Nativescript Angular: невозможно прочитать свойство undefined

Question

У меня есть следующий код с привязкой к latitude и longitude. Я хочу, чтобы мой Geolocation Tracker обновил значения. Но watchPosition не видит this.latitude и this.longitude.

@Component({ 
    selector: "my-app", 
    templateUrl: "app.component.html", 
}) 
export class AppComponent { 

latitude: number; 
longitude: number; 
watchId = 0; 

constructor() { 
    this.latitude = 0; 
    this.longitude = 0; 
} 

public startTracking() { 

    /* Get Location */ 
    this.watchId = geolocation.watchLocation(function (loc) { 
      if (loc) { 
       console.log("Current location is: " + loc.latitude + ", " + loc.longitude); 
       this.latitude = loc.latitude; 
       this.longitude = loc.longitude; 
      } 
     }, function (e) { 
      console.log("Error: " + e.message); 
     }, 
     {desiredAccuracy: enums.Accuracy.any, updateDistance: 10, minimumUpdateTime: 1000}); 
} 

public stopTracking() { 
    if (this.watchId) { 
     geolocation.clearWatch(this.watchId); 
    } 
} 

} 

Это ошибка я получаю:

com.tns.NativeScriptException: 
Calling js method onLocationChanged failed 

TypeError: Cannot set property 'latitude' of undefined 
File: "/data/data/org.nativescript.elektra/files/app/app.component.js, line: 44, column: 30 

StackTrace: 
Frame: function:'', file:'/data/data/org.nativescript.elektra/files/app/app.component.js', line: 44, column: 31 
Frame: function:'android.location.LocationListener.onLocationChanged', file:'/data/data/org.nativescript.elektra/files/app/tns_modules/nativescript-geolocation/nativescript-geolocation.js', line: 25, column: 17 


at com.tns.Runtime.callJSMethodNative(Native Method) 
at com.tns.Runtime.dispatchCallJSMethodNative(Runtime.java:861) 
at com.tns.Runtime.callJSMethodImpl(Runtime.java:726) 
at com.tns.Runtime.callJSMethod(Runtime.java:712) 
at com.tns.Runtime.callJSMethod(Runtime.java:693) 
at com.tns.Runtime.callJSMethod(Runtime.java:683) 
at com.tns.gen.android.location.LocationListener.onLocationChanged(LocationListener.java:11) 
at android.location.LocationManager$ListenerTransport._handleMessage(LocationManager.java:255) 
at android.location.LocationManager$ListenerTransport.access$000(LocationManager.java:184) 
at android.location.LocationManager$ListenerTransport$1.handleMessage(LocationManager.java:200) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:176) 
at android.app.ActivityThread.main(ActivityThread.java:5419) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:525) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862) 
at dalvik.system.NativeStart.main(Native Method) 

Answer 1

Это потому, что this невидим в анонимной функции. Перед началом анонимной функции вы должны сохранить ссылку this.

public startTracking() { 
let self = this; 
/* Get Location */ 
this.watchId = geolocation.watchLocation(function (loc) { 
     if (loc) { 
      console.log("Current location is: " + loc.latitude + ", " + loc.longitude); 
      self.latitude = loc.latitude; 
      self.longitude = loc.longitude; 
     } 
    }, function (e) { 
     console.log("Error: " + e.message); 
    }, 
    {desiredAccuracy: enums.Accuracy.any, updateDistance: 10, minimumUpdateTime: 1000}); 
}