Angular2实现自定义双向绑定属性

import { Component, OnInit, Output, Input, EventEmitter } from '@angular/core';@Component({  selector: 'twoway',  template: `    <input [(ngModel)]="username">    <p>Hello {{username}}!</p>  `})export class TwoWayComponent implements OnInit {  constructor() { }  usernameValue: string;  @Output() usernameChange = new EventEmitter();  @Input()  get username() {    return this.usernameValue;  }  set username(val) {    this.usernameValue = val;    this.usernameChange.emit(this.usernameValue);  }  ngOnInit() {  }}