理解iOS weak对象释放后置为nil原理

1、弱引用对象释放的浏览与普通对象释放流程基本一样，区别在于清除散列表。

1）清除散列表入口方法sidetable_clearDeallocating

void objc_object::sidetable_clearDeallocating()
{
    SideTable& table = SideTables()[this];
    table.lock();
    RefcountMap::iterator it = table.refcnts.find(this);
    if (it != table.refcnts.end()) {
        if (it->second & SIDE_TABLE_WEAKLY_REFERENCED) {
            weak_clear_no_lock(&table.weak_table, (id)this);
        }
        table.refcnts.erase(it);
    }
    table.unlock();
}

这里在散列表中找到当想对象的位置it，接着在弱引用表中删除弱引用对象，最后在散列表中清除it位置的数据；

2）weak_clear_no_lock方法分享

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) {
        return;
    }
    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);
}

首先根据weak_entry_for_referent(weak_table, referent)对象在弱引用表中找到实体entry，取出实体中弱引用数组referrers。referrers存储的是弱引用指针地址，循环数组置空，最后在弱引用表中移除实体weak_entry_remove(weak_table, entry)。