Enable Location Service on Onclick event in Android?

Question

In my application I want to get the location of the user. If the location service is on it will get the location. Otherwise it will call turnGPSon but this method it not working.

public class MainActivity extends Activity {

    Button btnShowLocation;

    GPSTracker gps;
    Context ctx;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btnShowLocation = (Button) findViewById(R.id.show_location);

        btnShowLocation.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                gps = new GPSTracker(MainActivity.this);

                if (gps.canGetLocation()) {
                    double latitude = gps.getLatitude();
                    double longitude = gps.getLongitude();

                    Toast.makeText(
                            getApplicationContext(),
                            "Your Location is -\nLat: " + latitude + "\nLong: "
                                    + longitude, Toast.LENGTH_LONG).show();
                } else {
                   // gps.showSettingsAlert();
                    turnGPSOn();

                }
            }
        });
    }
    public void turnGPSOn()
    {
        try{
            String provider = Settings.Secure.getString(getContentResolver(), Settings.Secure.LOCATION_PROVIDERS_ALLOWED);

            if(!provider.contains("gps"))
            { //if gps is disabled
                final Intent poke = new Intent();
                poke.setClassName("com.android.settings", "com.android.settings.widget.SettingsAppWidgetProvider");
                poke.addCategory(Intent.CATEGORY_ALTERNATIVE);
                poke.setData(Uri.parse("3"));
                sendBroadcast(poke);
            }
        }catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

this is my code to show dialog box for manually on Location Service

public void showSettingsAlert() {
    AlertDialog.Builder alertDialog = new AlertDialog.Builder(context);
    alertDialog.setTitle("GPS is settings");
    alertDialog.setMessage("GPS is not enabled. Do you want to go to settings menu?");

    alertDialog.setPositiveButton("Settings", new DialogInterface.OnClickListener() {           
        @Override
        public void onClick(DialogInterface dialog, int which) {
            Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
            context.startActivity(intent);
        }
    });

    alertDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
        }
    });

    alertDialog.show();
}