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import { Observable } from '../Observable';
import { SubscribableOrPromise } from '../types';
/**
* Decides at subscription time which Observable will actually be subscribed.
*
* <span class="informal">`If` statement for Observables.</span>
*
* `iif` accepts a condition function and two Observables. When
* an Observable returned by the operator is subscribed, condition function will be called.
* Based on what boolean it returns at that moment, consumer will subscribe either to
* the first Observable (if condition was true) or to the second (if condition was false). Condition
* function may also not return anything - in that case condition will be evaluated as false and
* second Observable will be subscribed.
*
* Note that Observables for both cases (true and false) are optional. If condition points to an Observable that
* was left undefined, resulting stream will simply complete immediately. That allows you to, rather
* then controlling which Observable will be subscribed, decide at runtime if consumer should have access
* to given Observable or not.
*
* If you have more complex logic that requires decision between more than two Observables, {@link defer}
* will probably be a better choice. Actually `iif` can be easily implemented with {@link defer}
* and exists only for convenience and readability reasons.
*
*
* ## Examples
* ### Change at runtime which Observable will be subscribed
* ```javascript
* 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 an access to an Observable
* ```javascript
* let accessGranted;
* const observableIfYouHaveAccess = iif(
* () => accessGranted,
* of('It seems you have an access...'), // Note that only one Observable is passed to the operator.
* );
*
* accessGranted = true;
* observableIfYouHaveAccess.subscribe(
* value => console.log(value),
* err => {},
* () => console.log('The end'),
* );
*
* // Logs:
* // "It seems you have an access..."
* // "The end"
*
* accessGranted = false;
* observableIfYouHaveAccess.subscribe(
* value => console.log(value),
* err => {},
* () => console.log('The end'),
* );
*
* // Logs:
* // "The end"
* ```
*
* @see {@link defer}
*
* @param {function(): boolean} condition Condition which Observable should be chosen.
* @param {Observable} [trueObservable] An Observable that will be subscribed if condition is true.
* @param {Observable} [falseObservable] An Observable that will be subscribed if condition is false.
* @return {Observable} Either first or second Observable, depending on condition.
* @static true
* @name iif
* @owner Observable
*/
export declare function iif<T, F>(condition: () => boolean, trueResult?: SubscribableOrPromise<T>, falseResult?: SubscribableOrPromise<F>): Observable<T | F>;
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