import {
  AfterViewInit,
  ChangeDetectorRef,
  Component,
  ElementRef,
  HostListener,
  OnDestroy,
  ViewChild
} from "@angular/core";
import {Observable, Subscription} from "rxjs";
import {distinctUntilChanged} from "rxjs/operators";

@Component({
  selector: '[page-content]',
  template: `
  <div id="page_content">
    <div #header id="page_content_header">
      <ng-content select="[header]"></ng-content>
    </div>
    <div id="page_content_inner" [ngStyle]="{'margin-top.px': (height)?height.toString():'0'}">
      <ng-content select="[inner]"></ng-content>
    </div>
  </div>
  `
})
export class PageContentComponent implements OnDestroy, AfterViewInit{
  public height = 0;
  private subscription: Subscription;
  @ViewChild('header') header: ElementRef;
  
  constructor(private cdr: ChangeDetectorRef) {
  }
  
  @HostListener('window:resize', ['$event'])
  onResize(event) {
    this.height = this.getHeight();
    this.cdr.detectChanges();
  }
  
  getHeight(): number{
    return this.header.nativeElement.offsetHeight;
  }
  
  setupHeightMutationObserver() {
    if (typeof document !== 'undefined') {
      const observable = new Observable<number>(observer => {
        const callback = (mutationsList, observer2) => {
          observer.next(this.getHeight());
        };

        // Create an observer instance linked to the callback function
        const elementObserver = new MutationObserver(callback);

        // Options for the observer (which mutations to observe)
        const config = {attributes: true, attributeFilter: ['class'], childList: true, subtree: true};
        // Start observing the target node for configured mutations
        elementObserver.observe(this.header.nativeElement, config);
      });

      this.subscription = observable
        .pipe(
          distinctUntilChanged()//if the value hasn't changed, don't continue
        )
        .subscribe(newHeight => {
          this.height = this.getHeight();
          this.cdr.detectChanges();
        });
    }
  }
  
  ngAfterViewInit() {
    this.setupHeightMutationObserver();
    this.height = this.getHeight();
    this.cdr.detectChanges();
  }
  
  ngOnDestroy() {
    if(this.subscription) {
      this.subscription.unsubscribe();
    }
  }
}