NativeScript with Angular2 data binding not working with geolocation - binding

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.