一.KVO

1. KVO的全称是Key-Value Observing，俗称“键值监听”，可以用于监听某个对象属性值的改变，不监听成员变量
KVO的使用不再赘述，见如下代码

#import "ViewController.h"

@interface Person : NSObject
{
    @public
    int age;
}

@property (nonatomic, copy) NSString *name;

@end

@implementation Person

- (void)setName:(NSString *)name{
    _name = name;
}

@end


@interface ViewController ()

@property (nonatomic, strong) Person *person1;
@property (nonatomic, strong) Person *person2;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.person1 = [[Person alloc] init];
    self.person2 = [[Person alloc] init];
    
    self.person1->age = 1;
    self.person1.name = @"person1";
    
    self.person2->age = 2;
    self.person2.name = @"person2";
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"name" options:options context:@"123"];
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:NULL];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    // NSKVONotifying_Person是使用Runtime动态创建的一个类，是Person的子类
    
    // self.person1.isa == NSKVONotifying_Person
    self.person1->age = 3;
    self.person1.name = @"Jack";

    // self.person2.isa = MJPerson
    self.person2->age = 4;
    self.person2.name = @"Marry";
}

- (void)dealloc {
    [self.person1 removeObserver:self forKeyPath:@"age"];
    [self.person1 removeObserver:self forKeyPath:@"name"];

}

// 当监听对象的属性值发生改变时，就会调用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context);
}

打印情况如下

截屏2021-09-12 上午10.42.44.png

可以看出只监听了属性name，没有监听成员变量age

2.从苹果官方文档可以看到KVO是使用isa-swizzling技术实现的，当观察者注册时，被观察者的isa指针被修改，指向一个中间类而不是真正的类。因此，isa指针指向的不一定是实例对象的真正类，您应该使用class方法来确定实例对象的类。

截屏2021-09-12 上午11.23.11.png

通过断点可以验证person1和person2的isa指向

截屏2021-09-12 上午11.35.18.png

3. 打印NSKVONotifying_Person的方法列表

 [self printClassAllMethod:objc_getClass("NSKVONotifying_Person")];

- (void)printClassAllMethod:(Class)cls{
    unsigned int count = 0;
    Method *methodList = class_copyMethodList(cls, &count);
    for (int i = 0; i<count; i++) {
        Method method = methodList[i];
        SEL sel = method_getName(method);
        IMP imp = class_getMethodImplementation(cls, sel);
        NSLog(@"%@-%p",NSStringFromSelector(sel),imp);
    }
    free(methodList);
}

从结果可以看出NSKVONotifying_Person重写了setName方法

截屏2021-09-12 下午12.25.42.png

4.在setName中打一个断点，可以看到_NSSetObjectValueAndNotify方法的调用，在setter方法前后，调用了NSKeyValueWillChange和NSKeyValueDidChangeBySetting

WX20210912-123044.png WX20210912-122804.png

实际上若被观察的类型是NSString,那么重写的方法的实现会指向_NSSetObjectValueAndNotify这个函数，若是Bool类型，那么重写的方法的实现会指向_NSSetBoolValueAndNotify这个函数，这个函数里会调用willChangeValueForKey:和didChangeValueForKey:,并且会在这2个方法调用之间，调用父类set方法的实现，didChangeValueForKey:内部会调用observer的observeValueForKeyPath:ofObject:change:context:方法

5.手动触发KVO
KVO属性发生变化时，会自动调用willChangeValueForKey:和didChangeValueForKey:，可以通过重写automaticallyNotifiesObserversForKey:返回NO关闭自动调用，并手动调用willChangeValueForKey:和didChangeValueForKey:

- (void)setName:(NSString *)name{
    [self willChangeValueForKey:@"name"];
    _name = name;
    [self didChangeValueForKey:@"name"];
}

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key{
    return NO;
}

6.总结
iOS用什么方式实现对一个对象的KVO？(KVO的本质是什么？)
答：利用RuntimeAPI动态生成一个子类，并且让instance对象的isa指向这个全新的子类，当修改instance对象的属性时，会调用Foundation的_NSSetXXXValueAndNotify函数，这个函数里会按顺序调用willChangeValueForKey:，父类原来的setter，didChangeValueForKey:，didChangeValueForKey:内部会触发监听器（Oberser）的监听方法( observeValueForKeyPath:ofObject:change:context:)

如何手动触发KVO？
答：手动调用willChangeValueForKey:和didChangeValueForKey:

直接修改成员变量会触发KVO吗？
答：不会触发KVO

二、KVC

KVC的全称是Key-Value Coding，俗称“键值编码”，可以通过一个key来访问某个属性，常见的API有

- (void)setValue:(id)value forKeyPath:(NSString *)keyPath;
- (void)setValue:(id)value forKey:(NSString *)key;
- (id)valueForKeyPath:(NSString *)keyPath;
- (id)valueForKey:(NSString *)key; 

KVC的取值和赋值原理可以参考苹果官方文档

setter
getter