src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<T> delaySubscription(ObservableSource<U> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableDelaySubscriptionOther<T, U>(this, other));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<T> delaySubscription(ObservableSource<U> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableDelaySubscriptionOther<T, U>(this, other));
    }
Returns an Observable that delays the subscription to this Observable
until the other Observable emits an element or completes normally.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/delaySubscription.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>This method does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <U> the value type of the other Observable, irrelevant
@param other the other Observable that should trigger the subscription
to this Observable.
@return an Observable that delays the subscription to this Observable
until the other Observable emits an element or completes normally.
@since 2.0