NativeScript with Angular2 data binding not working with geolocation

Question

I am developing a simple POC to display the current position. The program receives the information from the geolocation.watchLocation however the location is not bind and displayed on the screen until I press the button STOP Monitoring. To be noted that the log is correctly showing the coordinates

JS: Start Monitoring ...
JS: Location: 49.6411839:6.0040451
JS: Stop Monitoring ...

---

import {Component} from "@angular/core";
import {Router} from "@angular/router";
import geolocation = require("nativescript-geolocation");

@Component({
    selector: "TrackingPageSelector",
    template:`
    <StackLayout>
        <Button [text]="watchId==0 ? 'Start Monitoring' : 'Stop Monitoring'" (tap)="buttonStartStopTap()"></Button>
        <Label class="labelValue" [text]="latitude"> </Label>
        <Label class="labelValue" [text]="longitude"> </Label>
    </StackLayout>`
})

export class TrackingPageComponent {
    latitude: number = 0;
    longitude: number = 0;
    watchId: number = 0;
    constructor(private router: Router) {}

    ngOnInit() {
        if (!geolocation.isEnabled()) geolocation.enableLocationRequest();
    }

    public buttonStartStopTap() {
        if (this.watchId != 0) {
            console.log('Stop Monitoring ...');
            geolocation.clearWatch(this.watchId);
            this.watchId = 0;
        } else {
            console.log('Start Monitoring ...');
            let _self = this;
            this.watchId = geolocation.watchLocation(
                function (loc) {
                    if (loc) {
                        _self.latitude = loc.latitude;
                        _self.longitude = loc.longitude;
                        console.log(`Location: ${_self.latitude}:${_self.longitude}`);
                    }
                },
                function(e){
                    this.errorMsg = e.message;
                },
                {desiredAccuracy: 3, updateDistance: 10, minimumUpdateTime: 1000 * 3}); // Should update every X seconds
        }
    }
}

Answer 1

If you use ()=> instead of function () then you don't need the _self hack. It looks like you need to invoke change detection manually. NgZone seems not to cover geolocation.watchLocation()

 constructor(private cdRef:ChangeDetectorRef) {}

 ...

        this.watchId = geolocation.watchLocation(
            (loc) => {
                if (loc) {
                    this.latitude = loc.latitude;
                    this.longitude = loc.longitude;
                    this.cdRef.detectChanges();
                    console.log(`Location: ${this.latitude}:${this.longitude}`);
                }
            },
            (e) => {
                this.errorMsg = e.message;
            },

or alternatively zone.run(() => ...) like

 constructor(private zone:NgZone) {}

 ...

        this.watchId = geolocation.watchLocation(
            (loc) => {
                if (loc) {
                  this.zone.run(() => {
                    this.latitude = loc.latitude;
                    this.longitude = loc.longitude;

                    console.log(`Location: ${this.latitude}:${this.longitude}`);
                   });
                }
            },
            (e) => {
                this.errorMsg = e.message;
            },

If you only change local fields cdRef.detectChanges() is the better option, if you call methods that might modify some state outside of the current component, zone.run(...) is the better option.