Runtime objc4-779.1 通過runtime源碼對OC對象銷燬過程解析

Runtime objc4-779.1 通過runtime源碼對OC對象銷燬過程解析

Step1 NSObject.mm line 2340

// Replaced by NSZombies
- (void)dealloc {
	// Setp2
    _objc_rootDealloc(self);
}

Step2 NSObject.mm line 1814

void
_objc_rootDealloc(id obj)
{
// 判斷對象是否真是存在,不存在則結束
    ASSERT(obj);
	// Step3
    obj->rootDealloc();
}

Step3 objc-object.h line 433

inline void
objc_object::rootDealloc()
{
	// 如果是tagged pointer 則結束, 因爲它不按照對象的內存管理來,僅僅是一個指針,指針中含有對象相關的信息,包含它的值, 如果不知道什麼個tagged pointer 可以看之前的博文 https://blog.csdn.net/wxs0124/article/details/82712478
    if (isTaggedPointer()) return;  // fixme necessary?

	// 如果對象已成創建isa_t(初始化),沒有被weak修飾,沒有關聯其他對象,沒有實現C++析構方法,沒有因引用計數過多而存儲在sidetable中
    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    {
        //判定本對象不存在於sidetable中
        assert(!sidetable_present());
        //釋放
        free(this);
    } 
    else {
    	// 此步需要進一步解析
    	// Step4
        object_dispose((id)this);
    }
}

Step4 objc-runtime-new.mm line 7564

id 
object_dispose(id obj)
{
    if (!obj) return nil;
	// 處理c++析構過程,移除對象關聯,clear
	// Step5
    objc_destructInstance(obj);    
    free(obj);

    return nil;
}

Step5 objc-runtime-new.mm line 7542

void *objc_destructInstance(id obj) 
{
    if (obj) {
        // Read all of the flags at once for performance.
        // 是否有c++/OC析構函數
        bool cxx = obj->hasCxxDtor();
        // 是否有其他關聯
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        // 進行析構,詳見 Step6
        if (cxx) object_cxxDestruct(obj);
        // 移除所有相關的關聯對象,詳見 Step7
        if (assoc) _object_remove_assocations(obj);
        // 清理內存空間 詳見Step8
        obj->clearDeallocating();
    }

    return obj;
}

Step6 objc-class.mm line 463

void object_cxxDestruct(id obj)
{
    if (!obj) return;
    if (obj->isTaggedPointer()) return;
    // Step6.1
    object_cxxDestructFromClass(obj, obj->ISA());
}

6.1 objc-class.mm line 437

static void object_cxxDestructFromClass(id obj, Class cls)
{
    void (*dtor)(id);

    // Call cls's dtor first, then superclasses's dtors.

	// 從本類開始按照繼承鏈依次便利父類
    for ( ; cls; cls = cls->superclass) {
    
    	// 如果沒有實現c++/OC析構方法,則結束
        if (!cls->hasCxxDtor()) return; 
        
        // 在本類中找到析構方法並且記載到緩存中 , 方法實現在objc-class-old.mm line 578 此處不展開
        dtor = (void(*)(id))
            lookupMethodInClassAndLoadCache(cls, SEL_cxx_destruct);
        
        // 因爲lookupMethodInClassAndLoadCache只會在本類中尋找方法(不會去父類),找不到則會返回objc_msgForward, _objc_msgForward_impcache是編譯器的消息轉發標記,代表此方法要走消息轉發,如果析構方法在本類中被找到了,則此一定爲true,進入代碼塊,執行代碼.
        if (dtor != (void(*)(id))_objc_msgForward_impcache) {
            if (PrintCxxCtors) {
                _objc_inform("CXX: calling C++ destructors for class %s", 
                             cls->nameForLogging());
            }
            (*dtor)(obj);
        }
    }
}

Step7 objc-references.mm line 217

從關聯表中移除相關的關聯對象

void
_object_remove_assocations(id object)
{
    ObjectAssociationMap refs{};

    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.get());
        // 便利查找到和object相關的map並從其中移除記錄
        AssociationsHashMap::iterator i = associations.find((objc_object *)object);
        if (i != associations.end()) {
            refs.swap(i->second);
            associations.erase(i);
        }
    }

    // release everything (outside of the lock).
    for (auto &i: refs) {
        i.second.releaseHeldValue();
    }
}

這裏的代碼實在associationHashMap中尋找當前對象相關關聯,並且擦出
association是一塊相對獨立且重要的知識點.牽扯關聯表的存儲格式,存取對象方式,流程,內存管理等,我們會在後邊的博客中詳細探討.此處不再展開

Step8 objc-object.h line 417

inline void 
objc_object::clearDeallocating()
{
	// 如果是隻有isa指針,沒有優化成isa_t
    if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
        sidetable_clearDeallocating();
    }
    else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
    // 如果沒有被弱引用,或者 在sidetable中查不到(意味着引用計數沒有太大,在isa_t中存儲)
        // Slow path for non-pointer isa with weak refs and/or side table data.
        clearDeallocating_slow();
    }

    assert(!sidetable_present());
}

Step8.1 NSObject.mm line 1552

清空在sidetable(用於存儲引用計數過多導致isa存不下的對象的應用計數信息)的數據

void 
objc_object::sidetable_clearDeallocating()
{
    SideTable& table = SideTables()[this];

    // clear any weak table items
    // clear extra retain count and deallocating bit
    // (fixme warn or abort if extra retain count == 0 ?)
    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();
}

Step8.2 NSObject.mm line 1212

// Slow path of clearDeallocating() 
// for objects with nonpointer isa
// that were ever weakly referenced 
// or whose retain count ever overflowed to the side table.
NEVER_INLINE void
objc_object::clearDeallocating_slow()
{
    ASSERT(isa.nonpointer  &&  (isa.weakly_referenced || isa.has_sidetable_rc));

    SideTable& table = SideTables()[this];
    table.lock();
    if (isa.weakly_referenced) {
        weak_clear_no_lock(&table.weak_table, (id)this);
    }
    if (isa.has_sidetable_rc) {
        table.refcnts.erase(this);
    }
    table.unlock();
}

總結一下:

• 釋放對象的核心方法爲void *objc_destructInstance(id obj)
• 核心流程爲 1, 沿着繼承鏈調用c++/OC析構函數.2,移除與其相關聯的對象記錄.weak指針置爲nil.3,移除weak修飾的對象在weak_table的記錄並更新其引用計數數據.
• 萬變不離其宗,其中知識點依舊是老生常談的AssociationsHashMap,weak,引用計數.我們逐一擊破