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