09. Location

Location sources:

• GPS
  • ~5-10 m accuracy
  • Timing precision in the order of 20 ns
    • Requires special/general theories of relativity to be taken into effect
• Cell-ID (cell tower triangulation)
• Wi-Fi network

Android has three methods to obtain location:

• GPS
  • Most accurate and most energy intensive
  • Only works outdoors
• Network Location Provider: combines Cell-ID and Wi-Fi data
• Passive: piggybacks off other applications

Permissions:

<manifest>
  // Foreground: one-off access to location
  <uses-permission
    android:name="android.permission.ACCESS_COARSE_LOCATION"
  />
  // If you want fine access, you must also request coarse
  // Gives you accses to GPS location
  <uses-permission
    android:name="android.permission.ACCESS_FINE_LOCATION"
  />
  // Background: requires permission on API level 29 and higher
  <uses-permission
    android:name="android.permission.ACCESS_BACKGROUND_LOCATION"
  />
</manifest>

Older approach was easy to get wrong: forgetting to disable it and leading to large battery drain:

• Use LocationManager to get access to system location services
  • getSystemService(Context.LOCATION_SERVICE)
• Implement LocationListener subclass: override onLocationChanged, onStatusChanged, onProviderEnabled, onProviderDisabled methods

Newer approach uses Google Play Services and provides a higher-level API:

• Provides Fused Location Provider: combines multiple location sources in a unified interface

• fusedLocationClient = LocationServices.getFusedLocationProviderClient(this)
  fusedLocationClient.lastLocation.addOnSuccessListener {location: Location? -> 
    // Check to ensure accuracy hasn't dropped significantly (e.g. if phone switched from GPS to network)
  }
  

• Can also request location requests using LocationCallback:onLocationResult
• Geofencing
  • Set entrance and exit events
  • Max. 100 geofences per app
  • Requires fine location, background location permissions