Two way data binding

Two-way data binding combines the input and output binding into a single notation using the ngModeldirective.

<input [(ngModel)]="name" >

What this is doing behind the scenes is equivalent to:

<input [ngModel]="name" (ngModelChange)="name=$event">

To create your own component that supports two-way binding, you must define an @Output property to match an @Input, but suffix it with the Change. The code example below, inside class CounterComponent shows how to make property count support two-way binding.

app/counter.component.ts

import { Component, Input, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'rio-counter',
  templateUrl: 'app/counter.component.html'
})
export class CounterComponent {
  @Input() count = 0;
  @Output() countChange = EventEmitter<number>();

  increment() {
    this.count++;
    this.countChange.emit(this.count);
  }
}

app/counter.component.html

<div>
  <p>
    <ng-content></ng-content>    Count: {{ count }} -
    <button (click)="increment()">Increment</button>
  </p></div>

Source https://angular-2-training-book.rangle.io/handout/components/app_structure/two_way_data_binding.html

https://blog.thoughtram.io/angular/2016/10/13/two-way-data-binding-in-angular-2.html