respondsToSelector fails for appearance proxy

问题:

I’m trying to detect an iOS 6-specific appearance method, by running respondsToSelector on the [UIBarButtonItem appearance]. However, it always returns NO for me, whatever selector I specify:

// Should show NOPE in iOS 5, YEP in iOS 6. Shows NOPE always
NSLog(@"%@", [[UIBarButtonItem appearance] respondsToSelector:@selector(setBackgroundImage:forState:style:barMetrics:)] ? @"YEP" : @"NOPE"); 

// Should show YEP in both iOS 5 and iOS 6. Shows NOPE always
NSLog(@"%@", [[UIBarButtonItem appearance] respondsToSelector:@selector(setBackgroundImage:forState:barMetrics:)] ? @"YEP" : @"NOPE"); 

Actually using those methods works fine on their respective versions of iOS, but I can’t seem to detect which one is available to me. So how do I properly do that?

回答1:

Don't check the appearance proxy. You can never rely on that, since it's a proxy. Instead, directly check the item that has the new method, in this case, the UIBarButtonItem:

BOOL hasNewMethod = [UIBarButtonItem instancesRespondToSelector:@selector(setBackgroundImage:forState:style:barMetrics:)];
if( hasNewMethod )
  NSLog(@"Running iOS 6 with new method");
else
  NSLog(@"Current OS doesn't support method...");