Question

This is probably more of an objective-c question over iOS but I’ve seen some example code similar to the following that I’d like to better understand.

@interface MyMapView : MKMapView <MKMapViewDelegate> {
// ivars specific to derived class
}

@property(nonatomic,assign) id<MKMapViewDelegate> delegate;

@end

@implementation MyMapView
- (id) initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self)
    {
        // initialize the ivars specific to this class

        // Q1: Why invoke super on this delegate that's also a property of this class?    
        super.delegate = self;

        zoomLevel = self.visibleMapRect.size.width * self.visibleMapRect.size.height;
    }
    return self;
}
#pragma mark - MKMapViewDelegate methods
// Q2: Why intercept these callbacks, only to invoke the delegate?
- (void)mapView:(MKMapView *)mapView regionWillChangeAnimated:(BOOL)animated
{
    if( [delegate respondsToSelector:@selector(mapView:regionWillChangeAnimated:)] )
    {
        [delegate mapView:mapView regionWillChangeAnimated:animated];
    } 
}

@end

My two questions are:
1. Why would one invoke the super.delegate and also only declare the ‘delegate’ as a property?
2. Why intercept all of the delegate calls only to forward them back to the delegate?

I appreciate any insights.

Answer 1

Apple’s documentation explicitly states that you should avoid subclass MKMapView:

http://developer.apple.com/library/ios/#documentation/MapKit/Reference/MKMapView_Class/MKMapView/MKMapView.html#//apple_ref/doc/uid/TP40008205

Although you should not subclass the MKMapView class itself, you can
get information about the map view’s behavior by providing a delegate
object.

So i guess this delegate “forward” pattern is used to not break things.

I use a little different approach to subclass MKMapView. To minimize breakage i use two classes. One that subclass MKMapView and just override the init/dealloc method and assign/release the delegate property to a instance of the other class. The other class is a subclass of NSObject that implements the MKMapViewDelegate protocol and will be the one that does the real work.

MyMapView.h

@interface MyMapView : MKMapView
@end

MyMapView.m

// private map delegate class
@interface MapDelegate : NSObject <MKMapViewDelegate>
// instance is not alive longer then MKMapView so use assign to also solve
// problem with circular retain
@property(nonatomic, assign) MKMapView *mapView;
@end

@implementation MapDelegate
@synthesize mapView;

- (id)initWithMapView:(ReportsMapView *)aMapView {
  self = [super init];
  if (self == nil) {
    return nil;
  }

  self.mapView = aMapView;

  return self;
}

// MKMapViewDelegate methods and other stuff goes here

@end

@implementation MyMapView
- (id)init {
  self = [super init];
  if (self == nil) {
    return nil;
  }

  // delegate is a assign property
  self.delegate = [[MapDelegate alloc] initWithMapView:self];

  return self;
}

- (void)dealloc {
  ((MapDelegate *)self.delegate).mapView = nil;
  [self.delegate release];
  self.delegate = nil;

  [super dealloc];
}
@end

The mapView property for MapDelegate class is not strictly needed but is probably useful if want to do things to the map view that that is not a result of some MKMapViewDelegate method call, timers etc.