Question

So I’m pretty new to Xcode and Objective-C. I have several ArtPiece objects with various attributes stored in an array. I then add them as MKAnnotations to the mapview. What I need to be able to do is send a reference to the array position they came from on click. I believe MKAnnotations only have the data members title, image, and location, so when I make an MKAnnotation from the objects, the annotation does not keep any of the other attributes. So, my question is, how can I manage to keep a reference to the array position of the object the annotation was created from so that I can send it to other methods so they can retrieve the additional information about the object from the array for detail views etc. Is there any way to store a single int value in an annotation? I don’t think there is so any other ideas?

Here is my ArtPiece.h:

#import <Foundation/Foundation.h>
#import <MapKit/MapKit.h>
@interface ArtPiece : NSObject <MKAnnotation>{

    NSString *title;
    NSString *artist;
    NSString *series;
    NSString *description;
    NSString *location;
    NSString *area;
    NSNumber *latitude;
    NSNumber *longitude;
    UIImage *image;
}

@property (nonatomic, retain) NSString *title;
@property (nonatomic, retain) NSString *artist;
@property (nonatomic, retain) NSString *series;
@property (nonatomic, retain) NSString *description;
@property (nonatomic, retain) NSString *location;
@property (nonatomic, retain) NSString *area;
@property (nonatomic, retain) NSNumber *latitude;
@property (nonatomic, retain) NSNumber *longitude;
@property (nonatomic, retain) UIImage *image;


@end

and here is the .m:

#import "ArtPiece.h"
#import "FindArtController.h"
#import "ArtPiece.h"
#import <MapKit/MapKit.h>

@implementation ArtPiece

- (NSString *)description{
    return [NSString stringWithFormat:@"title: %@",title];
}

@synthesize title, artist, series, description, latitude, longitude, location, area, image;

- (CLLocationCoordinate2D)coordinate
{
    CLLocationCoordinate2D theCoordinate;
    theCoordinate.latitude = [self.latitude doubleValue];
    theCoordinate.longitude = [self.longitude doubleValue];
    return theCoordinate;
}




@end

I then go on to make objects of that class, set their values, and add them to an array in the AppDelegate. Then, in another class, I use:

[self.mapView addAnnotations:mainDelegate.mapAnnotations];

to add the annotations to the map. However, when I try to set the “artist” attribute in the viewforannotation method, I get “request for member artist in something not a structure or union.”

Obviously, I must be doing this subclassing thing wrong, but what do I change?

Here is the viewforannotation method I have now. The test.artist… line is not working.

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation{
    MKAnnotationView *test=[[MKAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"reuse"];
    test.image = [UIImage imageNamed:@"pao_pin.png"];
    [test setCanShowCallout:YES];
    [test setUserInteractionEnabled:YES];
    test.rightCalloutAccessoryView =
    [UIButton buttonWithType:UIButtonTypeDetailDisclosure];
    test.artist = annotation.artist;
    return test;    
}

So, should I: annotation = (ArtPiece *)annotation
In short, my goal is to be able to store a reference to the array position of the object in this annotation was made from so that I can send that to other methods who can access that object for detail views etc.

Answer 1

In the viewForAnnotation method, you can access your ArtPiece properties but to avoid compiler errors and warnings, you’ll need to first cast the annotation parameter to your custom class. The annotation parameter in that method is just defined as a id<MKAnnotation> so the compiler won’t know about the ArtPiece-specific properties (until you tell it that is an instance of ArtPiece).

You’ll need something like this:

ArtPiece *artPiece = (ArtPiece *)annotation;
NSString *artist = artPiece.artist;

Edit:
When an annotation view’s callout button is pressed, the map view calls the calloutAccessoryControlTapped delegate method. This method gets passed the MKAnnotationView in the view parameter. The annotation object itself is in the annotation property of the view parameter. You can cast that object to your custom class to access your ArtPiece properties:

- (void)mapView:(MKMapView *)mapView annotationView:(MKAnnotationView *)view calloutAccessoryControlTapped:(UIControl *)control
{
    ArtPiece *artPiece = (ArtPiece *)view.annotation;
    NSString *artist = artPiece.artist;
}

In other words, the callout button handler doesn’t need to access the original array. It gets a reference to the actual ArtPiece object itself.