javascript - How do I detect change to ngModel on a select tag (Angular 2)?

Question

I am attempting to detect a change on ngModel in a <select> tag. In Angular 1.x, we might solve this with a $watch on ngModel, or by using ngChange, but I've yet to understand how to detect a change to ngModel in Angular 2.

Full Example: http://plnkr.co/edit/9c9oKH1tjDDb67zdKmr9?p=info

import {Component, View, Input, } from 'angular2/core';
import {FORM_DIRECTIVES} from 'angular2/common';

@Component({
    selector: 'my-dropdown'
})
@View({
    directives: [FORM_DIRECTIVES],
    template: `
        <select [ngModel]="selection" (ngModelChange)="onChange($event, selection)" >
            <option *ngFor="#option of options">{{option}}</option>
        </select>
        {{selection}}
    `
})
export class MyDropdown {
    @Input() options;

    selection = 'Dog';

    ngOnInit() {
        console.log('These were the options passed in: ' + this.options);
  }

  onChange(event) {
    if (this.selection === event) return;
    this.selection = event;
    console.log(this.selection);
  }

}

As we can see, if we select a different value from the dropdown, our ngModel changes, and the interpolated expression in the view reflects this.

How do I get notified of this change in my class/controller?

Answer 1

Update:

Separate the event and property bindings:

<select [ngModel]="selectedItem" (ngModelChange)="onChange($event)">

onChange(newValue) {
    console.log(newValue);
    this.selectedItem = newValue;  // don't forget to update the model here
    // ... do other stuff here ...
}

You could also use

<select [(ngModel)]="selectedItem" (ngModelChange)="onChange($event)">

and then you wouldn't have to update the model in the event handler, but I believe this causes two events to fire, so it is probably less efficient.

---

Old answer, before they fixed a bug in beta.1:

Create a local template variable and attach a (change) event:

<select [(ngModel)]="selectedItem" #item (change)="onChange(item.value)">

plunker

See also How can I get new selection in "select" in Angular 2?