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