KVO 相关

• 1 . KVO 的基本使用
  kvo "键值监听"， 全称"Key-Value Observing" ,监听对象属性值的改变

// 监听对象
@interface Person : NSObject
// 监听属性
@property (nonatomic,copy)NSString * name;
@end

@implementation Person
@end

//----------------------------

@interface ViewController : UIViewController
@end

@interface ViewController ()

// 创建对象
@property (nonatomic,strong)Person *person;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.person = [Person new];
    self.person.name = @"jack";
    
// 配置监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld |NSKeyValueObservingOptionNew;
    [self.person addObserver:self forKeyPath:@"name" options:options context:nil];
}
  // 触发监听
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{

    self.person.name = @"rose";
}

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

// 移除监听
- (void)dealloc
{
    [self.person removeObserver:self forKeyPath:@"name"];
}
@end

/*
keyPaht ::name ; 
object ::<Person: 0x600003bfc340>  ; 
change ::{
    kind = 1;
    new = rose;
    old = jack;
} ;
context ：： (null)
*/

• 2 . kvo 的本质
  a. 属性值的改变，其本质是调用内部的set 方法；
  b. 利用runtime，动态生成了 NSKVONotifying_Person 类 继承于 Person类（NSKVONotifying_Person superclass 指向 person）,
  同时修改person 对象的isa 指向了 NSKVONotifying_Person 类 复写set 方法，class 方法
  (NSKVONotifying_Person 有自己的元类对象 ，所以 类对象的isa 指向自己的元类对象)
  c. 在set 方法中调用 willchange 等方法；

@interface NSKVONotifying_Person : Person
@end
/*新增
_isKVOA
dealloc
class // return 【Person class】
setName //调用Foundation 下的 _NSSetXXXValueAndNotify（c 函数）
*/

伪代码
@implementation NSKVONotifying_Person
-(void)setName(NSString *)name{
_NSSetXXXValueAndNotify()
}
void _NSSetIntValueAndNotify() {
[self willChangeValueForKey:@"name"];
[super setName:@"xx"];// 调用父类（person 类修改person 的值）
[self didChangeValueForKey:@"name"];
}
-（void）didChangeValueForKey(NSString *)name{
【observer observeValueForKeyPath... 】
}
@end

• 3 .手动触发 kvo

// 手动调用 

- (void)willChangeValueForKey:(NSString *)key;
- (void)didChangeValueForKey:(NSString *)key;

//因为在 didChange方... 法中判断 willChange... 是否有调用 ，所以要先调用willChange...

• 4 . 触发kvo的前提：对象的属性 必须有 set 方法；

@interface Person : NSObject{
    int age;
}
@property (nonatomic,copy)NSString * name;
@end

 // -----------------

@interface ViewController ()

@property (nonatomic,strong)Person *person;

@end

@implementation ViewController

- (void)viewDidLoad {
    self.person = [Person new];
    self.person.name = @"jack";
    self.person->age = 12;
   
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionOld |NSKeyValueObservingOptionNew;
    [self.person addObserver:self forKeyPath:@"name" options:options context:nil];
    [self.person addObserver:self forKeyPath:@"age" options:options context:nil];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.person.name = @"rose";
    self.person->age = 13;
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
   
    NSLog(@"keyPaht ::%@ ; object ::%@  ; change ::%@ ;context ;;%@ ",keyPath,object,change,context);
}

- (void)dealloc
{
    [self.person removeObserver:self forKeyPath:@"name"];
}

/*
这里只能监听到name属性 ； 无法监听 age 属性；
因为age 并没有set 方法，所以不会触发监听；
*/ 
@end

• 5 . 属性和成员变量得区别
  可以参照这篇博客

https://www.cnblogs.com/huangzs/p/7508583.html