How to add/remove class from directive

问题

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
    }
  }
}

回答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);
   }

}

回答2:

When you are using directives in Angular you would want to use @HostBinding, and bind to class.your-class in order to be able to add/remove your class based on a predicate. You don't need to DI in the Renderer2 to effectively add/remove classes.

For example, when using Bootstrap and Reactive Forms and you want to indicate a valid or invalid form field you can do something like:

import { Directive, Self, HostBinding, Input } from '@angular/core';
import { NgControl } from '@angular/forms';

@Directive({
  selector: '[appCheckFormFieldValidity]'
})
export class CheckFormFieldValidity{
  @Input() public class: string;

  constructor(
    @Self() private ngControl: NgControl
  ) { }

  @HostBinding('class.is-valid')
  public get isValid(): boolean {
    return this.valid;
  }

  @HostBinding('class.is-invalid')
  public get isInvalid(): boolean {
    return this.invalid;
  }

  public get valid(): boolean {
    return this.ngControl.valid && 
    (this.ngControl.dirty || this.ngControl.touched);
  }

  public get invalid(): boolean {
    return !this.ngControl.pending &&
      !this.ngControl.valid &&
      (this.ngControl.touched || this.ngControl.dirty);
  }
}

This is not a rigorous example, but it illustrates the use of @HostBinding, and I created the example in a StackBlitz

回答3:

export class CustomDirective {
   classname:string = "magenta";

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

   addClass(className: string, element: any) {
        // make sure you declare classname in your main style.css
        this.renderer.addClass(this.elementRef.nativeElement, className);
   }

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

}

来源：https://stackoverflow.com/questions/41520681/how-to-add-remove-class-from-directive