观察字典中的键/条目的可可键值

Question

可以观察字典中的特定键吗？如果是这样，我该怎么做？

Answer 1

是的（尽管只有在观察NSMutableDictionary时才有意义）。

@interface Foo : NSObject @end 
@implementation Foo 

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { 
    NSLog(@"observing: -[%@ %@]", object, keyPath); 
    NSLog(@"change: %@", change); 
} 

@end 

int main (int argc, const char * argv[]) { 
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; 

    Foo * f = [[Foo alloc] init]; 

    NSMutableDictionary * d = [NSMutableDictionary dictionary]; 
    [d addObserver:f forKeyPath:@"foo" options:0 context:NULL]; 
    [d setObject:@"bar" forKey:@"foo"]; 
    [d removeObjectForKey:@"foo"]; 
    [d removeObserver:f forKeyPath:@"foo"]; 
    [f release]; 

    [pool drain]; 
    return 0; 
} 

日志：

2010-12-21 17:39:53.758 EmptyFoundation[94589:a0f] observing: -[{ 
    foo = bar; 
} foo] 
2010-12-21 17:39:53.764 EmptyFoundation[94589:a0f] change: { 
    kind = 1; 
} 
2010-12-21 17:39:53.765 EmptyFoundation[94589:a0f] observing: -[{ 
} foo] 
2010-12-21 17:39:53.765 EmptyFoundation[94589:a0f] change: { 
    kind = 1; 
}