Ionic 2 Angular 2 - 不能從方法

Question

訪問NavController我想在用戶使用angularfire進行身份驗證後導航到另一個頁面。除了導航到其他頁面之外，一切都可以工作。

這裏是我的代碼：

constructor(public navCtrl: NavController, public menu: MenuController, public afAuth: AngularFireAuth, public db: AngularFireDatabase, private platform : Platform) { 
    this.navigateIfUserIsLogdIn(); 
    } 

    navigateIfUserIsLogdIn(){ 
     this.authState = this.afAuth.authState; 
     this.user = this.afAuth.auth.currentUser; 

     this.afAuth.auth.onAuthStateChanged(function(user) { 
      if (user) { 
      // User is signed in. 
      this.navCtrl.setRoot(HomePage); 
      } else { 
      // No user is signed in. 
      } 
     }); 
    } 

我得到的錯誤是：

錯誤：（在承諾）未捕獲：類型錯誤：無法讀取的不確定

財產 'navCtrl'

爲什麼不工作時，在裏面navigateIfUserIsLogdIn（）？

請大家幫忙，並提供了一個例子:)

Answer 1

您需要使用箭頭功能是這樣的：

navigateIfUserIsLogdIn(){ 
    this.authState = this.afAuth.authState; 
    this.user = this.afAuth.auth.currentUser; 

    this.afAuth.auth.onAuthStateChanged((user) => { 
     if (user) { 
     // User is signed in. 
     this.navCtrl.setRoot(HomePage); 
     } else { 
     // No user is signed in. 
     } 
    }); 
} 

注意，現在我們做

this.afAuth.auth.onAuthStateChanged((user) => {...});

代替

this.afAuth.auth.onAuthStateChanged(function(user) {

通過使用箭頭功能，的this屬性不被覆蓋，仍引用組件實例（否則，this關鍵字指向內的功能，和navCtrl不限定它）。

Answer 2

嘗試這樣做，

 constructor(public navCtrl: NavController, public menu: MenuController, public afAuth: AngularFireAuth, public db: AngularFireDatabase, private platform : Platform) { 
    this.navigateIfUserIsLogdIn(); 
    } 

    navigateIfUserIsLogdIn(){ 
     this.authState = this.afAuth.authState; 
     this.user = this.afAuth.auth.currentUser; 
     var that= this; 
     this.afAuth.auth.onAuthStateChanged(function(user) { 
      if (user) { 
      // User is signed in. 
      that.navCtrl.setRoot(HomePage); 
      } else { 
      // No user is signed in. 
      } 
     }); 
    }