iOS沒有提供原生方法去直接判斷對象是否包含某個觀察者，只能自己寫個方法去判斷

-(BOOL)targetObj:(NSObject *)obj hasObserverKey:(NSString *)keyPath{
    if (obj == nil) {
        return NO;
    }
    if (obj.observationInfo == nil) {
        return NO;
    }
    NSObject *observances = [((__bridge NSObject*)(obj.observationInfo))valueForKeyPath:@"observances"];
    if ([observances isKindOfClass:[NSArray class]]) {
        for (NSObject *observance in (NSArray *)observances) {
            NSString *inKeyPath = [NSString stringWithFormat:@"Key path: %@",keyPath];
            NSString *objString = [NSString stringWithFormat:@"%@",observance];
            if ([objString containsString:inKeyPath]) {
                return YES;
            }
        }
    }
    return NO;
}