UIResponder differences from iOS4->iOS5

[Jon Brooks]

I found a curious difference in behavior running my app on an iOS4 device vs. 
running the same app on an iOS5 device.  On iOS4, if I set a view (or really 
just a UIResponder) to be firstResponder, and then later, resignFirstResponder, 
there is no defined firstResponder; whatever was firstResponder previously is 
clobbered.   Our app needed a more defined approach to this, so when a new 
firstResponder was being set, we attempt to iterate through all views to find 
and cache any existing firstResponder so we could restore it when our new 
firstResponder resigned its firstResponder status.   I have found that on iOS 
5, this seems to happen automatically:  becomeFirstResponder seems to cache any 
existing firstResponder and resignFirstResponder restores the cached 
responder's firstResponder status.  I can't find any Apple documentation that 
addresses this behavior at all.  

Is this behavior that can be relied upon?  My reading of the UIResponder 
documentation is that I shouldn't rely on the state of the current 
firstResponder, and that if my app needs a specific view to be firstResponder 
at a particular point in time, it should set it explicitly. On the other hand, 
trying to determine the current firstResponder is clunky with current API's, 
and if Apple's doing it for me, I'd rather let them do it.

I realize iOS 4 is ancient history at this point, though my app still needs to 
support it.  (I also have no idea what the behavior is on iOS 6.)

Below is some sample code that illustrates the different behavior:
@interface testResponder : UIResponder
@end

@implementation testResponder

- (UIResponder*)nextResponder
{
        return [UIApplication sharedApplication].keyWindow.rootViewController;
}

- (BOOL)canBecomeFirstResponder
{
        return YES;
}

@end

@implementation ViewController

-(BOOL) canBecomeFirstResponder
{
        return YES;
}

-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
        [self becomeFirstResponder];
        NSLog(@"View controller is now %@", ([self isFirstResponder] ? 
@"FirstResponder" : @"NOT First Responder!"));
        
        testResponder *newResponder = [[testResponder alloc] init];
        
        [newResponder becomeFirstResponder];
        NSLog(@"View controller is now %@", ([self isFirstResponder] ? 
@"FirstResponder" : @"NOT First Responder!"));
        [newResponder resignFirstResponder];
        
        NSLog(@"View controller is now %@\n\n\n", ([self isFirstResponder] ? 
@"FirstResponder" : @"NOT First Responder!"));
        
        [newResponder release];
        
}

@end


Output on iOS4:
2013-01-07 12:45:44.999 testFirstResponderTwo[835:607] View controller is now 
FirstResponder
2013-01-07 12:45:45.007 testFirstResponderTwo[835:607] View controller is now 
NOT First Responder!
2013-01-07 12:45:45.010 testFirstResponderTwo[835:607] View controller is now 
NOT First Responder!

Output on iOS5:
2013-01-07 12:42:57.858 testFirstResponderTwo[1886:707] View controller is now 
FirstResponder
2013-01-07 12:42:57.862 testFirstResponderTwo[1886:707] View controller is now 
NOT First Responder!
2013-01-07 12:42:57.865 testFirstResponderTwo[1886:707] View controller is now 
FirstResponder


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