CAKeyframeAnimation *keyPosi=[CAKeyframeAnimation animationWithKeyPath:@"position"];
keyPosi.path=path.CGPath;
keyPosi.delegate=self;
[label.layer addAnimation:keyPosi forKey:@"x"];

- (void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag{}

[keyPosi setValue:label forKey:@"xx"];

- (void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag{

if (flag) {

UIView *vie=[anim valueForKey:@"xx"];

[vie removeFromSuperview];

}

}

NSObject *obj = [[NSObject alloc] init];
[obj setValue:@"asd"  forKey:@"xx"];

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<NSObject 0xf65f090> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key xx.'

setValue:forUndefinedKey:
Invoked by setValue:forKey: when it finds no property for a given key.- (void)setValue:(id)value forUndefinedKey:(NSString *)key
Discussion
Subclasses can override this method to handle the request in some other way. The default implementation raises an NSUndefinedKeyException.

#import <objc/runtime.h>
@interface NSObject (KVC)
- (id)valueForUndefinedKey:(NSString *)key;
- (void)setValue:(id)value forUndefinedKey:(NSString *)key;
@end@implementation NSObject(KVC)
- (id)valueForUndefinedKey:(NSString *)key{return objc_getAssociatedObject(self, key);
}
- (void)setValue:(id)value forUndefinedKey:(NSString *)key{if ([value isKindOfClass:[NSString class]]) {objc_setAssociatedObject(self, key, value, OBJC_ASSOCIATION_COPY_NONATOMIC);}else{objc_setAssociatedObject(self, key, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}}
@end

NSObject *obj = [[NSObject alloc] init];
[obj setValue:@"asd"  forKey:@"xx"];
NSLog(@"%@",[obj valueForKey:@"xx"]);

NSString *a=[NSString stringWithFormat:@"hello"];
NSString *b=[NSString stringWithFormat:@"hello"];
[self setValue:@"cannotfind" forKey:a];
NSString *result=[self valueForKey:b];
NSLog(@"%@",result);

static NSString *kHello=@"hello";

[self setValue:@"canfind" forKey:kHello];
NSString *result=[self valueForKey:kHello];
NSLog(@"%@",result);

static char kHello;- (id)valueForUndefinedKey:(NSString *)key{const void *newkey=nil;if ([key isEqualToString:@"hello"]) {newkey=&kHello;}else{NSAssert(false, @"undefined key");}return objc_getAssociatedObject(self, newkey);
}
- (void)setValue:(id)value forUndefinedKey:(NSString *)key{const void *newkey=nil;if ([key isEqualToString:@"hello"]) {newkey=&kHello;}else{NSAssert(false, @"undefined key");}if ([value isKindOfClass:[NSString class]]) {objc_setAssociatedObject(self, newkey, value, OBJC_ASSOCIATION_COPY_NONATOMIC);}else{objc_setAssociatedObject(self, newkey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}}