举个例子


@interface TPerson : NSObject
@property (assign, nonatomic) int age;
@end

@implementation TPerson
- (void)setAge:(int)age
{
    _age = age;
    NSLog(@"%@ useTPerson setAge: %d", self, age);
}
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[TPerson alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[TPerson alloc] init];
    self.person2.age = 2;
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"111"];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    [self.person1 setAge:21];
    [self.person2 setAge:22];
}

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

// 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);
}

//打印内容
2020-02-16 14:37:13.571758+0800 Interview01[7622:254950] <TPerson: 0x6000018643b0> useTPerson setAge: 1
2020-02-16 14:37:13.571859+0800 Interview01[7622:254950] <TPerson: 0x6000018643d0> useTPerson setAge: 2
2020-02-16 14:37:47.971643+0800 Interview01[7622:254950] <TPerson: 0x6000018643b0> useTPerson setAge: 21
2020-02-16 14:37:47.971943+0800 Interview01[7622:254950] 监听到<TPerson: 0x6000018643b0>的age属性值改变了 - {
    kind = 1;
    new = 21;
    old = 1;
} - 123
2020-02-16 14:37:47.972128+0800 Interview01[7622:254950] <TPerson: 0x6000018643d0> useTPerson setAge: 22

由刚开始打印setAge：的顺序可知：
0x6000018643b0 为person1的地址
0x6000018643d0 为person2的地址

问题

我们发现person1除了调用set方法之外还会另外执行监听器的observeValueForKeyPath方法，set方法调用的都是一样的可知问题不是出在set方法上，说明KVO对person1对象做了一些操作使得其调用set方法以后还调用了其他方法

KVO底层实现分析

在这里插入图片描述
打印其isa指向（也就是其class类）可得person1的class类是NSKVONotifying_TPerson，而person2的class类是TPerson
也就是说如果实例对象添加了KVO监听，类对象就会成为NSKVONotifying_TPerson

NSKVONotifyin_TPerson的superclass指针是指向TPerson类对象的，也就是说NSKVONotifyin_TPerson是TPerson的子类，NSKVONotifyin_TPerson是runtime在运行时生成的。那么person1对象在调用setAge：方法的时候，肯定会根据person1的isa找到NSKVONotifyin_TPerson，在NSKVONotifyin_TPerson中找setAge：的方法及实现。

NSKVONotifyin_TPerson中的setAge：方法中其实调用了 Fundation框架中C语言函数 _NSsetIntValueAndNotify，_NSsetIntValueAndNotify内部做的操作相当于，首先调用willChangeValueForKey 将要改变方法，之后调用父类的setAge：方法对成员变量赋值，最后调用didChangeValueForKey已经改变方法。didChangeValueForKey中会调用监听器的监听方法，最终来到监听者的observeValueForKeyPath方法中。

证明

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[MJPerson alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[MJPerson alloc] init];
    self.person2.age = 2;
    
    NSLog(@"person1添加KVO监听之前 - %p %p",
          [self.person1 methodForSelector:@selector(setAge:)],
          [self.person2 methodForSelector:@selector(setAge:)]);
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    NSLog(@"person1添加KVO监听之后 - %p %p",
          [self.person1 methodForSelector:@selector(setAge:)],
          [self.person2 methodForSelector:@selector(setAge:)]);
    
}

//打印情况
2020-02-16 15:13:09.960882+0800 Interview01[7944:269740] person1添加KVO监听之前 - 0x103d15630 0x103d15630
2020-02-16 15:13:09.961486+0800 Interview01[7944:269740] person1添加KVO监听之后 - 0x1040703d2 0x103d15630

可见person1 setAge：方法的地址不同了，也就是说person1添加KVO监听之后setAge：方法改变了
在这里插入图片描述
打断点后打印setAge：信息发现，没添加KVO的set方法就是我们写在TPerson里的方法，而添加KVO后则调用的是Foundation框架里的_NSSetIntValueAndNotify函数
再在添加KVO后打印类对象和元类对象

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[TPerson alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[TPerson alloc] init];
    self.person2.age = 2;
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    NSLog(@"类对象 - %@ %@",
          object_getClass(self.person1),  // self.person1.isa
          object_getClass(self.person2)); // self.person2.isa

    NSLog(@"元类对象 - %@ %@",
          object_getClass(object_getClass(self.person1)), // self.person1.isa.isa
          object_getClass(object_getClass(self.person2))); // self.person2.isa.isa
    
}

//打印结果
2020-02-16 15:23:07.905770+0800 Interview01[8049:274433] 类对象 - NSKVONotifying_TPerson TPerson
2020-02-16 15:23:07.905948+0800 Interview01[8049:274433] 元类对象 - NSKVONotifying_TPerson TPerson

说明NSKVONotifying_TPerson的isa指向它自己的元类对象NSKVONotifying_TPerson 而不是指向Tperson的元类对象

NSKVONotifying_TPerson类的内部方法

//获得类对象内部函数的方法
- (void)printMethodNamesOfClass:(Class)cls {
    unsigned int count;
    // 获得方法数组
    Method *methodList = class_copyMethodList(cls, &count);
    
    // 存储方法名
    NSMutableString *methodNames = [NSMutableString string];
    
    // 遍历所有的方法
    for (int i = 0; i < count; i++) {
        // 获得方法
        Method method = methodList[i];
        // 获得方法名
        NSString *methodName = NSStringFromSelector(method_getName(method));
        // 拼接方法名
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    
    // 释放
    free(methodList);
    
    // 打印方法名
    NSLog(@"%@ %@", cls, methodNames);
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[TPerson alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[TPerson alloc] init];
    self.person2.age = 2;
    
    // 给person1对象添加KVO监听
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    [self printMethodNamesOfClass:object_getClass(self.person1)];
    [self printMethodNamesOfClass:object_getClass(self.person2)];
}

//打印结果
2020-02-16 15:28:22.594827+0800 Interview01[8149:277599] NSKVONotifying_TPerson setAge:, class, dealloc, _isKVOA,
2020-02-16 15:28:22.594982+0800 Interview01[8149:277599] TPerson setAge:, age,

可得NSKVONotifying_TPerson类的内部方法有setAge:, class, dealloc, _isKVOA
但打印 [self.person1 class] 和 object_getClass(self.person1)得：

NSLog(@"[self.person1 class] = %@, object_getClass(self.person1) = %@", [self.person1 class], object_getClass(self.person1));

//打印结果
2020-02-16 15:32:42.118609+0800 Interview01[8189:279881] [self.person1 class] = TPerson, object_getClass(self.person1) = NSKVONotifying_TPerson

可知苹果公司并不希望把NSKVONotifying_TPerson类暴露出来，所以在内部重写了class方法，我们猜测class代码为：

- (Class)class
{
    return [TPerson class];
}

验证didChangeValueForKey:内部会调用KVO的实现方法

重写TPerson类的内部方法

- (void)setAge:(int)age {
    _age = age;
}

- (void)willChangeValueForKey:(NSString *)key {
    NSLog(@"willChangeValueForKey: - begin");
    [super willChangeValueForKey:key];
    NSLog(@"willChangeValueForKey: - end");
}

- (void)didChangeValueForKey:(NSString *)key {
    NSLog(@"didChangeValueForKey: - begin");
    [super didChangeValueForKey:key];
    NSLog(@"didChangeValueForKey: - end");
}

//打印情况
2020-02-16 15:39:01.220816+0800 Interview01[8245:282931] willChangeValueForKey: - begin
2020-02-16 15:39:01.221025+0800 Interview01[8245:282931] willChangeValueForKey: - end
2020-02-16 15:39:01.221141+0800 Interview01[8245:282931] didChangeValueForKey: - begin
2020-02-16 15:39:01.221404+0800 Interview01[8245:282931] 监听到<TPerson: 0x600002747b70>的age属性值改变了 - {
    kind = 1;
    new = 1;
    old = 1;
} - 123
2020-02-16 15:39:01.221545+0800 Interview01[8245:282931] didChangeValueForKey: - end

根据打印情况可知 observeValueForKeyPath: ofObject: change: context: 方法是在 didChangeValueForKey: 方法中被调用的

参考文献

小码哥底层班视频学习

iOS KVO原理

文章目录

举个例子

问题

KVO底层实现分析

证明

NSKVONotifying_TPerson类的内部方法

验证didChangeValueForKey:内部会调用KVO的实现方法

参考文献

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