You should listen to the AGSMapView's notifications for this.Here the doc from above link...The map component also broadcasts four notifications - MapViewDidLoad, MapTimeExtentChanged, MapDidEndPanning and MapDidEndZooming. Observers interested in these notifications can register for them using the default NSNotificationCenter.
Here is the code...//add this code line in your viewDidLoad or mapViewDidLoad
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(respondToZoom:) name:@"MapDidEndZooming" object:nil];
//add this to your .m file
- (void)respondToZoom:(NSNotification*)notification {
NSLog(@"map scale: %.6f",self.mapView.mapScale);
}
Hope this helps!Regards,Nimesh