybeFromSupplier<T>(supplier));
+    }
+
     /**
      * Returns a {@code Maybe} that emits a specified item.
      * <p>

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

@@ -1798,6 +1798,7 @@ public static <T> Observable<T> fromArray(T... items) {
      *         the type of the item emitted by the ObservableSource
      * @return an Observable whose {@link Observer}s' subscriptions trigger an invocation of the given function
      * @see #defer(Supplier)
+     * @see #fromSupplier(Supplier)
      * @since 2.0
      */
     @CheckReturnValue
@@ -2021,6 +2022,43 @@ public static <T> Observable<T> fromPublisher(Publisher<? extends T> publisher)
         return RxJavaPlugins.onAssembly(new ObservableFromPublisher<T>(publisher));
     }
 
+    /**
+     * Returns an Observable that, when an observer subscribes to it, invokes a supplier function you specify and then
+     * emits the value returned from that function.
+     * <p>
+     * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/fromCallable.png" alt="">
+     * <p>
+     * This allows you to defer the execution of the function you specify until an observer subscribes to the
+     * ObservableSource. That is to say, it makes the function "lazy."
+     * <dl>
+     *   <dt><b>Scheduler:</b></dt>
+     *   <dd>{@code fromSupplier} does not operate by default on a particular {@link Scheduler}.</dd>
+     *   <dt><b>Error handling:</b></dt>
+     *   <dd> If the {@link Supplier} throws an exception, the respective {@link Throwable} is
+     *   delivered to the downstream via {@link Observer#onError(Throwable)},
+     *   except when the downstream has disposed this {@code Observable} source.
+     *   In this latter case, the {@code Throwable} is delivered to the global error handler via
+     *   {@link RxJavaPlugins#onError(Throwable)} as an {@link io.reactivex.exceptions.UndeliverableException UndeliverableException}.
+     *   </dd>
+     * </dl>
+     * @param supplier
+     *         a function, the execution of which should be deferred; {@code fromSupplier} will invoke this
+     *         function only when an observer subscribes to the ObservableSource that {@code fromSupplier} returns
+     * @param <T>
+     *         the type of the item emitted by the ObservableSource
+     * @return an Observable whose {@link Observer}s' subscriptions trigger an invocation of the given function
+     * @see #defer(Supplier)
+     * @see #fromCallable(Callable)
+     * @since 3.0.0
+     */
+    @CheckReturnValue
+    @NonNull
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public static <T> Observable<T> fromSupplier(Supplier<? extends T> supplier) {
+        ObjectHelper.requireNonNull(supplier, "supplier is null");
+        return RxJavaPlugins.onAssembly(new ObservableFromSupplier<T>(supplier));
+    }
+
     /**
      * Returns a cold, synchronous and stateless generator of values.
      * <p>

src/main/java/io/reactivex/Single.java

@@ -613,6 +613,8 @@ public static <T> Single<T> error(final Throwable exception) {
      * @param <T>
      *         the type of the item emitted by the {@link Single}.
      * @return a {@link Single} whose {@link SingleObserver}s' subscriptions trigger an invocation of the given function.
+     * @see #defer(Supplier)
+     * @see #fromSupplier(Supplier)
      */
     @CheckReturnValue
     @NonNull
@@ -811,6 +813,44 @@ public static <T> Single<T> fromObservable(ObservableSource<? extends T> observa
         return RxJavaPlugins.onAssembly(new ObservableSingleSingle<T>(observableSource, null));
     }
 
+    /**
+     * Returns a {@link Single} that invokes passed supplierfunction and emits its result
+     * for each new SingleObserver that subscribes.
+     * <p>
+     * Allows you to defer execution of passed function until SingleObserver subscribes to the {@link Single}.
+     * It makes passed function "lazy".
+     * Result of the function invocation will be emitted by the {@link Single}.
+     * <p>
+     * <img width="640" height="467" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.fromCallable.png" alt="">
+     * <dl>
+     *   <dt><b>Scheduler:</b></dt>
+     *   <dd>{@code fromSupplier} does not operate by default on a particular {@link Scheduler}.</dd>
+     *   <dt><b>Error handling:</b></dt>
+     *   <dd> If the {@link Supplier} throws an exception, the respective {@link Throwable} is
+     *   delivered to the downstream via {@link SingleObserver#onError(Throwable)},
+     *   except when the downstream has disposed this {@code Single} source.
+     *   In this latter case, the {@code Throwable} is delivered to the global error handler via
+     *   {@link RxJavaPlugins#onError(Throwable)} as an {@link io.reactivex.exceptions.UndeliverableException UndeliverableException}.
+     *   </dd>
+     * </dl>
+     *
+     * @param supplier
+     *         function which execution should be deferred, it will be invoked when SingleObserver will subscribe to the {@link Single}.
+     * @param <T>
+     *         the type of the item emitted by the {@link Single}.
+     * @return a {@link Single} whose {@link SingleObserver}s' subscriptions trigger an invocation of the given function.
+     * @see #defer(Supplier)
+     * @see #fromCallable(Callable)
+     * @since 3.0.0
+     */
+    @CheckReturnValue
+    @NonNull
+    @SchedulerSupport(SchedulerSupport.NONE)
+    public static <T> Single<T> fromSupplier(final Supplier<? extends T> supplier) {
+        ObjectHelper.requireNonNull(supplier, "supplier is null");
+        return RxJavaPlugins.onAssembly(new SingleFromSupplier<T>(supplier));
+    }
+
     /**
      * Returns a {@code Single} that emits a specified item.
      * <p>

src/main/java/io/reactivex/internal/operators/completable/CompletableFromSupplier.java

@@ -0,0 +1,53 @@
+/**
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * 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 io.reactivex.internal.operators.completable;
+
+import io.reactivex.*;
+import io.reactivex.disposables.*;
+import io.reactivex.exceptions.Exceptions;
+import io.reactivex.functions.Supplier;
+import io.reactivex.plugins.RxJavaPlugins;
+
+/**
+ * Call a Supplier for each incoming CompletableObserver and signal completion or the thrown exception.
+ * @since 3.0.0
+ */
+public final class CompletableFromSupplier extends Completable {
+
+    final Supplier<?> supplier;
+
+    public CompletableFromSupplier(Supplier<?> supplier) {
+        this.supplier = supplier;
+    }
+
+    @Override
+    protected void subscribeActual(CompletableObserver observer) {
+        Disposable d = Disposables.empty();
+        observer.onSubscribe(d);
+        try {
+            supplier.get();
+        } catch (Throwable e) {
+            Exceptions.throwIfFatal(e);
+            if (!d.isDisposed()) {
+                observer.onError(e);
+            } else {
+                RxJavaPlugins.onError(e);
+            }
+            return;
+        }
+        if (!d.isDisposed()) {
+            observer.onComplete();
+        }
+    }
+}

src/main/java/io/reactivex/internal/operators/flowable/FlowableFromCallable.java

@@ -19,11 +19,12 @@
 
 import io.reactivex.Flowable;
 import io.reactivex.exceptions.Exceptions;
+import io.reactivex.functions.Supplier;
 import io.reactivex.internal.functions.ObjectHelper;
 import io.reactivex.internal.subscriptions.DeferredScalarSubscription;
 import io.reactivex.plugins.RxJavaPlugins;
 
-public final class FlowableFromCallable<T> extends Flowable<T> implements Callable<T> {
+public final class FlowableFromCallable<T> extends Flowable<T> implements Supplier<T> {
     final Callable<? extends T> callable;
     public FlowableFromCallable(Callable<? extends T> callable) {
         this.callable = callable;
@@ -51,7 +52,7 @@ public void subscribeActual(Subscriber<? super T> s) {
     }
 
     @Override
-    public T call() throws Exception {
+    public T get() throws Throwable {
         return ObjectHelper.requireNonNull(callable.call(), "The callable returned a null value");
     }
 }