大概思路：創(chuàng)建一個子類，然后把父類的 isa 指向子類 (object_setClass)，在子類里新增一個setter方法，并在這個setter里調(diào)用父類的setter，調(diào)用完，執(zhí)行block回調(diào);

typedef void (^YCCallback)(id object, NSString *key, id newValue);

@interface NSObject (KVO)


-(void)addObserver:(NSObject *)target forKeyPath:(NSString *)keyPath callBack:(YCCallback)callback;

-(void)removeObserverForKeyPath:(NSString *)keyPath;

@end

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

#define kSetterPrefix @"set"
#define kGenObserverClsName(clsName) [NSString stringWithFormat:@"YCObserver_%@",clsName]

static NSString *const kMethodStorageKey = @"kMethodStorageKey";
static NSString *const kCallbackStorageKey = @"kCallbackStorageKey";



@implementation NSObject (KVO)


-(void)addObserver:(NSObject *)target forKeyPath:(NSString *)keyPath callBack:(YCCallback)callback{
    
    if (target == nil || keyPath == nil || keyPath.length == 0) return;
   
    Class origionCls = object_getClass(self);
    NSString *origionClsName = NSStringFromClass(origionCls);
    NSString *observerClsName = kGenObserverClsName(origionClsName);
    
    SEL origionSetter = [self generateSetter:keyPath];
    //Method 的應用場景：獲取方法簽名
    Method origionSetterMethod = class_getInstanceMethod(origionCls, origionSetter);
    
    Class allocatedNewClass = [self generateNewClass:self.class className:observerClsName];
    
    //修改self的isa ，讓他指向新的子類
    object_setClass(self, allocatedNewClass);
    
    //存儲父類的 method，以后會用到
    initMethodStorage(self);
    storeMethod(self, NSStringFromSelector(origionSetter),origionSetterMethod);

    
    initCallBackStorage(self);
    storeCallBack(self, NSStringFromSelector(origionSetter), callback);
    
    //給子類新增setter方法，覆蓋父類的setter，子類方法名不變，只是IMP是自定義的
    class_addMethod(allocatedNewClass, origionSetter, (IMP)yc_setter, method_getTypeEncoding(origionSetterMethod));
    
}

-(Class)generateNewClass:(Class)superClass className:(NSString *)className{
    //objc_allocateClassPair 和 objc_registerClassPair 成對出現(xiàn)
    Class allocatedNewClass = objc_allocateClassPair(superClass, [className UTF8String], 0);
    objc_registerClassPair(allocatedNewClass);
    return allocatedNewClass;
}


//生成setter方法：setWhat:
-(SEL)generateSetter:(NSString *)keyPath {
    NSString *firstKeyPathChar = [keyPath substringToIndex:1];
    NSString *otherKeyPathChar = [keyPath substringFromIndex:1];

    NSMutableString *setterSELName = [kSetterPrefix mutableCopy];
    [setterSELName appendString:[firstKeyPathChar uppercaseString]];
    [setterSELName appendString:otherKeyPathChar];
    [setterSELName appendString:@":"];

    return NSSelectorFromString(setterSELName);
}

//setWhat: -> what
NSString* revertSetter(SEL sel){
    NSString *selName = NSStringFromSelector(sel);
    if ([selName hasPrefix:kSetterPrefix]) {
        selName = [selName stringByReplacingOccurrencesOfString:kSetterPrefix withString:@""];
        NSString *firstChar = [selName substringToIndex:1];
        NSRange range = NSMakeRange(1, selName.length - 2);
        NSString *lastChars = [selName substringWithRange:range];
        selName = [[firstChar lowercaseString] stringByAppendingString:lastChars];
    }
    return selName;
}


void yc_setter(id target, SEL sel, id newValue){
    
    //自定義setter 一定要調(diào)用父類的 setter
    NSString *selName = NSStringFromSelector(sel);
    
    //拿到父類的 IMP ，再調(diào)用
    Method superMethod = loadMethod(target,selName);
    IMP superImp = method_getImplementation(superMethod);
    SEL superSEL = method_getName(superMethod);
    ((id(*)(id, SEL, id))superImp)(target, superSEL, newValue);

    YCCallback callback = loadCallback(target, selName);
    if (callback) {
        callback([target superclass],revertSetter(sel),newValue);
    }
}



void initMethodStorage(id target){
    if (!objc_getAssociatedObject(target, &kMethodStorageKey)) {
        objc_setAssociatedObject(target, &kMethodStorageKey, [NSMutableDictionary new], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
}

void initCallBackStorage(id target){
    if (!objc_getAssociatedObject(target, &kCallbackStorageKey)) {
        objc_setAssociatedObject(target, &kCallbackStorageKey, [NSMutableDictionary new], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
}


void storeMethod(id target,NSString *key,Method method){
    if (!target || !key || !method) {
        return;
    }
    NSMutableDictionary *storage = objc_getAssociatedObject(target, &kMethodStorageKey);
    [storage setObject:(__bridge id _Nonnull)(method) forKey:key];
}


void storeCallBack(id target,NSString *key,YCCallback callback){
    if (!target || !key) {
        return;
    }
    NSMutableDictionary *storage = objc_getAssociatedObject(target, &kCallbackStorageKey);
    if (callback == nil) {
        [storage removeObjectForKey:key];
    }else{
        [storage setObject:callback forKey:key];
    }
}



Method loadMethod(id target,NSString *key){
    if (!target || !key) {
        return nil;
    }
    NSMutableDictionary *storage = objc_getAssociatedObject(target, &kMethodStorageKey);
    return (__bridge Method)([storage objectForKey:key]);
}


YCCallback loadCallback(id target,NSString *key){
    if (!target || !key) {
        return nil;
    }
    NSMutableDictionary *storage = objc_getAssociatedObject(target, &kCallbackStorageKey);
    return (YCCallback)[storage objectForKey:key];
}



-(void)removeObserverForKeyPath:(NSString *)keyPath{
    SEL deleteSel = [self generateSetter:keyPath];
    storeCallBack(self, NSStringFromSelector(deleteSel), nil);//刪除callback
}



@end

@interface ViewController ()


@property (nonatomic,strong) What *w;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.w = [What new];
    
    [self.w addObserver:self forKeyPath:@"what" callBack:^(id  _Nonnull object, NSString * _Nonnull key, id  _Nonnull newValue) {

        
        
    }];
}



-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.w.what = @"xxx";
    
    [self.w removeObserverForKeyPath:@"what"];
}

@end

@interface What : NSObject


@property(strong,nonatomic) NSString *what;

@end