iOS - How to limit the MapView to a specific region?

I have the following problem:

I have a "drawn map" (image) which I add to the MapView as an Overlay. No Problem with that.. but I need to limit the MapView to the region of the Overlay, so a user isn't able to scroll/zoom outside of this region.. but it should be possible to scroll/zoom inside the "bounds" of the overlay - means I cannot just disable zoom/scrolling for the MapView.

Are there any ideas/solution on this topic? The reason for using the MapView/-Kit is that I need to add various POIs to the custom map. This may become more complex when just using an ImageView+ScrollView for presenting the custom map.

I've researched alot on this topic, but I didn't found a nice solution.

Any help is appreciated!

Best Regards, Christian

Edit: This is our solution: You supply a topleft and a bottomright coordinate to limit the map. The (minimum) zoomlevel is also limited. I've deactivated decelerating and you are able to bounce a bit out of the map (for better performance/ux). I added a ~1km grey border to the overlay so the user isnÄt able to see the orignal worldmap of google.

LimitedMapView.h:

#import <Foundation/Foundation.h>
#import <MapKit/MapKit.h>


@interface LimitedMapView : MKMapView <UIScrollViewDelegate>{

}

@property (nonatomic, assign) CLLocationCoordinate2D topLeftCoordinate;
@property (nonatomic, assign) CLLocationCoordinate2D bottomRightCoordinate;


@end

LimitedMapView.m:

#import "LimitedMapView.h"

@implementation LimitedMapView

@synthesize topLeftCoordinate, bottomRightCoordinate;

- (void)scrollViewDidZoom:(UIScrollView *)scrollView{

    if([super respondsToSelector:@selector(scrollViewDidZoom:)]) [super scrollViewDidZoom:scrollView];

    if ([self region].span.latitudeDelta > 0.002401f || [self region].span.longitudeDelta > 0.003433f) {

        CLLocationCoordinate2D center = self.centerCoordinate;
        MKCoordinateSpan span = MKCoordinateSpanMake(0.002401f, 0.003433f);

        self.region = MKCoordinateRegionMake(center, span);

    }

}

-(void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{

    if([super respondsToSelector:@selector(scrollViewDidEndDragging:)]) [super scrollViewDidEndDragging:scrollView willDecelerate:decelerate];

    MKCoordinateRegion currentRegion = self.region;
    bool changeRegionLong = YES;
    bool changeRegionLat = YES;

    // LONGITUDE    
    if((currentRegion.center.longitude - (currentRegion.span.longitudeDelta/2)) < self.topLeftCoordinate.longitude) {

        currentRegion.center.longitude = (topLeftCoordinate.longitude + (currentRegion.span.longitudeDelta/2));

    } else if((currentRegion.center.longitude + (currentRegion.span.longitudeDelta/2)) > self.bottomRightCoordinate.longitude) {

        currentRegion.center.longitude = (bottomRightCoordinate.longitude - (currentRegion.span.longitudeDelta/2));

    } else {

        changeRegionLong = NO;

    }

    // LATITUDE    
    if((currentRegion.center.latitude + (currentRegion.span.latitudeDelta/2)) > self.topLeftCoordinate.latitude) {

        currentRegion.center.latitude = (topLeftCoordinate.latitude - (currentRegion.span.latitudeDelta/2));

    } else if((currentRegion.center.latitude - (currentRegion.span.latitudeDelta/2)) < self.bottomRightCoordinate.latitude) {

        currentRegion.center.latitude = (bottomRightCoordinate.latitude + (currentRegion.span.latitudeDelta/2));

    } else {

        changeRegionLat = NO;

    }

    if(changeRegionLong || changeRegionLat) [self setRegion:currentRegion animated:YES];

}

-(void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView{

    [scrollView setContentOffset:scrollView.contentOffset animated:YES];
}

@end