iOS KVO （3）KVO键之间的依赖

依赖，故名思义，有单项依赖，双向依赖，多项依赖。在项目开发中有的时候我们会遇到这样的问题，我们需要监控变量A的变化，而变量A的变化依赖于变量B，C的变化，变量B，C不改变，A就不会改变。变量B，C任一改变，A就会发生改变。是的我们可以同事监控变量B，C但是如果是100对象呢 需要对100个变量进行监控吗？ 问题已经清晰我们直接上代码吧。

#import "ViewController.h"


@interface ViewController ()
@property (nonatomic,copy)NSString *fullName;
@property (nonatomic,copy)NSString *firstName;
@property (nonatomic,copy)NSString *lastName;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    [self addObserver:self forKeyPath:@"fullName" options:NSKeyValueObservingOptionNew context:nil];
    self.firstName = @"WANG";
    self.lastName = @"Gang";
    self.firstName = @"LI";
    self.lastName = @"QIANG";
}
- (NSString *)fullName{
    return [NSString stringWithFormat:@"%@ %@",self.firstName,self.lastName];
}
+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
    
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    
    if ([key isEqualToString:@"fullName"]) {
        NSArray *affectingKeys = @[@"lastName", @"firstName"];
        keyPaths = [keyPaths setByAddingObjectsFromArray:affectingKeys];
    }
    return keyPaths;
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    NSLog(@"%@",change);
}
- (void)dealloc{
    [self removeObserver:self forKeyPath:@"fullName"];
}

打印数据

2018-05-22 16:00:42.137928+0700 KVODemo[60579:2029281] {
    kind = 1;
    new = "WANG (null)";
}
2018-05-22 16:00:42.138129+0700 KVODemo[60579:2029281] {
    kind = 1;
    new = "WANG Gang";
}
2018-05-22 16:00:42.138259+0700 KVODemo[60579:2029281] {
    kind = 1;
    new = "LI Gang";
}
2018-05-22 16:00:42.138383+0700 KVODemo[60579:2029281] {
    kind = 1;
    new = "LI QIANG";
}

总结：
1.看到这些时候我想你已经明白了 fullName的改变依赖于firstName和lastName 只有lastName和lastName改变fullName才会改变，需要注意的是，虽然KeyPath是fullName 但是你对fullName进行赋值，也不会调用监控的方法，因为已经讲监控对象指向了依赖对象。

keyPathsForValuesAffectingValueForKey添加依赖的方法，此方法同样遵循KVC的命名查找规则
你也可以通过实现 keyPathsForValuesAffecting<Key> 方法来达到前面同样的效果，这里的<Key>就是属性名，不过第一个字母要大写，用前面的例子来说就是这样：

+ (NSSet *)keyPathsForValuesAffectingFullName {
    return [NSSet setWithObjects:@"lastName", @"firstName", nil];
}

未完，待续