3期_iOS-weak内部实现

weak修饰的属性，内部会执行

初始化weak指针，这个方法传递了两个参数值，一个是要指向弱引用对象的对象，一个是需要被弱引用的对象

id
objc_initWeak(id *location, id newObj)
{
    if (!newObj) {
        *location = nil;
        return nil;
    }

    return storeWeak<DontHaveOld, DoHaveNew, DoCrashIfDeallocating>
        (location, (objc_object*)newObj);
}

id
objc_storeWeak(id *location, id newObj)
{
    return storeWeak<DoHaveOld, DoHaveNew, DoCrashIfDeallocating>
        (location, (objc_object *)newObj);
}

大体流程为

被weak修饰的变量，系统会在编译时调用objc_initWeak方法，然后调用storeWeak，再调用weak_register_no_lock，在这个方法中，会根据对象的地址hash算出位置，然后插入弱引用表中

如何被销毁呢？

dealloc方法中，标志有弱引用关系，执行weak_clear_no_lock，这个方法会在弱引用表中找到要销毁的对象，并把当前对象的弱应用都取出来，置为nil

此方法传入弱引用计数表，被清除的弱应用对象

void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    objc_object *referent = (objc_object *)referent_id;

    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    if (entry == nil) {
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }

    // zero out references
    weak_referrer_t *referrers;
    size_t count;
    
    if (entry->out_of_line()) {
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }
    
    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p. "
                             "This is probably incorrect use of "
                             "objc_storeWeak() and objc_loadWeak(). "
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();
            }
        }
    }
    
    weak_entry_remove(weak_table, entry);
}