滚动时的Angular 5粘滞菜单

Question

I would change menu position to fixed when menu reach top of page. here's how I proceed.

import { Component, OnInit, Inject, HostListener } from '@angular/core';
import {Location} from '@angular/common';
import { DOCUMENT } from "@angular/platform-browser";
....
export class DashboardComponent implements OnInit {
  constructor(private _location: Location, @Inject(DOCUMENT) private doc: Document) { }
  public fixed: boolean = false; 
  @HostListener("window:scroll", [])
  onWindowScroll() {
    let num = this.doc.body.scrollTop;
    console.log("scroll top" , this.doc.body.scrollTop)
    if ( num > 50 ) {
        this.fixed = true;
    }else if (this.fixed && num < 5) {
        this.fixed = false;
    }
  }

in HTML

<div [class.menus]="fixed">
  <div class="left-menu ">
    <app-left-menu></app-left-menu>
  </div>
  <div class="second-menu" >
    <app-second-menu (display)="onDisplay($event)" [expanded]=expanded ></app-second-menu>
  </div>
</div>

CSS

.menus{
  position: fixed;
}

The problem is that scrollTop is not getting changed. when I do console.log(this.doc.body.scrollTop) and scroll, value is 0 and don't change.

Answer 1

Most likely the body is not not the scroll container, the HTML element is

Try document.documentElement.scrollTop

In browsers the document.documentElement is a reference to the root HTML element