Using a directive to add class to host element

Question

I am currently learning Angular 2. I understood how to use the Angular Renderer to set an ElementStyle, but now I would like to use the Renderer method:

setElementClass(renderElement: any, className: string, isAdd: boolean) : void

My question is how can I import a CSS class to my attribute directive? Do I have to convert my CSS class to JSON?

Answer 1

Original OP was asking how to use Renderer. I've included the @HostBinding for completeness.

Using @HostBinding

To add a class to an element you can use @HostBinding

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

@Directive({
  selector: '[myDirective]',
})
export class MyDirective {

  @HostBinding('class')
  elementClass = 'custom-theme';

  constructor() {
  }
}

Using @HostBinding with multiple classes

To make multiple classes more comfortable to use, you can use the ES6 getter and join the classes together before returning them:

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

@Directive({
  selector: '[myDirective]',
})
export class MyDirective {
  protected _elementClass: string[] = [];

  @Input('class')
  @HostBinding('class')
  get elementClass(): string {
      return this._elementClass.join(' ');
  }
  set(val: string) {
      this._elementClass = val.split(' ');
  }

  constructor() {
      this._elementClass.push('custom-theme');
      this._elementClass.push('another-class');
  }
}

Using Renderer

The more low level API is Renderer2. Renderer2 is useful when you have a dynamic set of classes you would like to apply to an element.

Example:

import { Directive, ElementRef, Renderer2 } from '@angular/core';

@Directive({
  selector: '[myDirective]',
})
export class MyDirective {

  constructor(private renderer: Renderer2, hostElement: ElementRef) {
    renderer.addClass(hostElement.nativeElement, 'custom-theme');
  }
}