1、KVO简介

KVO全称是Key-Value Observing，俗称“键值监听”，可用于监听某个对象属性值的改变。
添加监听：

self.person = [[Person alloc] init];
self.person.age = 10;
NSKeyValueObservingOptions options =  NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[self.person addObserver:self forKeyPath:@"age" options:options context:nil];

监听回调方法：

// 当监听属性值发生改变时调用
- (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 - %@", object, keyPath, change);
}

移除监听：

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

2、KVO本质

给Person的实例对象添加KVO后，RunTime会在运行过程中创建一个继承于Person的子类NSKVONOtifying_Person，并将Person实例对象的isa指针指向这个子类。子类内部会将被观察属性的set方法实现替换为_NSSetIntValueAndNotify方法，如图所示：

伪代码：

// 伪代码
- (void)setAge:(int)age {
    // 调用Foundation框架里的方法
    _NSSetIntValueAndNotify();
}

_NSSetIntValueAndNotify内部会调用willChangeValueForKey:、父类setAge:、didChangeValueForKey:方法：

// 伪代码
void _NSSetIntValueAndNotify() {
    [self willChangeValueForKey:@"age"];
    [super setAge:age];
    [self didChangeValueForKey:@"age"];
}

didChangeValueForKey内部会调用observeValueForKeyPath:方法：

// 伪代码
- (void)didChangeValueForKey:(NSString *)key {
    // 通知监听器，某某属性值发生了改变
    [oberser observeValueForKeyPath:key ofObject:self change:nil context:nil];
}

总结：
当属性的set方法出发后，执行子类内部的_NSSetIntValueAndNotify方法
_NSSetIntValueAndNotify内部调用：
willChangeValueForKey:—->super setAge:—->didChangeValueForKey:
didChangeValueForKey:内部调用：observeValueForKeyPath:
从而完成整个KVO监听流程。

3、验证

3.1、验证NSKVONotifying_Person类的存在

创建Person的两个实例对象，给其中一对象的age属性添加KVO，打印两个实例对象的isa指针

self.person1 = [[Person alloc] init];
self.person1.age = 11;
self.person2 = [[Person alloc] init];
self.person2.age = 12;
NSKeyValueObservingOptions options =  NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[self.person1 addObserver:self forKeyPath:@"age" options:options context:nil];
NSLog(@"person1.isa = %@, person2.isa = %@",object_getClass(self.person1),object_getClass(self.person2));

打印结果：

～：添加KVO前 - person1.isa = Person, person2.isa = Person
～：添加KVO后 - person1.isa = NSKVONotifying_Person, person2.isa = Person

从打印结果可知，添加KVO后person1.isa指针指向了NSKVONotifying_Person

3.2、验证_NSSetIntValueAndNotify方法存在

通过methodForSelector方法，获取setAge:方法的IMP（实现）

NSLog(@"添加KVO前 - person1.setAge = %p, person2.setAge = %p",[self.person1 methodForSelector:@selector(setAge:)],[self.person2 methodForSelector:@selector(setAge:)]);
NSKeyValueObservingOptions options =  NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[self.person1 addObserver:self forKeyPath:@"age" options:options context:nil];
NSLog(@"添加KVO后 - person1.setAge = %p, person2.setAge = %p",[self.person1 methodForSelector:@selector(setAge:)],[self.person2 methodForSelector:@selector(setAge:)]);

打印结果：

添加KVO前 - person1.setAge = 0x108041e10, person2.setAge = 0x108041e10
添加KVO后 - person1.setAge = 0x10835779f, person2.setAge = 0x108041e10

从打印结果可知，添加KVO后的setAge:方法的IMP地址发生了改变，再通过LLDB命令打印IMP：

(lldb) p (IMP)0x10efcae10
(IMP) $0 = 0x000000010efcae10 (Study`-[Person setAge:] at Person.h:14)
(lldb) p (IMP)0x10835779f
(IMP) $1 = 0x000000010835779f (Foundation`_NSSetIntValueAndNotify)

得出0x10835779f对应的方法实现是Foundation框架里的_NSSetIntValueAndNotify方法。

3.3、验证_NSSetXXValueAndNotify方法内部调用顺序

在Person类中添加打印，并重新设置age的值：

- (void)setAge:(int)age {
    _age = age;
    NSLog(@"setAge");
}
- (void)willChangeValueForKey:(NSString *)key {
    [super willChangeValueForKey:key];
    NSLog(@"willChangeValueForKey");
}
- (void)didChangeValueForKey:(NSString *)key {
    NSLog(@"didChangeValueForKey - Begin");
    [super didChangeValueForKey:key];
    NSLog(@"didChangeValueForKey - End");
}

打印结果：

～：willChangeValueForKey
～：setAge
～：didChangeValueForKey - Begin
～：监听到<Person: 0x600000d5c270>的age属性值改变 - {
            kind = 1;
            new = 20;
            old = 11;
        }
～：didChangeValueForKey - End

得出_NSSetIntValueAndNotify内部执行顺序是:
willChangeValueForKey:—->setAge:—->didChangeValueForKey:—->observeValueForKeyPath:

*这里之所以调用_NSSetIntValueAndNotify方法是因为被添加KVO的属性是int类型，Foundation框架内部还有_NSSetDoubleValueAndNotify、_NSSetRangeValueAndNotify等方法，会根据被添加观察的属性类型决定调用哪个方法。

3.4、NSKVONotifying_*子类内部方法

打印两个对象person1（添加了KVO）和person2的类对象方法列表，进行对比

[self printClassMethodList:object_getClass(self.person1)];
[self printClassMethodList:object_getClass(self.person2)];

打印方法列表方法：

- (void)printClassMethodList:(Class)cls {
    unsigned int count;
    Method *methodList = class_copyMethodList(cls, &count);
    NSMutableString *methodNames = [[NSMutableString alloc] init];
    for (int i = 0; i < count; i++) {
        Method method = methodList[i];
        [methodNames appendString:NSStringFromSelector(method_getName(method))];
        [methodNames appendString:@", "];
    }
    free(methodList);
    NSLog(@"%@的方法有%@", NSStringFromClass(cls), methodNames);
}

打印结果：

～：NSKVONotifying_Person的方法有setAge:, class, dealloc, _isKVOA
～：Person的方法有setAge:, age

由打印结果得出，子类中不仅有set方法，还有class、dealloc、_isKVOA方法

子类重新class方法是为了不想让开发者知道子类的存在，所以返回的仍然是Person的类对象。内部可能实现方式：

- (Class)class {
    // 返回自己的父类
    return class_getSuperclass(object_getClass(self));
}

4、手动触发KVO

由于observeValueForKeyPath:是在didChangeValueForKey:方法内部调用的，所以需要调用didChangeValueForKey:方法来手动触发，且在调用didChangeValueForKey:前需要调用willChangeValueForKey:方法：

[self.person1 willChangeValueForKey:@"age"];
[self.person1 didChangeValueForKey:@"age"];