@acutmore/rxjs
Version:
Reactive Extensions for modern JavaScript
131 lines • 4.8 kB
JavaScript
import { Observable } from '../Observable';
import { subscribeToResult } from '..//util/subscribeToResult';
import { OuterSubscriber } from '../OuterSubscriber';
/**
* We need this JSDoc comment for affecting ESDoc.
* @extends {Ignored}
* @hide true
*/
export class IfObservable extends Observable {
constructor(condition, thenSource, elseSource) {
super();
this.condition = condition;
this.thenSource = thenSource;
this.elseSource = elseSource;
}
/**
* Decides at subscription time which Observable will actually be subscribed.
*
* <span class="informal">`If` statement for Observables.</span>
*
* `if` 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 `if` can be easily implemented with {@link defer}
* and exists only for convenience and readability reasons.
*
*
* @example <caption>Change at runtime which Observable will be subscribed</caption>
* let subscribeToFirst;
* const firstOrSecond = Rx.Observable.if(
* () => subscribeToFirst,
* Rx.Observable.of('first'),
* Rx.Observable.of('second')
* );
*
* subscribeToFirst = true;
* firstOrSecond.subscribe(value => console.log(value));
*
* // Logs:
* // "first"
*
* subscribeToFirst = false;
* firstOrSecond.subscribe(value => console.log(value));
*
* // Logs:
* // "second"
*
*
* @example <caption>Control an access to an Observable</caption>
* let accessGranted;
* const observableIfYouHaveAccess = Rx.Observable.if(
* () => accessGranted,
* Rx.Observable.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 if
* @owner Observable
*/
static create(condition, thenSource, elseSource) {
return new IfObservable(condition, thenSource, elseSource);
}
_subscribe(subscriber) {
const { condition, thenSource, elseSource } = this;
return new IfSubscriber(subscriber, condition, thenSource, elseSource);
}
}
class IfSubscriber extends OuterSubscriber {
constructor(destination, condition, thenSource, elseSource) {
super(destination);
this.condition = condition;
this.thenSource = thenSource;
this.elseSource = elseSource;
this.tryIf();
}
tryIf() {
const { condition, thenSource, elseSource } = this;
let result;
try {
result = condition();
const source = result ? thenSource : elseSource;
if (source) {
this.add(subscribeToResult(this, source));
}
else {
this._complete();
}
}
catch (err) {
this._error(err);
}
}
}
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