Typescript getCurrent location and pass to backend to get the result and pass to Material Table data source

Question

I have my Angular 2 project that tries to get user's current location, therefore in my class I have the following code:

export class CurrentLocation {
  constructor(private http: Http) { }
  private lat : any;
  private lon : any;
  private params  = new URLSearchParams();
  private url = 'api/search/location';


  getPosition = (lat, lon) => {
    navigator.geolocation.getCurrentPosition((position) => { 
      this.lat = position.coords.latitude; 
      this.lon = position.coords.longitude;
     });
  }
  getCurrentLocation(): Observable<any> {
    this.getPosition(this.lat, this.lon);
    console.log(this.lat+ "," + this.lon);
    //console.log(this.lat + ":" + this.lon);
    this.params.set('lat', this.lat);
    this.params.set('lon', this.lon);
    //console.log(this.params.toString());
    var result = this.http.get(this.url, { search: this.params });
    result.toPromise();
    return result;
  }
}

But lat and lon return undefined.. The expected behaviour I want is once I got those latitude and longitude, I will send it to my backend address like following url http://localhost:8080/api/search/location?lat=123&lon=123

But it seems like setting value for lat and lon are failed.

How can I properly set the latitude and longitude in this class?

EDIT on August 3:

According libertyernie's answer below, I can successfully pass the current lat and lon into backend, but I don't know how to convert to Observable, since I try to using Table module in Angular Material, in that module for data source it must be Observable...

Here is the code after I try, but still not working...

import { Component, OnInit } from '@angular/core';
import { Http, Response, URLSearchParams } from '@angular/http';
import { DataSource } from '@angular/cdk';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/startWith';
import 'rxjs/add/observable/merge';
import 'rxjs/add/operator/map';
import { Restaurant } from '../restaurant/restaurant';
import { Category } from '../category/category';
import { RestaurantService } from '../restaurant/restaurant.service';


@Component({
  selector: 'app-home',
  templateUrl: './home.component.html',
  styleUrls: ['./home.component.css']
})
export class HomeComponent implements OnInit {
  displayedColumns = ['Id', 'Name', 'Category', 'Address', 'City'];
  exampleDatabase: ExampleHttpDatabase | null;
  dataSource: ExampleDataSource | null;
  location: CurrentLocation | null;
  lat: any;
  lon: any;
  result: Promise<any>;

  constructor(http: Http) {
    this.exampleDatabase = new ExampleHttpDatabase(http);
    this.dataSource = new ExampleDataSource(this.exampleDatabase);
    this.location = new CurrentLocation(http);
  }

  ngOnInit() {
    this.result = this.location.getCurrentLocation(this.lat, this.lon);
    this.result.then(function(result){
      console.log(result._body);
    })
    console.log(this.lat, this.lon);
    this.dataSource.connect();
  }
}

export class ExampleHttpDatabase {
  private restaurantUrl = 'api/restaurant'; // URL to web API
  getRestaurants(): Observable<Restaurant[]> {
    var result = this.http.get(this.restaurantUrl)
      .map(this.extractData);
    result.toPromise();
    return result;
  }

  extractData(result: Response): Restaurant[] {
    return result.json().map(restaurant => {
      return {
        id: restaurant.id,
        name: restaurant.restaurant_name,
        category: restaurant.category.map(c => c.categoryName).join(','),
        address: restaurant.address.address,
        city: restaurant.address.city.city_name
      }
    });
  }
  constructor(private http: Http) { }
}

export class CurrentLocation {
  constructor(private http: Http) { }
  private lat: any;
  private lon: any;
  private params = new URLSearchParams();
  private url = 'api/search/location';


  getPosition = () => {
    var latitude, longitude;
    return new Promise((resolve, reject) => {
      navigator.geolocation.getCurrentPosition((position) => {
        resolve(position.coords);
      }, (err) => {
        reject(err);
      });
    })
  }
  async getCurrentLocation(lat, lon): Promise<any> {
    let coords = await this.getPosition();
    lat = this.lat = coords['latitude'];
    lon = this.lon = coords['longitude'];
    this.params.set('lat', this.lat);
    this.params.set('lon', this.lon);
    var result = this.http.get(this.url, { search: this.params });
    return await result.toPromise();
  }
}

export class ExampleDataSource extends DataSource<Restaurant> {
  constructor(private _exampleDatabase: ExampleHttpDatabase) {
    super();
  }

  /** Connect function called by the table to retrieve one stream containing the data to render. */
  connect(): Observable<Restaurant[]> {
    return this._exampleDatabase.getRestaurants();
  }

  disconnect() { }
}

Sorry about the code is a little bit confusing.. But I don't know how to pass the result into connect() function to make Material Table works..

Full code also in Github: https://github.com/zhengye1/Eatr/tree/dev

Answer 1

All of the code in getCurrentLocation() will run before the callback given to navigator.geolocation.getCurrentPosition has a chance to run. You can see this by putting another console.log statement inside the callback:

navigator.geolocation.getCurrentPosition((position) => { 
  console.log("Got position", position.coords);
  this.lat = position.coords.latitude; 
  this.lon = position.coords.longitude;
});

There are a couple of reasons that getCurrentPosition needs to use a callback:

• The geolocation lookup might require the browser to query some online service to figure out your location
• Users might be given a prompt asking if they want to share their location with the website (this is the default behavior in most browsers)

The solution probably involves making a wrapper around getCurrentPosition so that it returns a promise. I don't know how Angular 2 observables work so I won't be able to help much in that part, but you could have getPosition return a promise like so:

getPosition = () => {
  return new Promise((resolve, reject) => {
    navigator.geolocation.getCurrentPosition((position) => { 
      resolve(position.coords);
    }, (err) => {
      reject(err);
    });
  });

And maybe getCurrentLocation could look like this (just guessing):

async getCurrentLocation(): PromiseLike<any> {
  let coords = await this.getPosition(this.lat, this.lon);
this.lat = coords['latitude'];
this.lon = coords['longitude'];
  this.params.set('lat', this.lat);
  this.params.set('lon', this.lon);
  var result = this.http.get(this.url, { search: this.params });
  return await result.toPromise();
}