memorizing-subject.ts

import { Subject } from 'rxjs/Subject';
import { Subscriber } from 'rxjs/Subscriber';
import { ISubscription, Subscription } from 'rxjs/Subscription';
import { ObjectUnsubscribedError } from 'rxjs/util/ObjectUnsubscribedError';

/**
 * MemorizingSubject acts like a BehaviorSubject, except that it is seeded
 * with no value, calls to getValue() will fail before the first `next` call,
 * and it does not push a value to subscribers if `next` has not been called.
 */
export class MemorizingSubject<T> extends Subject<T> {
    private value: T;
    private hasSet = false;

    public hasValue(): boolean {
        return this.hasSet;
    }

    public getValue(): T {
        if (this.hasError) {
            throw this.thrownError;
        }
        if (this.closed) {
            throw new ObjectUnsubscribedError();
        }
        if (!this.hasSet) {
            throw new Error(
                'Attempted to call `getValue` on a MemorizingSubject' +
                'which did not previous have its value set.',
            );
        }
        return this.value;
    }

    public next(value: T): void {
        this.hasSet = true;
        super.next(this.value = value);
    }

    protected _subscribe(subscriber: Subscriber<T>): Subscription { // tslint:disable-line
        const subscription = super._subscribe(subscriber);
        if (subscription && this.hasSet && !(<ISubscription>subscription).closed) {
            subscriber.next(this.value);
        }
        return subscription;
    }
}