Angular 2 - Apply conditional style to a directive's child HTML element

Add a style to the child-component

@Component({
  selector: 'child-component',
  inputs: ['item'],
  template: `
    <div class="This is where I really want to apply the style">  
      {{ item.val }}
    </div>
  `,
  styles: [`
    :host(.bordered) > div {
    // if this selector doesn't work use instead
    // child-component.bordered > div {
      border: 3px solid red;
    }
  `],
})
export class ChildComponent {}

I found a better way to solve this making good use of Angular2 features.

Specifically instead of doing trickery with :host and CSS functionality you can just pass in a variable to the child component by changing:

[class.bordered]='isSelected(item)'

being set in the element of the child class change it to

[isBordered]='isSelected(item)'

Then on the div you'd like to apply the bordered class to in the template of the child component just add:

[ngClass]='{bordered: isBordered}'

Here is the full code with the change:

@Component({
  selector: 'parent-component',
  directives: [ChildComponent],
  template: `
    <child-component
      *ngFor='#item of items'
      [item]='item'
      (click)='clicked(item)'
      [isBordered]='isSelected(item)'>
    </child-component>
  `
})
export class ParentComponent {
  items: Item[];
  currentItem: item;

  constructor(private i: ItemService) {
    this.items = i.items;
  }

  clicked(item: Item): void {
    this.currentItem = item;
  }

  isSelected(item: Items): boolean {
    if (!item || !this.currentItem) {
      return false;
    }
    return item.val === this.currentItem.val;
  }
}


@Component({
  selector: 'child-component',
  inputs: ['item'],
  template: `
    <div [ngClass]='{bordered: isBordered}'>
      {{ item.val }}
    </div>
  `
})
export class ChildComponent {}