iOS Google Map Construct the ruler to show the scale

I am trying to create the map scale ruler on Google Map to show the scale ratio (1 cm to XXXX m ). The custom ruler should works like what Google map scale ruler is. When it comes to the testing, I have found that the number is always changing and increases linearly when zooming in or out. I know the Google map ruler does not want this happen to report fixed value until zoom level overflow happens. Could you please tell me how to enhance it ?

The below is my working

- (double ) mapViewRation :(GMSMapView*)mapView idleAtCameraPosition:(GMSCameraPosition*)position
{
    CLLocationCoordinate2D topLeft = mapView.projection.visibleRegion.farLeft;
    CLLocationCoordinate2D bottomLeft = mapView.projection.visibleRegion.nearLeft;
    double lat = fabs(topLeft.latitude - bottomLeft.latitude);
    double mpp = cos(lat * M_PI / 180) * 2 * M_PI * 6378137 / (256 * pow(2, mapView.camera.zoom));
    double distance = mpp * mapView.frame.size.width;
    return distance; 
}




-(void)mapView:(GMSMapView *)mapView didChangeCameraPosition:(GMSCameraPosition*)position {

    // handle you zoom related logic

    NSLog(@"zoom :  %f"  , position.zoom );

    double asK =   [self mapViewRation : self.myMapView  idleAtCameraPosition : position ];

    double aasK=   fmod(asK, 300);
    UIView* lineView;
    if(lineView==nil){
        lineView = [[UIView alloc] initWithFrame:CGRectMake(100, 200, 400 , 1)];
        lineView.backgroundColor = [UIColor blackColor];
    }

    CGRect make = CGRectMake(100, 200, 400-aasK , 1) ;
    [lineView setFrame:make ];
        UILabel* zeroLabel;

    if(zeroLabel==nil){
        zeroLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 10, 400-aasK, 10)];
        [zeroLabel setTextColor:[UIColor blackColor]];
        [zeroLabel setBackgroundColor:[UIColor clearColor]];
        [zeroLabel setFont:[UIFont fontWithName: @"Trebuchet MS" size: 14.0f]];
    }
    [zeroLabel setText:[NSString stringWithFormat:@"%.1f" ,1.0f]];

        UILabel* numberLabel;
    if(numberLabel==nil){
        numberLabel = [[UILabel alloc] initWithFrame:CGRectMake(380-aasK, 10, 400-aasK, 10)];
        [numberLabel setTextColor:[UIColor blackColor]];
        [numberLabel setBackgroundColor:[UIColor clearColor]];
        [numberLabel setFont:[UIFont fontWithName: @"Trebuchet MS" size: 14.0f]];
    }
    CGRect maaake = CGRectMake(380-aasK, 10, 400-aasK , 10) ;
    [numberLabel setFrame:maaake ];
    [numberLabel setText:[NSString stringWithFormat:@"%f" ,position.zoom]];

    [lineView addSubview:numberLabel];
    [lineView addSubview:zeroLabel];
    [self.view addSubview:lineView];




}

via Chebli Mohamed