Angular LAB: بیایید یک دستورالعمل دید ایجاد کنیم

  visibility
  [visibilityMonitor]="true"
  (visibilityChange)="onVisibilityChange($event)"
>
  I'm being observed! Can you see me yet?

type VisibilityChange =
  | {
      isVisible: true;
      target: HTMLElement;
    }
  | {
      isVisible: false;
      target: HTMLElement | undefined;
    };

@Directive({
  selector: "[visibility]",
  standalone: true
})
export class VisibilityDirective implements OnInit, OnChanges, AfterViewInit, OnDestroy {
  private element = inject(ElementRef);

  /**
   * Emits after the view is initialized.
   */
  private afterViewInit$ = new Subject<void>();

  /**
   * The IntersectionObserver for this element.
   */
  private observer: IntersectionObserver | undefined;

  /**
   * Last known visibility for this element.
   * Initially, we don't know.
   */
  private isVisible: boolean = undefined;

  /**
   * If false, once the element becomes visible there will be one emission and then nothing.
   * If true, the directive continuously listens to the element and emits whenever it becomes visible or not visible.
   */
  visibilityMonitor = input(false);

  /**
   * Notifies the listener when the element has become visible.
   * If "visibilityMonitor" is true, it continuously notifies the listener when the element goes in/out of view.
   */
  visibilityChange = output<VisibilityChange>();
}

ngOnInit(): void {
  this.reconnectObserver();
}

ngOnChanges(): void {
  this.reconnectObserver();
}

ngAfterViewInit(): void {
  this.afterViewInit$.next();
}

ngOnDestroy(): void {
  // Disconnect and if visibilityMonitor is true, notify the listener
  this.disconnectObserver();
  if (this.visibilityMonitor) {
    this.visibilityChange.emit({
      isVisible: false,
      target: undefined
    });
  }
}

private reconnectObserver(): void {}
private disconnectObserver(): void {}

private reconnectObserver(): void {
    // Disconnect an existing observer
    this.disconnectObserver();
    // Sets up a new observer
    this.observer = new IntersectionObserver((entries, observer) => {
      entries.forEach(entry => {
        const { isIntersecting: isVisible, target } = entry;
        const hasChangedVisibility = isVisible !== this.isVisible;
        const shouldEmit = isVisible || (!isVisible && this.visibilityMonitor);
        if (hasChangedVisibility && shouldEmit) {
          this.visibilityChange.emit({
            isVisible,
            target: target as HTMLElement
          });
          this.isVisible = isVisible;
        }
        // If visilibilyMonitor is false, once the element is visible we stop.
        if (isVisible && !this.visibilityMonitor) {
          observer.disconnect();
        }
      });
    });
    // Start observing once the view is initialized
    this.afterViewInit$.subscribe(() => {
        this.observer?.observe(this.element.nativeElement);
    });
  }

 private disconnectObserver(): void {
    if (this.observer) {
      this.observer.disconnect();
      this.observer = undefined;
    }
  }

type VisibilityChange =
  | {
      isVisible: true;
      target: HTMLElement;
    }
  | {
      isVisible: false;
      target: HTMLElement | undefined;
    };

@Directive({
  selector: "[visibility]",
  standalone: true
})
export class VisibilityDirective
  implements OnChanges, OnInit, AfterViewInit, OnDestroy {
  private element = inject(ElementRef);

  /**
   * Emits after the view is initialized.
   */
  private afterViewInit$ = new Subject<void>();

  /**
   * The IntersectionObserver for this element.
   */
  private observer: IntersectionObserver | undefined;

  /**
   * Last known visibility for this element.
   * Initially, we don't know.
   */
  private isVisible: boolean = undefined;

  /**
   * If false, once the element becomes visible there will be one emission and then nothing.
   * If true, the directive continuously listens to the element and emits whenever it becomes visible or not visible.
   */
  visibilityMonitor = input(false);

  /**
   * Notifies the listener when the element has become visible.
   * If "visibilityMonitor" is true, it continuously notifies the listener when the element goes in/out of view.
   */
  visibilityChange = output<VisibilityChange>();

  ngOnInit(): void {
    this.reconnectObserver();
  }

  ngOnChanges(): void {
    this.reconnectObserver();
  }

  ngAfterViewInit(): void {
    this.afterViewInit$.next(true);
  }

  ngOnDestroy(): void {
    // Disconnect and if visibilityMonitor is true, notify the listener
    this.disconnectObserver();
    if (this.visibilityMonitor) {
      this.visibilityChange.emit({
        isVisible: false,
        target: undefined
      });
    }
  }

  private reconnectObserver(): void {
    // Disconnect an existing observer
    this.disconnectObserver();
    // Sets up a new observer
    this.observer = new IntersectionObserver((entries, observer) => {
      entries.forEach(entry => {
        const { isIntersecting: isVisible, target } = entry;
        const hasChangedVisibility = isVisible !== this.isVisible;
        const shouldEmit = isVisible || (!isVisible && this.visibilityMonitor);
        if (hasChangedVisibility && shouldEmit) {
          this.visibilityChange.emit({
            isVisible,
            target: target as HTMLElement
          });
          this.isVisible = isVisible;
        }
        // If visilibilyMonitor is false, once the element is visible we stop.
        if (isVisible && !this.visibilityMonitor) {
          observer.disconnect();
        }
      });
    });
    // Start observing once the view is initialized
    this.afterViewInit$.subscribe(() => {
        this.observer?.observe(this.element.nativeElement);
    });
  }

  private disconnectObserver(): void {
    if (this.observer) {
      this.observer.disconnect();
      this.observer = undefined;
    }
  }
}