11-iOS关联对象实现weak属性

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
    OBJC_ASSOCIATION_ASSIGN = 0, //assign
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, //strong, nonatomic
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3, //copy, nonatomic
    OBJC_ASSOCIATION_RETAIN = 01401, //strong, atomic
    OBJC_ASSOCIATION_COPY = 01403 //copy, atomic
};

//定义Person类
@interface Person : NSObject
@end
@implementation Person
- (void)dealloc {
    NSLog(@"Person dealloc");
}
@end
@interface Person (Test)
//在分类中声明UIViewController属性，用assign修饰
@property(assign, nonatomic) UIViewController *viewController;
@end
@implementation Person (Test)
- (void)setViewController:(UIViewController *)viewController {
    //利用objc_setAssociatedObject设置值，policy为OBJC_ASSOCIATION_ASSIGN
    objc_setAssociatedObject(self, @selector(viewController), viewController, OBJC_ASSOCIATION_ASSIGN);
}
- (UIViewController *)viewController {//取值
    return objc_getAssociatedObject(self, _cmd);
}
@end

@interface WeakObjWrapper : NSObject
@property(weak, nonatomic) id weakObj;
@end
@implementation WeakObjWrapper
- (instancetype)initWithWeakObject:(id)weakObj {
    if (self = [super init]) {
        _weakObj = weakObj;
    }
    return self;
}
@end

@interface Person (Test)
@property(weak, nonatomic) UIViewController *viewController;
@end
@implementation Person (Test)
- (void)setViewController:(UIViewController *)viewController {
    WeakObjWrapper *warpper = objc_getAssociatedObject(self, @selector(viewController));
    //用warpper保存传递进来的值
    if (!warpper) {//warpper不存在则创建
        warpper = [[WeakObjWrapper alloc] initWithWeakObject:viewController];
    }else {//已经存在直接赋值
        warpper.weakObj = viewController;
    }
    //保存的实际上是warpper对象
    objc_setAssociatedObject(self, @selector(viewController), warpper, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (UIViewController *)viewController {
    //获取到warpper
    WeakObjWrapper *warpper = objc_getAssociatedObject(self, _cmd);
    //取出warpper中的值
    return warpper.weakObj;
}
@end

int main(int argc, char * argv[]) {
    {
        Person *person = [[Person alloc] init];
        {
            UIViewController *viewController = [UIViewController new];
            person.viewController = viewController;
            NSLog(@"in - %@", person.viewController); //打印结果: in - <UIViewController: 0x101204370>
        }
        NSLog(@"end - %@", person.viewController);
    }
    return 0;
}

in - <UIViewController: 0x7fea1a404080>
end - (null)

void weak_setAssociatedObject(id _Nonnull object,
                              const void * _Nonnull key,
                              id _Nullable value) {
    //派生一个子类，类名为WeakObjWrapper+value对应的类名
    const char *clsName = [[NSString stringWithFormat:@"WeakObjWrapper%@", [value class]] UTF8String];
    //获取派生的子类
    Class childCls = objc_getClass(clsName);
    //如果子类不存在，利用runtime动态的创建一个子类
    if (!childCls) {
        childCls = objc_allocateClassPair([value class], clsName, 0);
        objc_registerClassPair(childCls);
    }
    //注册dealloc方法SEL
    SEL sel = sel_registerName("dealloc");
    //获取dealloc对应的类型编码
    const char *deallocEncoding = method_getTypeEncoding(class_getInstanceMethod([value class], sel));
    // 注意：内部持有value此处需要弱引用处理一下
    __weak typeof(value) weakValue = value;
    // 创建一个指向在调用dealloc方法时调用指定block的函数指针
    IMP deallocImp = imp_implementationWithBlock(^(id _childCls) {
        //在子类的dealloc方法中将value设置为nil，避免崩溃
        objc_setAssociatedObject(object, key, nil, OBJC_ASSOCIATION_ASSIGN);
        //派生的子类的dealloc方法会被调用，父类的不再被调用，故在此处调用一下父类的
        ((void (*)(id, SEL))(void *)objc_msgSend)(weakValue, sel);
    });
    //给子类添加dealloc方法
    class_addMethod(childCls, sel, deallocImp, deallocEncoding);
    //将value对应的isa指向子类
    object_setClass(value, childCls);
    //设置关联对象
    objc_setAssociatedObject(object, key, value, OBJC_ASSOCIATION_ASSIGN);
}

@interface Person (Test)
@property(assign, nonatomic) UIViewController *viewController;
@end
@implementation Person (Test)
- (void)setViewController:(UIViewController *)viewController {
    weak_setAssociatedObject(self, @selector(viewController), viewController);
}
- (UIViewController *)viewController {
    return objc_getAssociatedObject(self, _cmd);
}
@end

int main(int argc, char * argv[]) {
    {
        Person *person = [[Person alloc] init];
        {
            UIViewController *viewController = [UIViewController new];
            person.viewController = viewController;
            NSLog(@"in - %@", person.viewController); //打印结果: in - <UIViewController: 0x101204370>
        }
        NSLog(@"end - %@", person.viewController);
    }
    getchar();
    return 0;
}

in - <UIViewController: 0x7ff113704080>
end - (null)