The @HostBinding decorator allows us to programatically set a property value on the directive's host element. It works similarly to a property binding defined in a template, except it specifically targets the host element. The binding is checked for every change detection cycle, so it can change dynamically if desired. For example, lets say that we want to create a directive for buttons that dynamically adds a class when we press on it. That could look something like:

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

@Directive({
  selector: '[appButtonPress]'
})
export class ButtonPressDirective {
  @HostBinding('attr.role') role = 'button';
  @HostBinding('class.pressed') isPressed: boolean;

  @HostListener('mousedown') hasPressed() {
    this.isPressed = true;
  }
  @HostListener('mouseup') hasReleased() {
    this.isPressed = false;
  }
}

Notice that for both use cases of @HostBinding we are passing in a string value for which property we want to affect. If we don't supply a string to the decorator, then the name of the class member will be used instead. In the first @HostBinding, we are statically setting the role attribute to button. For the second example, the pressed class will be applied when isPressed is true