src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Completable fromRunnable(final Runnable run) {
        ObjectHelper.requireNonNull(run, "run is null");
        return RxJavaPlugins.onAssembly(new CompletableFromRunnable(run));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static Completable fromRunnable(final Runnable run) {
        ObjectHelper.requireNonNull(run, "run is null");
        return RxJavaPlugins.onAssembly(new CompletableFromRunnable(run));
    }
Returns a Completable instance that runs the given Runnable for each subscriber and
emits either its exception or simply completes.
<p>
<img width="640" height="297" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.fromRunnable.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code fromRunnable} does not operate by default on a particular {@link Scheduler}.</dd>
<dt><b>Error handling:</b></dt>
<dd> If the {@link Runnable} 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 run the runnable to run for each subscriber
@return the new Completable instance
@throws NullPointerException if run is null