Android Tutorial GPS tutorial

Erel · Nov 14, 2010

This example shows how to work with the GPS library.

The GPS object is declared in the Starter service. It is easier to put the GPS in a service and not in an Activity as the services life cycle is simpler.

When the program starts we check whether the location features are enabled:

B4X:

Sub Activity_Resume
   If Starter.GPS1.GPSEnabled = False Then
       ToastMessageShow("Please enable the GPS device.", True)
       StartActivity(Starter.GPS1.LocationSettingsIntent) 'Will open the relevant settings screen.
   Else
       Starter.rp.CheckAndRequest(Starter.rp.PERMISSION_ACCESS_FINE_LOCATION)
       Wait For Activity_PermissionResult (Permission As String, Result As Boolean)
       If Result Then CallSubDelayed(Starter, "StartGPS")
   End If
End Sub

If not then we open the relevant settings screen.

The next step is to check for the ACCESS_FINE_LOCATION permission (runtime permissions tutorial).
If we have permission then we call the StartGPS sub from the Starter service.

Now all that is left is to delegate the LocationChanged event from the service to the activity and show the information.

B4X:

'Starter service
Sub GPS_LocationChanged (Location1 As Location)
   CallSub2(Main, "LocationChanged", Location1)
End Sub

'Main activity
Public Sub LocationChanged(Location1 As Location)
   lblLat.Text = "Lat = " & Location1.ConvertToMinutes(Location1.Latitude)
   lblLon.Text = "Lon = " & Location1.ConvertToMinutes(Location1.Longitude)
   lblSpeed.Text = $"Speed = $1.2{Location1.Speed} m/s "$
End Sub

Due to Google policy change you also need to add this line to the manifest editor:

B4X:

AddManifestText(<uses-feature android:name="android.hardware.location.gps"/>)

This will prevent the app from being installed on devices without GPS hardware.

The example is attached.

Modified example with NMEA listener: https://www.b4x.com/android/forum/t...-not-firing-on-android-10.112140/#post-699351

Erel · Aug 4, 2012

Can you post your code? Is your time zone set correctly?

GaNdAlF89 · Aug 4, 2012

How I can set my time zone?

In my code, I've modified only with

B4X:

DateTime.DateFormat = "dd/MM/yyyy"
today=DateTime.Date(GPS_Timestamp)
...

Erel · Aug 4, 2012

DateTime.Date adds the system time zone. If the device time zone is wrong then the date string might also be wrong. You can set it to 0 by calling DateTime.SetTimeZone.

GaNdAlF89 · Aug 4, 2012

ok..thanks for all!

mjtaryan · Aug 4, 2012

I think this is quite useful. However, for my current purposes I'm wondering if there is a way to read the current device location from the system? If so, how? Thanks.

Erel · Aug 4, 2012

You can use ABWifi library to get the last known location.

GaNdAlF89 · Aug 6, 2012

B4X:

Sub Service_Create
    DateTime.SetTimeZone(1)
End Sub


Sub MyGPS_LocationChanged (MyLocation As Location)

   TimestampGPS = MyLocation.Time
   
   DateGPS= DateTime.Date(TimestampGPS)
   TimeGPS= DateTime.Time(TimestampGPS)
End Sub

Timezone of device is set to +1. I receive the date of tomorrow, and the time of an hour ago. Please help me!

Erel · Aug 6, 2012

It seems like a problem in your device.

I tested the following code on multiple devices and it works as expected:

B4X:

Sub Process_Globals
   Dim g As GPS
End Sub

Sub Globals

End Sub
Sub Activity_Create(FirstTime As Boolean)
   If FirstTime Then
      g.Initialize("g")
      g.Start(0, 0)
   End If
End Sub
Sub G_LocationChanged (Location1 As Location)
   ToastMessageShow(DateTime.Date(Location1.Time), True)
End Sub

GaNdAlF89 · Aug 6, 2012

What can I do to resolve this problem?

Erel · Aug 7, 2012

Which device are you using (type and OS version)?

GaNdAlF89 · Aug 7, 2012

my device is:

Samsung Galaxy Tab 7.0 Plus - Android 3.2 Honeycomb

Erel · Aug 9, 2012

Not sure if and how it can be resolved.

phast · Aug 14, 2012

I am very new to this, so sorry if this is completely wrong. I am trying to find the distance between a set lat and lon against where I am currently located. My current code is:

B4X:

Sub GPS_LocationChanged (Location1 As Location)
   lblLat.Text = "Lat = " & Location1.ConvertToMinutes(Location1.Latitude)
   lblLon.Text = "Lon = " & Location1.ConvertToMinutes(Location1.Longitude)
   Location2.Latitude="40.4406"
   Location2.Longitude="79.9961"
   lblDist.Text = Location1.DistanceTo(Location2)
   'lblSpeed.Text = "Speed = " & Location1.Speed
End Sub

I have dim Location2 as Location

When I start the app it says "invalid double"

EDIT::

Sorry, I solved my own issue by removing the "" which I now assume makes the value a string?

michelg · Aug 22, 2012

Erel said:
You can't enable or disable the GPS device. However if it is enabled it will turn on when it needed and off when it is not needed (to save battery).

Hi,

I was trying to run the GPS app in the emulator. when I run the app,
on the emulator's screen this is what I see:
Lat:
Lon:
Speed:
and a quick message saying
GPS device enable=True

However I do not see any location information.

what should I do to see the location information?

Erel · Aug 22, 2012

You should test it on a real device.

GaNdAlF89 · Aug 22, 2012

@Erel

I have reset my device, I set the timezone in settings to +1 (I'm italian).

In Main Activity:

B4X:

Sub Activity_Create
    DateTime.SetTimeZone(1)
    DateTime.DateFormat = "dd/MM/yyyy" 'I prefer this format
End Sub

In GPS_Service

B4X:

Sub MyGPS_LocationChanged (MyLocation As Location)
   GlobalModule.Latitudine = MyLocation.Latitude
   GlobalModule.Longitudine = MyLocation.Longitude   
   GlobalModule.Velocita = MyLocation.Speed*3.6 'I convert to km/h
   GlobalModule.TimestampGPS = MyLocation.Time

   'now I convert timestampGPS in date and time
   GlobalModule.DateGPS = DateTime.Date(GlobalModule.TimestampGPS)
   GlobalModule.TimeGPS = DateTime.Time(GlobalModule.TimestampGPS)
End Sub

I receive the date of tomorrow, and the time of an hour ago...I don't understand what's the problem!
Please help me

Erel · Aug 23, 2012

I'm sorry but it seems to be a problem in your device. Maybe updating the OS will fix it.

GaNdAlF89 · Aug 23, 2012

I tried to convert the gps timestamp with an online converter, and I obtain the same result of my app...why??

Erel · Aug 23, 2012

Because your device returns the wrong result.

michelg · Aug 24, 2012

Erel said:
The GPS is an important feature of many Android devices.
Fortunately it is pretty easy to work with it.

Hi,

when I run the GPS program in the emulator this is what I see:

Lat:
Lon:
Speed:
Satellites:

I do not see any information:

I used b4a bridge to run it in my phone and the result is the same.

Can you please give me an idea of what could be wrong.

Thank you,

Android Tutorial GPS tutorial

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