src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> doOnSubscribe(final Consumer<? super Disposable> onSubscribe) {
        ObjectHelper.requireNonNull(onSubscribe, "onSubscribe is null");
        return RxJavaPlugins.onAssembly(new SingleDoOnSubscribe<T>(this, onSubscribe));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> doOnSubscribe(final Consumer<? super Disposable> onSubscribe) {
        ObjectHelper.requireNonNull(onSubscribe, "onSubscribe is null");
        return RxJavaPlugins.onAssembly(new SingleDoOnSubscribe<T>(this, onSubscribe));
    }
Calls the shared consumer with the Disposable sent through the onSubscribe for each
SingleObserver that subscribes to the current Single.
<p>
<img width="640" height="347" src="https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.doOnSubscribe.png" alt="">
</p>
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code doOnSubscribe} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param onSubscribe the consumer called with the Disposable sent via onSubscribe
@return the new Single instance
@since 2.0