iOS weak指针置nil具体过程

先看一下delloc的具体实现：

dealloc的实现

- (void)dealloc {
    _objc_rootDealloc(self);
}

    进入_objc_rootDealloc

void
_objc_rootDealloc(id obj)
{
    assert(obj);

    obj->rootDealloc();
}

    进入rootDealloc

inline void
objc_object::rootDealloc()
{
    if (isTaggedPointer()) return;  // fixme necessary?
        //如果是优化的isa
    //并且对应的没有关联对象、weak应用、以及c++析构函数和sideTable的引用计数，就直接free掉
    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    {
        assert(!sidetable_present());
        free(this);
    } 
    else {
        //进入销毁流程
        object_dispose((id)this);
    }
}

    进入object_dispose


id 
object_dispose(id obj)
{
    if (!obj) return nil;

    objc_destructInstance(obj);    
    free(obj);

    return nil;
}

    接着进入objc_destructInstance

void *objc_destructInstance(id obj) 
{
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj);//清除成员变量
        if (assoc) _object_remove_assocations(obj);//移除关联对象
        obj->clearDeallocating();//将指向当前的弱指针置为nil
    }

    return obj;
}

weak指针置nil

• 当weak引用指向的对象被释放时，我们需要把指针置为nil

• 我们在前面已经讲解了，当一个对象释放时，需要调用objc_release方法，如果引用计数为0时，会执行dealloc方法，在把weak指针置nil的过程会调用clearDeallocating

void 
objc_clear_deallocating(id obj) 
{
    assert(obj);

    if (obj->isTaggedPointer()) return;
    obj->clearDeallocating();
}

• 在函数内部调用了clearDeallocating函数

inline void 
objc_object::clearDeallocating()
{
    //如果是没有优化过的isa指针
    if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
        sidetable_clearDeallocating();
    }
    //是优化过的isa指针，并且是否有弱引用过，或者引用计数是否存在sideTable中
    else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
        // Slow path for non-pointer isa with weak refs and/or side table data.
        clearDeallocating_slow();
    }
    assert(!sidetable_present());
}

• 不论我们是哪种方式，我们都会调用weak_clear_no_lock方法

void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    //1、拿到被销毁对象的指针
    objc_object *referent = (objc_object *)referent_id;
    //2、通过 指针 在weak_table中查找出对应的entry
    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    if (entry == nil) {
        return;
    }

    //3、将所有的引用设置成nil
    weak_referrer_t *referrers;
    size_t count;
    
    if (entry->out_of_line()) {
        //3.1、如果弱引用超过4个则将referrers数组内的弱引用都置成nil。
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        //3.2、不超过4个则将inline_referrers数组内的弱引用都置成nil
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }
    //循环设置所有的引用为nil
    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();
            }
        }
    }
    //4、从weak_table中移除entry
    weak_entry_remove(weak_table, entry);
}