前言

之前只是了解KVO的原理，但是从未自己手动实现过KVO，主要是因为之前对runtime的操作函数没有那么熟练，所以一直耽搁了。我希望在不参考网上代码的之提下自己实现KVO, 有问题自己解决，除非碰到一些坎。

自己手动实现KVO

过程是这样的：

1. 创建目标对象的子类，前面加上"MyObserver_"的前缀作标识。
2. 保存原来setter的对应的IMP，将其存到"orgi_"+setter的方法下
3. 用我们的IMP替换原来setter对应的IMP
4. 注册子类
5. 替换isa
6. 保存监听的属性和block值到一个静态字典（这个最后废弃而用了关联引用）

@implementation NSObject (Observe)
static NSMutableDictionary *myobserveDic;

- (void)addMyObserver:(id)object propertyName:(NSString *)propertyName block:(void(^)(void))block{
    propertyName = @"name";
    Class cls = [object class];
    NSString *observeClassName = [NSString stringWithFormat:@"MyObserver_%@", NSStringFromClass(cls)];
    Class MyObserveClass = objc_allocateClassPair(cls, [observeClassName UTF8String], 0);
    NSString *setter = [NSString stringWithFormat:@"set%@:", [propertyName capitalizedString]];
    NSString *orginSetter = [NSString stringWithFormat:@"orgi_%@", setter];
    SEL setterSEL = sel_registerName([setter UTF8String]);
    SEL orginSetterSEL = sel_registerName([orginSetter UTF8String]);
    
    Method method = class_getInstanceMethod(MyObserveClass, setterSEL);
    //保存旧的IMP 名字前加orgi_
    class_addMethod(MyObserveClass, orginSetterSEL, method_getImplementation(method), method_getTypeEncoding(method));
    //替换IMP
    class_replaceMethod(MyObserveClass, setterSEL, (IMP)setterXXX, method_getTypeEncoding(method));
    

    objc_registerClassPair(MyObserveClass);//注册类
    object_setClass(object, MyObserveClass);//替换isa
    
    myobserveDic = myobserveDic ?: [NSMutableDictionary dictionary];
    NSValue *objectValue = [NSValue valueWithNonretainedObject:object];
    NSMutableArray *names = [myobserveDic objectForKey:objectValue];
    if (!names) {
        names = [NSMutableArray array];
        [myobserveDic setObject:names forKey:objectValue];
    }
    
    
    [names addObject:setter];
    [names addObject:@{@"setter":setter, @"block": block}];
}

用一个静态全局的字典myobserveDic存储数据，key是object, value是sel、block等信息。但是这样的话，运行会报错。

1. 因为它的Key要支持<NSCopy>协议
2. object的强引用会出现问题
   解决方法：
   想到了用NSValue在object外包一层，这样就解可以解决这些问题了。

全局字典和关联引用哪个更好？

如果用全局字典的话会有一个问题，就是在dealloc里从全局字典移除相关信息，相比关联引用多了一个步骤。所以我后来选择了用关联引用实现。
改进后的代码如下：

@implementation NSObject (Observe) 

- (void)addMyObserver:(id)object propertyName:(NSString *)propertyName block:(void(^)(void))block{
    propertyName = @"name";
    Class cls = [object class];
    NSString *observeClassName = [NSString stringWithFormat:@"MyObserver_%@", NSStringFromClass(cls)];
    Class MyObserveClass = objc_allocateClassPair(cls, [observeClassName UTF8String], 0);
    NSString *setter = [NSString stringWithFormat:@"set%@:", [propertyName capitalizedString]];
    NSString *orginSetter = [NSString stringWithFormat:@"orgi_%@", setter];
    SEL setterSEL = sel_registerName([setter UTF8String]);
    SEL orginSetterSEL = sel_registerName([orginSetter UTF8String]);
    
    Method method = class_getInstanceMethod(MyObserveClass, setterSEL);
    //保存旧的IMP 名字前加orgi_
    class_addMethod(MyObserveClass, orginSetterSEL, method_getImplementation(method), method_getTypeEncoding(method));
    //替换IMP
    class_replaceMethod(MyObserveClass, setterSEL, (IMP)setterXXX, method_getTypeEncoding(method));
    

    objc_registerClassPair(MyObserveClass);//注册类
    object_setClass(object, MyObserveClass);//替换isa
    //监听属性的数组
    NSMutableArray *infoArray = objc_getAssociatedObject(self, &observerInfoArrayKey);
    if (!infoArray) {
        infoArray = [NSMutableArray array];
        objc_setAssociatedObject(self, &observerInfoArrayKey, infoArray, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }

    MyObserveInfo *info = [[MyObserveInfo alloc] init];
    info.setter = setter;
    info.block = block;
    
    [infoArray addObject:info];
}

static void setterXXX(id self, SEL _cmd, id value) {
    NSMutableArray *observeInfoArray = objc_getAssociatedObject(self, &observerInfoArrayKey);

    for (MyObserveInfo *info in observeInfoArray) {
        if ([info.setter isEqualToString:@(sel_getName(_cmd))]) {
            //调用原始setter方法
            NSString *orgiSELString = [NSString stringWithFormat:@"orgi_%@", @(sel_getName(_cmd))];
            SEL orginSetterSEL = sel_registerName([orgiSELString UTF8String]);
            #pragma clang diagnostic push
            #pragma clang diagnostic ignored "-Warc-performSelector-leaks"
            [self performSelector:orginSetterSEL withObject:value];
            #pragma clang diagnostic pop
            
            if (info.block) {
                info.block();
            }
        }
    }
}

@end

MyObserveInfo类的定义：

@interface MyObserveInfo : NSObject

@property(nonatomic, strong) NSString *setter;
@property(nonatomic, copy) void(^block)(void);

@end

在setterXXX方法调用时，会用performSelector先调用原始的setter方法，然后再调用block。
但是会有leak的黄色警告，用-Warc-performSelector-leaks就可以去除掉。

不过我看网上的一些实现方法不是用的performSelector，而是用的objc_msgSendSuper方法，如下：

 // 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);

刚开始看了有点不解，为什么要调supper的方法，后来一想是因为我添加了一个叫orgin方法来存储原始的IMP, 所以它是现成的，可以直接用performSelector调用。

替换class方法

忘了添加class的方法替换了, 这里补上

//覆盖class方法
    Method classMethod = class_getInstanceMethod(MyObserveClass, sel_registerName("class"));
    class_addMethod(MyObserveClass, sel_registerName("class"), (IMP)(myClass), method_getTypeEncoding(classMethod));

myClass实现如下：

Class myClass(id self, SEL _cmd) {
    Class cls = object_getClass(self);
    Class supCls = class_getSuperclass(cls);
    return supCls;
}

这里我踩了个坑，千万不要直接调用superClass方法：

Class myClass(id self, SEL _cmd) {
  return [self superClass];
}

这样会导致无限递归，我看了一下runtime源码才知道superClass里面又调用了[self class]方法，所以。。。

- (Class)superclass {
    return [self class]->superclass;
}