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support-portal-frontend/src/app/shared/scrollspy.directive.ts

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+import { Directive, Input, EventEmitter, Inject, Output, ElementRef, HostListener } from '@angular/core';
+import { DOCUMENT } from '@angular/common';
+
+@Directive({
+  selector: '[appScrollspy]'
+})
+export class ScrollspyDirective {
+
+  @Input() public spiedTags = [];
+  @Output() public sectionChange = new EventEmitter<string>();
+  private currentSection: string;
+
+  // tslint:disable-next-line: variable-name
+  constructor(private _el: ElementRef, @Inject(DOCUMENT) private document: Document,) { }
+
+  @HostListener('window:scroll', ['$event'])
+  /**
+   * Window scroll method
+   */
+  onScroll(event: any) {
+    let currentSection: string;
+    const children = this._el.nativeElement.querySelectorAll('section');
+    const scrollTop = this.document.documentElement.scrollTop;
+    const parentOffset = this.document.documentElement.offsetTop;
+
+    // tslint:disable-next-line: prefer-for-of
+    for (let i = 0; i < children.length; i++) {
+      const element = children[i];
+      if (this.spiedTags.some(spiedTag => spiedTag === element.tagName)) {
+        if ((element.offsetTop - parentOffset) <= scrollTop) {
+          currentSection = element.id;
+        }
+      }
+    }
+    if (currentSection !== this.currentSection) {
+      this.currentSection = currentSection;
+      this.sectionChange.emit(this.currentSection);
+    }
+  }
+}