什么是 "non-weak zeroing reference"

Question

在 OS X v10.11 beta release notes 中，我找到以下内容：

NSNotificationCenter and NSDistributedNotificationCenter no longer send notifications to registered observers that may be deallocated. If the observer is able to be stored as a zeroing-weak reference the underlying storage stores the observer as a zeroing weak reference. Alternatively, if the object cannot be stored weakly (because it has a custom retain/release mechanism that would prevent the runtime from being able to store the object weakly) the object is stored as a non-weak zeroing reference. This means that observers are not required to un-register in their deallocation method. [emphasis mine]

这对我来说没有意义。如果是非弱引用，那岂不是强引用？所以 NSNotificationCenter 仍然是所有者，所以对象不会解除分配（直到手动取消注册），所以在这种情况下说它是 "zeroing".

是荒谬的

如果这是指某种 __unsafe_unretained 引用，那么问题是…… NSNotificationCenter 如何避免向僵尸发送消息？

Answer 1

这个问题的答案深藏于 objective-c 运行时，以及 __weak 变量的实际工作方式。为了解释，让我们稍微看一下 objc_weak.mm:

id 
weak_read_no_lock(weak_table_t *weak_table, id *referrer_id) 
{
    ...

    if (! referent->ISA()->hasCustomRR()) {
        if (! referent->rootTryRetain()) {
            return nil;
        }
    }
    else {
        BOOL (*tryRetain)(objc_object *, SEL) = (BOOL(*)(objc_object *, SEL))
            object_getMethodImplementation((id)referent, 
                                           SEL_retainWeakReference);
        if ((IMP)tryRetain == _objc_msgForward) {
            return nil;
        }
        if (! (*tryRetain)(referent, SEL_retainWeakReference)) {
            return nil;
        }
    }

    return (id)referent;
}

如您所见，当对象使用自定义 -retain 和 -release 方法时，根本不能保证它们支持弱引用（另请注意，您可以使用完全不同的对象的弱引用的对象，虽然那是另一个话题）。

这是因为弱引用被objc_destructInstance清除，后者调用objc_clearDeallocating，后者调用weak_clear_no_lock。

现在，自定义对象实现不需要调用 objc_destructInstance，尽管大多数对象都会调用它。

因此，运行时允许您实现方法 -allowsWeakReference（和 retainWeakReference）来禁用对您的对象的弱引用，在这种情况下，它很可能会通过 swizzling -dealloc 在对象上。当然，这是所有的实现细节，所以 NSNotificationCenter 可以有它自己创新的做事方式，但这是我最好的猜测，没有尝试反汇编 NSNotificationCenter。

Answer 2

Declaring a property as strong makes that property a strong reference. Declaring it as weak uses a zeroing weak reference. The unsafe_unretained modifier uses a non-zeroing weak reference

简述：non-weak zeroing reference == unsafe_unretained refernce

参考：

https://mikeash.com/pyblog/friday-qa-2011-09-30-automatic-reference-counting.html http://www.jessesquires.com/UIKit-changes-in-iOS-9/

什么是 "non-weak zeroing reference"

What is a "non-weak zeroing reference"

cocoa

memory-management

objective-c

nsnotificationcenter

osx-elcapitan