How to add/remove class from directive

Question

Using a custom directive how would you add/remove a class on the host element based on a specific conditions?

Example:

@Directive({
  selector: '[customDirective]'
})
export class CustomDirective {
  constructor(service: SomService) {
    // code to add class

    if (service.someCondition()) {
        // code to remove class
    }
  }
}

Answer 1

If you don't want to use the ngClass directive (Hint: you can pass a function to [ngClass]="myClasses()" if it would be to messy inline in your template) you can just utilize the Renderer2 for it to add one or more classes:

export class CustomDirective {

   constructor(private renderer: Renderer2,
               private elementRef: ElementRef,
               service: SomService) {
   }

   addClass(className: string, element: any) {
       this.renderer.addClass(element, className);
       // or use the host element directly
       // this.renderer.addClass(this.elementRef.nativeElement, className);
   }

   removeClass(className: string, element: any) {
       this.renderer.removeClass(element, className);
   }

}