import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';
import { Observable } from 'rxjs';

/**
 * Service that is used to indicate whether the {@link PageLoadingComponent} should display
 * a loading symbol or not.  Used when the data needed to show the page is not yet ready.
 */
@Injectable()
export class PageLoadingService {
  loadingCount = 0;
  loadingCountSource = new Subject<boolean>();

  /**
   * Called to indicate the start of an operation that requires the display to be temporarily
   * hidden.
   */
  public setLoadingStart(): void {
    this.loadingCount++;
    this.evaluateLoading();
  }

  /**
   * Called to indicate the end of an operation that requires the display to be temporarily
   * hidden.
   */
  public setLoadingEnd(): void {
    if (--this.loadingCount < 0) {
      this.loadingCount = 0;
    }
    this.evaluateLoading();
  }

  /**
   * @return An rxjs/Observable that indicates whether to show/hide the loading indicator.
   */
  public watchLoading(): Observable<boolean> {
    return this.loadingCountSource.asObservable();
  }

  /**
   * Internal function for re-evaluating the need to show the loading indicator.
   * Will trigger hiding the loading indicator if all callers that have called
   * {@link setLoadingStart} have also called {@link setLoadingEnd}, otherwise the indicator
   * is shown.
   */
  private evaluateLoading(): void {
    this.loadingCountSource.next((this.loadingCount > 0));
  }

}