KVO 存在 key 依赖的情况

重写方法
keyPathsForValuesAffectingValueForKey
或者
keyPathsForValuesAffecting<Key> (推荐)
上面两个方法可以解决 key 依赖的情况

key 依赖

类 KVO_DependentKeys 有三个属性，fullName,firstName,lastName
fullName 依赖于其他两个属性 firstName,lastName

KVO_DependentKeys.m 实现

    - (NSString *)fullName {
        return [NSString stringWithFormat:@"%@ %@",_firstName, _lastName];
    }
    // 推荐使用 keyPathsForValuesAffecting<Key> ， 比下面的方法要好 keyPathsForValuesAffectingValueForKey
    + (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key{
        // 必须先调用父类方法
        NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
        if ([key isEqualToString:@"fullName"]) {
            NSArray *affectingKeys = @[@"lastName",@"firstName"];
            keyPaths = [keyPaths setByAddingObjectsFromArray:affectingKeys];
        }
        return keyPaths;
    }
    //keyPathsForValuesAffecting<Key>
    + (NSSet<NSString *> *)keyPathsForValuesAffectingFullName{
        return [NSSet setWithObjects:@"lastName",@"firstName", nil];
    }

外部调用

 KVO_DependentKeys *obj = [[KVO_DependentKeys alloc] init];
    [obj addObserver:self forKeyPath:@"fullName" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    
    obj.firstName = @"wang";
    obj.lastName = @"bin bin";
    NSLog(@"fullName = %@",obj.fullName);
    
    [obj removeObserver:self forKeyPath:@"fullName"];

keyPath 依赖

如果 KVO_DependentKeys 的属性 information 依赖于 另外一个属性内部的属性 target.age 和 target.grade ,也可以使用上面的机制实现 kvo

KVO_target 的属性

@property (nonatomic, readwrite) int age;
@property (nonatomic, readwrite) int grade;

KVO_DependentKeys 的属性

@property (nonatomic, strong) NSString *information;  /// 信息
@property (nonatomic, strong) KVO_target *target;  /// keyPath 依赖

KVO_DependentKeys .m 文件

+ (NSSet<NSString *> *)keyPathsForValuesAffectingInformation{
    return [NSSet setWithObjects:@"target.age",@"target.grade", nil];
}

外部调用

// keyPath 依赖
    KVO_target *target;
    {
        target = [[KVO_target alloc] init];
        target.age = 20;
        target.grade = 5;
    }
    
    KVO_DependentKeys *objKeyPath = [[KVO_DependentKeys alloc] init];
    {
        objKeyPath.target = target;
        [objKeyPath addObserver:self forKeyPath:@"information" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
        target.age = 22;
        target.grade = 7;
        NSLog(@"information = %@",objKeyPath.information);
        [objKeyPath removeObserver:self forKeyPath:@"information" ];
    }