caching whether a delegate responds to a selector

delegate_responds_cache.m

@protocol SomeDelegate <NSObject>
- (void)someMethod:(id)item;
@end

@interface SomeInterface : NSObject
@property (nonatomic, weak) id <SomeDelegate> delegate;
@end

@implementation SomeInterface {
  struct {
    unsigned int someMethod:1;
  } delegateRespondsTo;
}

- (void)setDelegate:(id <SomeDelegate>)delegate {
  if (_delegate != delegate) {
    _delegate = delegate;
    
    delegateRespondsTo.someMethod = [_delegate respondsToSelector:@selector(someMethod:)];
  }
}
@end