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Basic KVC/KVO Implementation
#import <Foundation/Foundation.h>
static void* theContext = &theContext;
/// KVC/KVO 호환 클래스.
@interface Foo: NSObject
@property (copy, nonatomic) NSString* moo;
@implementation Foo
- (NSString *)moo
if(!_moo) {
_moo = @"hello";
return _moo;
/// 문자열을 두번 반복하도록 확장하는 메소드.
/// 수동으로 KVO 호환이 되도록 한다.
[self willChangeValueForKey:@"bar"];
_moo = [NSString
stringWithFormat:@"%@%@", _moo, _moo];
[self didChangeValueForKey:@"bar"];
/// 옵저버 클래스
@interface Bar: NSObject
@implementation Bar
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
if(context == theContext) {
NSLog(@"the foo's bar has changed from:%@ to:%@",
[change objectForKey:NSKeyValueChangeOldKey],
[change objectForKey:NSKeyValueChangeNewKey]
} else {
[super observeValueForKeyPath:keyPath
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
Foo* foo = [[Foo alloc] init];
NSLog(@"foo's bar is set for initial time.");
foo.moo = @"one";
Bar* x = [[Bar alloc] init];
[foo addObserver:x
foo.moo = @"two";
[foo doubleBar];
[foo removeObserver:x forKeyPath:@"bar"];
return 0;
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