import { Observable } from '../Observable';
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import { defer } from './defer';
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import { ObservableInput } from '../types';
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/**
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* Checks a boolean at subscription time, and chooses between one of two observable sources
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*
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* `iif` expects a function that returns a boolean (the `condition` function), and two sources,
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* the `trueResult` and the `falseResult`, and returns an Observable.
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*
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* At the moment of subscription, the `condition` function is called. If the result is `true`, the
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* subscription will be to the source passed as the `trueResult`, otherwise, the subscription will be
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* to the source passed as the `falseResult`.
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*
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* If you need to check more than two options to choose between more than one observable, have a look at the {@link defer} creation method.
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*
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* ## Examples
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*
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* Change at runtime which Observable will be subscribed
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*
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* ```ts
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* import { iif, of } from 'rxjs';
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*
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* let subscribeToFirst;
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* const firstOrSecond = iif(
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* () => subscribeToFirst,
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* of('first'),
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* of('second')
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* );
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*
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* subscribeToFirst = true;
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* firstOrSecond.subscribe(value => console.log(value));
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*
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* // Logs:
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* // 'first'
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*
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* subscribeToFirst = false;
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* firstOrSecond.subscribe(value => console.log(value));
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*
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* // Logs:
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* // 'second'
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* ```
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*
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* Control access to an Observable
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*
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* ```ts
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* import { iif, of, EMPTY } from 'rxjs';
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*
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* let accessGranted;
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* const observableIfYouHaveAccess = iif(
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* () => accessGranted,
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* of('It seems you have an access...'),
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* EMPTY
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* );
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*
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* accessGranted = true;
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* observableIfYouHaveAccess.subscribe({
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* next: value => console.log(value),
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* complete: () => console.log('The end')
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* });
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*
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* // Logs:
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* // 'It seems you have an access...'
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* // 'The end'
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*
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* accessGranted = false;
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* observableIfYouHaveAccess.subscribe({
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* next: value => console.log(value),
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* complete: () => console.log('The end')
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* });
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*
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* // Logs:
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* // 'The end'
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* ```
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*
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* @see {@link defer}
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*
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* @param condition Condition which Observable should be chosen.
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* @param trueResult An Observable that will be subscribed if condition is true.
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* @param falseResult An Observable that will be subscribed if condition is false.
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* @return An observable that proxies to `trueResult` or `falseResult`, depending on the result of the `condition` function.
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*/
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export function iif<T, F>(condition: () => boolean, trueResult: ObservableInput<T>, falseResult: ObservableInput<F>): Observable<T | F> {
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return defer(() => (condition() ? trueResult : falseResult));
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}
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