如何检查Google API下的设备是否启用了位置服务

以下代码检查是否启用了位置.如果未启用,则显示警报对话框.

LocationManager lm = (LocationManager)getSystemService(Context.LOCATION_SERVICE);
    try {
        gps_enabled = lm.isProviderEnabled(LocationManager.GPS_PROVIDER);
    }catch (Exception ex){}
    try{
        network_enabled = lm.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
    }catch (Exception ex){}
    if(!gps_enabled && !network_enabled){
        AlertDialog.Builder dialog = new AlertDialog.Builder(this);
        dialog.setMessage(getResources().getString(R.string.gps_network_not_enabled));
        dialog.setPositiveButton(getResources().getString(R.string.open_location_settings), new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface paramDialogInterface, int paramInt) {                 
                Intent myIntent = new Intent( Settings.ACTION_LOCATION_SOURCE_SETTINGS);
                Startup.this.startActivity(myIntent);                    
            }
        });
        dialog.setNegativeButton(getString(R.string.Cancel), new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface paramDialogInterface, int paramInt) {
                // TODO Auto-generated method stub

            }
        });
        dialog.show();
    }