angular 2 (click) passing iteration variable and [class.selected] passing function

Question

The following is taken from ng-book 2

  @Component({
  selector: 'products-list',
  directives: [ProductRow],
  inputs: ['productList'],
  outputs: ['onProductSelected'],
  template: `
  <div class="ui items">
    <product-row 
      *ngFor="let myProduct of productList" 
      [product]="myProduct" 
      (click)='clicked(myProduct)'
      [class.selected]="isSelected(myProduct)">
    </product-row>
  </div>
  `
})
class ProductsList {
  /**
   * @input productList - the Product[] passed to us
   */
  productList: Product[];

  /**
   * @ouput onProductSelected - outputs the current 
   *          Product whenever a new Product is selected
   */
  onProductSelected: EventEmitter<Product>;

  /**
   * @property currentProduct - local state containing 
   *             the currently selected `Product`
   */
  currentProduct: Product;

  constructor() {
    this.onProductSelected = new EventEmitter();
  }

  clicked(product: Product): void {
    this.currentProduct = product;
    this.onProductSelected.emit(product);
  }

  isSelected(product: Product): boolean {
    if (!product || !this.currentProduct) {
      return false;
    }
    return product.sku === this.currentProduct.sku;
  }

}

@Component({
  selector: 'product-row',
  inputs: ['product'],
  ... not relevant to the question
  `
})
class ProductRow {
  product: Product;
}

Two questions,

Q1. where it says

(click)='clicked(myProduct)'

is the argument for clicked the same as the product property on ProductRow component? I am used to passing $event to clicked. Why pass "myProduct" instead?

Q2. where it says

[class.selected]="isSelected(myProduct)"

it seems like we are doing

[class.selected]="false"

for all products cuz none of them is selected initially. How is angular able to call isSelected(myProduct) again and again? How does angular decide when to call isSelected?

Answer 1

this is an example of parent-child communication in angular2.

when you are using (click)='clicked(myProduct)' event, what essentially you are doing is emitting the value of the selected product using @ouput onProductSelected property as shown here :

this.onProductSelected.emit(product);
$event` here would be equivalent to `#myProduct.value

when any event gets executed, angular2's change detections kicks-in; checking all the template expression values. It then updates the property binding as per value changes. now everytime value of myProduct changes, the property binding [class.selected] need to get updated , hence isSelected method gets called.