objective c - Subclassing UIButton but can't access my properties

Question

I've created a sub class of UIButton:

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

@interface MyDetailButton : UIButton {
    NSObject *annotation;
}

@property (nonatomic, retain) NSObject *annotation;


@end

//
//  DetailButton.m
//


#import "MyDetailButton.h"


@implementation MyDetailButton


@synthesize annotation;

@end

I figured that I can then create this object and set the annotation object by doing the following:

MyDetailButton* rightButton = [MyDetailButton buttonWithType:UIButtonTypeDetailDisclosure];
rightButton.annotation = localAnnotation;

localAnnotation is an NSObject but it is really an MKAnnotation. I can't see why this doesn't work but at runtime I get this error:

 2010-05-27 10:37:29.214 DonorMapProto1[5241:207] *** -[UIButton annotation]: unrecognized selector sent to instance 0x445a190
2010-05-27 10:37:29.215 DonorMapProto1[5241:207] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[UIButton annotation]: unrecognized selector sent to instance 0x445a190'

'

I can't see why it's even looking at UIButton because I've subclassed that so it should be looking at the MyDetailButton class to set that annotation property. Have I missed something really obvious. It feels like it :)

Thanks in advance for any help you can provide

Ross

Answer 1

UIButton is a class cluster, which implies that Apple's implementation of buttonWithType: probably looks something like this:

+(id)buttonWithType:(UIButtonType)t {
  switch (t) {
    case UIButtonTypeDetailDisclosure:
      return [[[PrivateDetailDisclosureButtonClass alloc] init] autorelease];
    case ...
  }
}

So when you call [MyDetailButton buttonWithType:UIButtonTypeDetailDisclosure]; you don't get an instance of MyDetailButton, you get an instance of PrivateDetailDisclosureButtonClass (or whatever Apple actually calls it).

Note, however, that you can get buttonWithType to instantiate a subclass if you call it with UIButtonTypeCustom (At least in the simulator running v3.0):

// LGButton is a straightforward subclass of UIButton
LGButton *testBtn = [LGButton buttonWithType:UIButtonTypeCustom];
LGButton *testBtn2 = [LGButton buttonWithType:UIButtonTypeDetailDisclosure];
NSLog(@"testBtn: %@, testBtn2: %@", [testBtn class], [testBtn2 class]);
// Output: testBtn: LGButton, testBtn2: UIButton