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import { MonoTypeOperatorFunction, SchedulerLike } from '../types';
import { operate } from '../util/lift';
/**
* Asynchronously subscribes Observers to this Observable on the specified {@link SchedulerLike}.
*
* With `subscribeOn` you can decide what type of scheduler a specific Observable will be using when it is subscribed to.
*
* Schedulers control the speed and order of emissions to observers from an Observable stream.
*
* 
*
* ## Example
*
* Given the following code:
*
* ```ts
* import { of, merge } from 'rxjs';
*
* const a = of(1, 2, 3);
* const b = of(4, 5, 6);
*
* merge(a, b).subscribe(console.log);
*
* // Outputs
* // 1
* // 2
* // 3
* // 4
* // 5
* // 6
* ```
*
* Both Observable `a` and `b` will emit their values directly and synchronously once they are subscribed to.
*
* If we instead use the `subscribeOn` operator declaring that we want to use the {@link asyncScheduler} for values emitted by Observable `a`:
*
* ```ts
* import { of, subscribeOn, asyncScheduler, merge } from 'rxjs';
*
* const a = of(1, 2, 3).pipe(subscribeOn(asyncScheduler));
* const b = of(4, 5, 6);
*
* merge(a, b).subscribe(console.log);
*
* // Outputs
* // 4
* // 5
* // 6
* // 1
* // 2
* // 3
* ```
*
* The reason for this is that Observable `b` emits its values directly and synchronously like before
* but the emissions from `a` are scheduled on the event loop because we are now using the {@link asyncScheduler} for that specific Observable.
*
* @param scheduler The {@link SchedulerLike} to perform subscription actions on.
* @param delay A delay to pass to the scheduler to delay subscriptions
* @return A function that returns an Observable modified so that its
* subscriptions happen on the specified {@link SchedulerLike}.
*/
export function subscribeOn<T>(scheduler: SchedulerLike, delay: number = 0): MonoTypeOperatorFunction<T> {
return operate((source, subscriber) => {
subscriber.add(scheduler.schedule(() => source.subscribe(subscriber), delay));
});
}