Angular Dropdown directive doesn't work

I am using Angular 6.0.5 alongside with Bootstrap 4.1.1 and I added a directive for dropdowns. But I can't make it work in no way. I have seen a lot of similar problems here but none was for Bootstrap 4. this is the dropdown html:

<div class="btn-group" appDropdown>
  <button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">
    Manage
    <span class="caret"></span></button>
  <ul class="dropdown-menu">
    <li><a class="dropdown-item" style="cursor: pointer"
           (click)="OnAddToShoppingList()">To Shopping List</a></li>
    <li><a class="dropdown-item" href="#">Edit Recipe</a></li>
    <li><a class="dropdown-item" href="#">Delete Recipe</a></li>
  </ul>
</div>

and this is the directive:

import {Directive, HostBinding, HostListener} from '@angular/core';

@Directive({
  selector: '[appDropdown]'
})
export class DropdownDirective {
  @HostBinding('class.open') isOpen = false;

  @HostListener('click') toggleopen() {
    this.isOpen = !this.isOpen;
  }
}

I suspected about the "open" class. and I think it was a depreacted class from Bootstrap 3. so what is alternative in Bootstrap 4?

In order to work drop down in bootstrap you need to add show class in two places which you can achieve by get the reference to isOpen variable using angular template ref

If you want to get the reference to directives you need to export the directive first

@Directive({
  selector: '[appDropDown]',
exportAs:'appDropDown'
})

Then you can use the template ref method to achive dropdown

<div class="btn-group" appDropDown #r="appDropDown" >
  <button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown" >
    Manage
    <span class="caret"></span></button>
  <ul class="dropdown-menu" [ngClass]="{'show':r.isOpen}" >
    <li><a class="dropdown-item" style="cursor: pointer"
           >To Shopping List</a></li>
    <li><a class="dropdown-item" href="#">Edit Recipe</a></li>
    <li><a class="dropdown-item" href="#">Delete Recipe</a></li>
  </ul>
</div>

I have include example here check out: https://stackblitz.com/edit/angular-h9rgmn