export class SubscribableObserver<T, P> {
    public completed: boolean = false

    private subscriptions = new Set<{
        onNext?: (next: T) => void
        onComplete?: (complete: P) => void
        onError?: (error: any) => void
    }>()

    public subscribe(
        onNext?: (next: T) => void,
        onComplete?: (complete: P) => void,
        onError?: (error: any) => void
    ) {
        if (this.completed) {
            throw new Error('Observer completed.')
        }
        const subscription = { onNext, onComplete, onError }
        this.subscriptions.add(subscription)

        return {
            unsubscribe: () => this.subscriptions.delete(subscription)
        }
    }

    public next(next?: T): void {
        this.emit('onNext', next)
    }

    public complete(resolve?: P): void {
        this.emit('onComplete', resolve)
        this.unsubscribe()
    }

    public error(error?: any): void {
        this.emit('onError', error)
        this.unsubscribe()
    }

    private emit(type: 'onNext' | 'onComplete' | 'onError', value: any) {
        Array.from(this.subscriptions)
            .map((subscription) => subscription[type])
            .filter((callback: any) => callback && typeof callback === 'function')
            .forEach((callback: any) => callback(value))
    }

    private unsubscribe() {
        this.completed = true
        this.subscriptions.clear()
    }
}