KVO底层实现原理，仿写KVO

#import <Foundation/Foundation.h>

@interface Person : NSObject

//字符串类型的属性name
@property (nonatomic, strong) NSString *name;

@end

#import "Person.h"

@implementation Person
- (void)setName:(NSString *)name
{ //别问为什么（下面有用处），就是要自己处理set方法
    _name = [NSString stringWithFormat:@"%@aaaa",name];
}
@end

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@end

#import "ViewController.h"
#import "Person.h"

@interface ViewController ()

@property (nonatomic, strong) Person *p;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    Person *p = [[Person alloc] init];
    
    // 监听name属性有没有改变
    [p addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    self.p = p;
}

//点击屏幕修改self.p的name属性的值
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    static int i = 0;
    i++;
    self.p.name = [NSString stringWithFormat:@"%d",i];
}

// 所有的KVO监听都会来到这个方法
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context
{
    NSLog(@"%@",self.p.name);
}

@end

#import <Foundation/Foundation.h>

@interface Person : NSObject
{
    @public
    NSString *_name;//为了验证KVO监听的是setter方法
}

@end

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    Person *p = [[Person alloc] init];
    
    // 监听_name有没有改变
    [p addObserver:self forKeyPath:@"_name" options:NSKeyValueObservingOptionNew context:nil];
    self.p = p;
}

#import <Foundation/Foundation.h>

@interface NSKVONotifying_Person : NSObject

@end

#import "NSKVONotifying_Person.h"

@implementation NSKVONotifying_Person

@end

#import <Foundation/Foundation.h>

@interface NSObject (KVO)

//添加监听
- (void)ey_addObserver:(NSObject *_Nullable)observer forKeyPath:(NSString *_Nullable)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;

//移除监听
- (void)ey_removeObserver:(NSObject *_Nullable)observer forKeyPath:(NSString *_Nullable)keyPath context:(nullable void *)context;

@end

    //
    //  NSObject+KVO.m
    //  KVO底层实现
    //
    //  Created by lieryang on 2017/6/17.
    //  Copyright ? 2017年 lieryang. All rights reserved.
    //

#import "NSObject+KVO.h"
#import <objc/message.h>

static NSString * const EYKVONotifying_ = @"EYKVONotifying_";
static NSString * const observerKey = @"observer";
@implementation NSObject (KVO)

    //添加监听
- (void)ey_addObserver:(NSObject *_Nullable)observer forKeyPath:(NSString *_Nullable)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context
{
    if (keyPath.length == 0) {//如果传进来的keyPath为@""或者为nil 直接返回
        return;
    }
    
    // 1. 检查对象的类有没有相应的 setter 方法。
    SEL setterSelector = NSSelectorFromString([self setterForGetter:keyPath]);
    
    Method setterMethod = class_getInstanceMethod([self class], setterSelector);
    
    if (!setterMethod) {//如果没有直接返回，不需要任何处理
        NSLog(@"找不到该方法");
        return;
    }
    
    // 2. 检查对象 isa 指向的类是不是一个 KVO 类。如果不是，新建一个继承原来类的子类，并把 isa 指向这个新建的子类
    Class clazz = object_getClass(self);
    NSString *className = NSStringFromClass(clazz);
    
    if (![className hasPrefix:EYKVONotifying_]) {
        clazz = [self ey_KVOClassWithOriginalClassName:className];
        object_setClass(self, clazz);
    }
    
    // 3. 为EYKVONotifying_XXX添加setter方法的实现
    const char *types = method_getTypeEncoding(setterMethod);
    class_addMethod(clazz, setterSelector, (IMP)ey_setter, types);
    
    // 4. 添加该观察者到观察者列表中
    NSMutableArray *observers = objc_getAssociatedObject(self, (__bridge const void * _Nonnull)(observerKey));
    if (!observers) {
        observers = [NSMutableArray array];
        objc_setAssociatedObject(self, (__bridge const void * _Nonnull)(observerKey), observers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    if ([observers indexOfObject:observer] == NSNotFound) {
        [observers addObject:observer];
    }
}

    //移除监听
- (void)ey_removeObserver:(NSObject *_Nullable)observer forKeyPath:(NSString *_Nullable)keyPath context:(nullable void *)context
{
    
    NSMutableArray* observers = objc_getAssociatedObject(self, (__bridge const void *)(observerKey));
    
    [observers removeObject:observer];
}
#pragma mark - 注册自己的EYKVONotifying_XXX
- (Class)ey_KVOClassWithOriginalClassName:(NSString *)className
{
    // 生成EYKVONotifying_XXX的类名
    NSString *kvoClassName = [EYKVONotifying_ stringByAppendingString:className];
    Class kvoClass = NSClassFromString(kvoClassName);
    
    // 如果EYKVONotifying_XXX已经被注册过了, 则直接返回
    if (kvoClass) {
        return kvoClass;
    }
    
    // 如果EYKVONotifying_XXX不存在, 则创建这个类
    Class originClass = object_getClass(self);
    kvoClass = objc_allocateClassPair(originClass, kvoClassName.UTF8String, 0);
    
    // 修改EYKVONotifying_XXX方法的实现, 学习Apple的做法, 隐瞒这个EYKVONotifying_XXX
    Method classMethod = class_getInstanceMethod(kvoClass, @selector(class));
    const char *types = method_getTypeEncoding(classMethod);
    class_addMethod(kvoClass, @selector(class), (IMP)ey_class, types);
    
    // 注册EYKVONotifying_XXX
    objc_registerClassPair(kvoClass);
    
    return kvoClass;
}

Class ey_class(id self, SEL cmd)
{
    Class clazz = object_getClass(self); // EYKVONotifying_XXX
    Class superClazz = class_getSuperclass(clazz); // origin_class
    return superClazz; // origin_class
}

/**
 *  重写setter方法, 新方法在调用原方法后, 通知每个观察者
 */
static void ey_setter(id self, SEL _cmd, id newValue)
{
    NSString *setterName = NSStringFromSelector(_cmd);
    NSString *getterName = [self getterForSetter:setterName];
    
    if (!getterName) {
        NSLog(@"找不到getter方法");
    }
    
    // 调用原类的setter方法
    struct objc_super superClazz = {
        .receiver = self,
        .super_class = class_getSuperclass(object_getClass(self))
    };
    // 这里需要做个类型强转, 否则会报too many argument的错误
    ((void (*)(void *, SEL, id))objc_msgSendSuper)(&superClazz, _cmd, newValue);
    
    // 找出观察者的数组
    NSArray *observers = objc_getAssociatedObject(self, (__bridge const void * _Nonnull)(observerKey));
        // 遍历数组
    for (id observer in observers) {
        // 调用监听者observer的observeValueForKeyPath 方法，因为observer为id类型，所以就偷懒调用了系统的监听回调，要是自己定义方法，会报找方法的错误，可以在添加监听的时候，传进来一个block代码块，在此处回调block，更方便外界的调用
        [observer observeValueForKeyPath:getterName ofObject:self change:nil context:nil];
    }
}

#pragma mark - 生成对应的setter方法字符串
- (NSString *)setterForGetter:(NSString *)key
{
    // 1. 首字母转换成大写
    NSString * firstString = [[key substringToIndex:1] uppercaseString];
    // 2. 剩下的字母
    NSString * remainingString = [key substringFromIndex:1];
    
    // 3. 最前增加set, 最后增加: setName:
    NSString *setter = [NSString stringWithFormat:@"set%@%@:", firstString, remainingString];
    
    return setter;
    
}
#pragma mark - 生成对应的getter方法字符串
- (NSString *)getterForSetter:(NSString *)key
{
    // setName
    if (key.length <=0 || ![key hasPrefix:@"set"] || ![key hasSuffix:@":"]) {
        return nil;
    }
    
    // 移除set和:
    NSRange range = NSMakeRange(3, key.length - 4);
    NSString *getter = [key substringWithRange:range];
    
    // 小写
    NSString *firstString = [[getter substringToIndex:1] lowercaseString];
    getter = [getter stringByReplacingCharactersInRange:NSMakeRange(0, 1)
                                       withString:firstString];
    
    return getter;
}
@end

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic, copy) NSString * name;

@end

#import "Person.h"

@implementation Person

@end

#import "ViewController.h"
#import "Person.h"
#import "NSObject+KVO.h"

@interface ViewController ()

@property (nonatomic, strong) Person * p;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    Person * p = [[Person alloc] init];
    [p ey_addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    self.p = p;
}

// 监听的回调
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context
{
    NSLog(@"%@", self.p.name);
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    [super touchesBegan:touches withEvent:event];
    
    self.p.name = [NSString stringWithFormat:@"%u", arc4random_uniform(20)];
}

@end