问离子和角度5表单构建器和组未读取值

EN

<form [formGroup]="login">
    <ion-item>
      <ion-label>
        <ion-icon android="android-contacts" name="contacts"></ion-icon>
      </ion-label>
      <ion-input type="text" value="" name="username" [formControlName]="'username'" ></ion-input>
    </ion-item>
    <ion-item>
      <ion-label>
        <ion-icon android="android-unlock" md="md-unlock"></ion-icon>
      </ion-label>
      <ion-input type="password" value="" name="password" [formControlName]="'password'" placeholder="Password"></ion-input>
    </ion-item>
  </form>
  <button ion-button full (click)="onLoginClick()" [disabled]="login.valid" >LOGIN</button>

constructor(public fb: FormBuilder, public authService: ApiAuthProvider, public altCtrl: AlertController, public navCtrl: NavController, 

    public navParams: NavParams) {

    this.login = fb.group({
      username: new FormControl(['u', Validators.required]),
      password: new FormControl(['p', Validators.required])
    })
  }

onLoginClick(){
    console.log(this.login['username']);
}