KVC 相关

1 .KVC 的基本使用
kvc 相关的基本方法包括

设置属性值：：：
setValue: forKey: ;
setValue: forKeyPath: ;
获取属性值：：：
valueForKey: ;
valueForKeyPath: ;

keyPath 是指可以多层通过路径设置属性或者获取属性值

@interface Dog : NSObject
{// 这里的成员变量没有 set/ get 方法
    @public
    int dog_age; // 一般成员变量
    NSString * dog_food; // 实例变量
}
@property (nonatomic,copy)NSString * dog_name ; // 属性
@end

@interface Person : NSObject
{
    @public
    int person_age;
    NSString * person_food;
}
@property (nonatomic,copy)NSString * person_name;

@property (nonatomic,strong)Dog * dog;
@end

// -------------------vc 中调用
   self.person = [Person new];
   
    [self.person setValue:@"jack" forKey:@"person_name"];
    [self.person setValue:@"三明治" forKey:@"person_food"];
    [self.person setValue:@12 forKey:@"person_age"];

    NSLog(@"person name value is :: %@",[self.person valueForKey:@"person_name"]);
    NSLog(@"person name is :: %@",self.person.person_name);
   
    NSLog(@"person food value is :: %@",[self.person valueForKey:@"person_food"]);
    NSLog(@"person food is :: %@",self.person->person_food);
  
    NSLog(@"person age value is :: %@",[self.person valueForKey:@"person_age"]);
    NSLog(@"person age is :: %d",self.person->person_age);
    
    self.person.dog = [Dog new];
   
    [self.person setValue:@"pretty" forKeyPath:@"dog.dog_name"];
    [self.person setValue:@"狗粮" forKeyPath:@"dog.dog_food"];
    [self.person setValue:@2 forKeyPath:@"dog.dog_age"];

    NSLog(@"person dog name value is :: %@",[self.person valueForKeyPath:@"dog.dog_name"]);
    NSLog(@"person dog name is :: %@",self.person.dog.dog_name);

    NSLog(@"person dog food value is :: %@",[self.person valueForKeyPath:@"dog.dog_food"]);
    NSLog(@"person dog food is :: %@",self.person.dog->dog_food);

    NSLog(@"person age value is :: %@",[self.person valueForKeyPath:@"dog.dog_age"]);
    NSLog(@"person dog age is :: %d",self.person.dog->dog_age);

/*
key::
person name value is :: jack
person name is :: jack
person food value is :: 三明治
person food is :: 三明治
person age value is :: 12
person age is :: 12

keyPath::
person dog name value is :: pretty
person dog name is :: pretty
person dog food value is :: 狗粮
person dog food is :: 狗粮
person age value is :: 2
person dog age is :: 2
*/

通过代码验证可以得到相关的结果：： kvc 通过key /keypath 设置成员变量得值

2 .kvc的调用顺序
赋值过程：

KVC调用顺序.png

** 1 . 是否包含 setKey：、 _setKey：的方法
（包含：传参赋值 ; 不包含 -> 2）;

** 2 . 通过判断 + (BOOL)accessInstanceVariablesDirectly 的值（是否可以直接访问实例变量 ,默认值 true）
（ture：查找相关成员变量 -> 3 ; false -> 报错：：undefined key）;

** 3 . 成员变量得查找顺序 _key , _isKey , key , isKey
（发现变量：传参赋值 ; 未发现变量 -> 报错：：undefined key）;

取值过程：

kvc取值.png

3 . kvc 是否调用kvo？？？
解答：：（kvc修改成员变量、属性都会调用kvo）

@interface Person : NSObject
{
    @public
    int person_age;
}
@property (nonatomic,copy)NSString * person_name;
@end

//________________________ VC 调用

@interface ViewController ()

@property (nonatomic,strong)Person *person;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.person = [Person new];
    // 设置初始值
    [self.person setValue:@"jack" forKey:@"person_name"];
    [self.person setValue:@12 forKey:@"person_age"];
    
// 添加监听 name age
    [self.person addObserver:self forKeyPath:@"person_name" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:@"name"];
    [self.person addObserver:self forKeyPath:@"person_age" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:@"age"];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
// 修改成员变量 、 属性的值
    [self.person setValue:@"rose" forKey:@"person_name"];
    [self.person setValue:@13 forKey:@"person_age"];
}

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

- (void)dealloc
{
    [self.person removeObserver:self forKeyPath:@"person_age"];
    [self.person removeObserver:self forKeyPath:@"person_name"];
}
@end
/*
// kvo:: name 
keyPath:: person_name ; object:: <Person: 0x60000371c3c0> ; change :: {
    kind = 1;
    new = rose;
    old = jack;
} ; context :: name

 kvo:: age
keyPath:: person_age ; object:: <Person: 0x60000371c3c0> ; change :: {
    kind = 1;
    new = 13;
    old = 12;
} ; context :: age
*/

这里可以推断一个结论：
kvo的实现依赖于方法：：willChangeValueForKey 、didChangeValueForKey
推断 kvc 修改值得时候调用顺序：：

1 . willChangeValueForKey
2 . 根据前面讲到的查找顺序修改 value
3 . didChangeValueForKey