前言

这篇写一下KVO的原理及手动实现

正文

先看这样一段代码

KVO

isa是真实类型，class是系统提供方法获取的类型。可见，对person进行观察后，从第三句输出可以看到，person的真实类型发生了改变，并且中间类继承自原类，即NSKVONtifying_Person -> Person
，而苹果欺骗了我们。结合前文写过的KVO内部通知，可以来看看KVO究竟做了那些事

• 重写被观察对象属性对应的setter
• 改变被观察对象的真实类型，且与原类为继承关系
• 重写被观察对象的- (Class)class

知道苹果都干了啥，自己就可以开工了。

这里模仿如何自己动手实现 KVO写了一份代码

首先调试一下环境

Snip20151026_18.png

@interface Person : NSObject
@property (copy, nonatomic) NSString *name;
@end

@implementation Person
- (instancetype)init {
    if (self = [super init]) {
        _name = @"default";
    }
    return self;
}
@end

简单的搞个Person类，这没啥好说的

然后搞个NSObject的分类（利用block回调更方便，个人也不是很习惯苹果的API）

typedef void(^JKKVOBlock)(id observedObj, NSString *observedKey, id oldValue, id newValue);

@interface NSObject (JKKVO)

- (void)jk_removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;

- (void)jk_addObserver:(NSObject *)observer forKey:(NSString *)key withBlock:(JKKVOBlock)block;

@end

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

NSString *const JKKVOClassPrefix = @"JKKVONotifying_";
const char * JKKVOAssociateKey;

@interface JKObserverInfo : NSObject

@property (weak, nonatomic) NSObject *observer;
@property (copy, nonatomic) NSString *key;
@property (copy, nonatomic) JKKVOBlock block;

@end

接着就是最重要的部分，实现KVO。文中关键部分有注释，耐心点看应该不难懂

@implementation JKObserverInfo

- (instancetype)initWithObserver:(NSObject *)observer Key:(NSString *)key block:(JKKVOBlock)block {
    if (self = [super init]) {
        _observer = observer;
        _key = key;
        _block = block;
    }
    return self;
}

@end

@implementation NSObject (JKKVO)

- (NSString *)setterFromKey:(NSString *)key {
    
    NSMutableString *mutableKey = [NSMutableString stringWithString:key];
    [mutableKey replaceCharactersInRange:NSMakeRange(0, 1) withString:[[key substringToIndex:1] uppercaseString]];
    
    return [NSString stringWithFormat:@"set%@:", mutableKey];
}

- (Class)kvoObservedClassFromOriginClass:(Class)cls {
    
    NSString *clsStr = NSStringFromClass(cls);
    NSString *newClsStr = [JKKVOClassPrefix stringByAppendingString:clsStr];
    
    Class newCls = NSClassFromString(newClsStr);
    if (newCls) return newCls;
    
    newCls = objc_allocateClassPair(cls, newClsStr.UTF8String, 0);
    
    [self rewriteClassMethodWithNewClass:newCls];
    
    objc_registerClassPair(newCls);
    return newCls;
}

- (void)rewriteClassMethodWithNewClass:(Class)newCls {
    
    Method clsMethod = class_getInstanceMethod([self class], @selector(class));
    const char *types = method_getTypeEncoding(clsMethod);
    class_addMethod(newCls, @selector(class), (IMP)newClass, types);
}

Class newClass(id self, SEL _cmd) {
    return class_getSuperclass(object_getClass(self));
}

- (void)addObservers:(NSObject *)obs {
    // 设置关联属性用于添加观察者（因为在分类中，不能直接设置属性）
    NSMutableArray *observers = objc_getAssociatedObject(self, JKKVOAssociateKey);
    if (!observers) {
        observers = [NSMutableArray array];
        objc_setAssociatedObject(self, JKKVOAssociateKey, observers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    [observers addObject:obs];
}

void newSetter(id self, SEL _cmd, id newValue) {
    NSString *setter = NSStringFromSelector(_cmd);
    NSString *getter = [self getterFromSetter:setter];
    
    if (!getter) {
        NSString *reason = [NSString stringWithFormat:@"找不到%@对应属性%@的setter", self, getter];
        @throw [NSException exceptionWithName:NSInvalidArgumentException reason:reason userInfo:nil];
        return;
    }
    
    NSString *key = [[getter componentsSeparatedByString:@":"] firstObject];
    
    id oldValue = [self valueForKey:key];
    
    struct objc_super supercls = {
        .receiver = self,
        .super_class = class_getSuperclass(object_getClass(self))
    };
    
    objc_msgSendSuper(&supercls, _cmd, newValue);

    NSMutableArray *observers = objc_getAssociatedObject(self, JKKVOAssociateKey);
    for (JKObserverInfo *obs in observers) {
        if ([obs.key isEqualToString:key]) {
            obs.block(self, key, oldValue, newValue);
        }
    }
}

- (NSString *)getterFromSetter:(NSString *)setter {
    
    NSString *getter = [setter substringFromIndex:3];
    NSString *firstLow = [getter substringToIndex:1].lowercaseString;
    
    return [getter stringByReplacingCharactersInRange:NSMakeRange(0, 1) withString:firstLow];
}

- (void)jk_addObserver:(NSObject *)observer forKey:(NSString *)key withBlock:(JKKVOBlock)block {
    
    // 获取观察对象属性的setter
    NSString *setter = [self setterFromKey:key];
    
    SEL setterS = NSSelectorFromString(setter);
    Method setterM = class_getInstanceMethod([self class], setterS);
    if (!setterM) {
        NSString *reason = [NSString stringWithFormat:@"找不到%@对应属性%@的setter", self, key];
        @throw [NSException exceptionWithName:NSInvalidArgumentException reason:reason userInfo:nil];
        return;
    }
    
    // 生成中间类并重写 - (Class)class
    Class newCls = [self kvoObservedClassFromOriginClass:[self class]];
    // self 重定向
    object_setClass(self, newCls);
    
    // 重写setter
    const char *types = method_getTypeEncoding(setterM);
    class_addMethod(newCls, setterS, (IMP)newSetter, types);
    
    // 添加观察者
    JKObserverInfo *observerInfo = [[JKObserverInfo alloc] initWithObserver:observer Key:key block:block];
    [self addObservers:observerInfo];
    
}

- (void)jk_removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath {
    
    NSMutableArray *observers = objc_getAssociatedObject(self, JKKVOAssociateKey);
    for (JKObserverInfo *obsInfo in observers) {
        if (obsInfo.observer == observer && [obsInfo.key isEqualToString:keyPath]) {
            [observers removeObject:obsInfo];
            break;
        }
    }
}

@end

OK，大功告成！来测试一下吧

Snip20151026_19.png

然而并没有什么卵用，有些东西自己写着玩就好，要么就搞个开源框架，否则你永远不知道自己的代码有多烂😔