銷毀的源頭

調(diào)用-release改览，release會(huì)調(diào)用:
uintptr_t objc_object::sidetable_release(bool performDealloc)

sidetable_release():

以下是runtime源碼 NSObject.mm 1584行

bjc_object::sidetable_release(bool performDealloc)
{
#if SUPPORT_NONPOINTER_ISA
    assert(!isa.nonpointer);
#endif
    SideTable& table = SideTables()[this];

    bool do_dealloc = false;
    //加鎖
    table.lock();
//獲取當(dāng)前對(duì)象所在的sidetable(一個(gè)hash表),在sidetable.refcnts(RefcountMap,一個(gè)map)中查到當(dāng)前對(duì)象的迭代器
    RefcountMap::iterator it = table.refcnts.find(this);
    //接著判斷迭代器是否是指向了sidetable的end
    //如果是就代表找不到:
    if (it == table.refcnts.end()) {
      //將對(duì)象標(biāo)記為“正在析構(gòu)”
      //標(biāo)記需要dealloc
        do_dealloc = true;
        table.refcnts[this] = SIDE_TABLE_DEALLOCATING;
    } else if (it->second < SIDE_TABLE_DEALLOCATING) {
      //判斷之前存儲(chǔ)的引用計(jì)數(shù)值是否為 0,避免負(fù)數(shù)
      //將對(duì)象標(biāo)記為“正在析構(gòu)”
      //標(biāo)記需要dealloc
        // SIDE_TABLE_WEAKLY_REFERENCED may be set. Don't change it.
        do_dealloc = true;
        it->second |= SIDE_TABLE_DEALLOCATING;
    } else if (! (it->second & SIDE_TABLE_RC_PINNED)) {
       //將引用計(jì)數(shù)減一
        it->second -= SIDE_TABLE_RC_ONE;
    }
   //解鎖
    table.unlock();
    if (do_dealloc  &&  performDealloc) {
    //調(diào)用對(duì)象的dealloc方法:
        ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
    }
    return do_dealloc;
}

如果調(diào)用了dealloc:

進(jìn)行以下過程時(shí)低滩，不能再創(chuàng)建有新的 __weak引用,否則會(huì)crash

遞歸調(diào)用父類的-dealloc

如果是 MRC 代碼,則需要手動(dòng)釋放實(shí)例變量

最后調(diào)用NSObject的dealloc

NSObject的dealloc會(huì)調(diào)用 _objc_rootDealloc(self);

_objc_rootDealloc(id obj) NSObject.mm 1838行

_objc_rootDealloc(id obj)
{
    //是否還活著 斷言
    assert(obj);
   //調(diào)用 objc_object::rootDealloc()
    obj->rootDealloc();
}

objc_object::rootDealloc() objc_object.h 415行

objc_object::rootDealloc()
{
    //是否使用TaggedPointer優(yōu)化 是直接人突然
    if (isTaggedPointer()) return;  // fixme necessary?

   //不需要處理object_dispose的所有內(nèi)容
    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    {
        assert(!sidetable_present());
        free(this);
    } 
    else {
       //調(diào)用object_dispose
        object_dispose((id)this);
    }
}

object_dispose(id obj) objc-runtime-new.mm 6313行

id object_dispose(id obj)
{
    //是否為nil 直接返回
    if (!obj) return nil;
    //調(diào)用 void *objc_destructInstance(id obj)
    objc_destructInstance(obj);    
    free(obj);
    return nil;
}

void *objc_destructInstance(id obj) objc-runtime-new.mm 6290行

void *objc_destructInstance(id obj) 
{
//如果不為nil 才處理
    if (obj) {
        //是否有析構(gòu)函數(shù)  這個(gè)bool值取決于當(dāng)前類以及父類往上是否有實(shí)例變量，如果有實(shí)例變量當(dāng)前類就有.cxxDestruct欺抗，當(dāng)前類或父類有此方法值=YES售碳，都沒有才=NO
        bool cxx = obj->hasCxxDtor();
       //是否有關(guān)聯(lián)對(duì)象
        bool assoc = obj->hasAssociatedObjects();
        // This order is important.
      //如果有析構(gòu)函數(shù) 調(diào)用 void object_cxxDestruct(id obj)
        if (cxx) object_cxxDestruct(obj);
       //如果有關(guān)聯(lián)對(duì)象 移除關(guān)聯(lián)對(duì)象
        if (assoc) _object_remove_assocations(obj);
        //調(diào)用objc_clear_deallocating()清空引用計(jì)數(shù)表
        obj->clearDeallocating();
    }
    return obj;
}

void object_cxxDestruct(id obj) objc-class.mm 473行

void object_cxxDestruct(id obj)
{
   //如果為nil 直接retun
    if (!obj) return;
   //是否使用TaggedPointer優(yōu)化 是直接return
    if (obj->isTaggedPointer()) return;
   //調(diào)用 static void object_cxxDestructFromClass(id obj, Class cls)
    object_cxxDestructFromClass(obj, obj->ISA());
}

static void object_cxxDestructFromClass(id obj, Class cls) objc-class 447行

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

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

    //往父類遞歸調(diào)用.cxxDestruct 直到hasCxxDtor=NO return結(jié)束
    for ( ; cls; cls = cls->superclass) {
        if (!cls->hasCxxDtor()) return; 
        dtor = (void(*)(id))
            lookupMethodInClassAndLoadCache(cls, SEL_cxx_destruct);
        if (dtor != (void(*)(id))_objc_msgForward_impcache) {
            if (PrintCxxCtors) {
                _objc_inform("CXX: calling C++ destructors for class %s", 
                             cls->nameForLogging());
            }
            (*dtor)(obj);
        }
    }
}

.cxx_destruct

ARC下?lián)碛袑?shí)例變量才會(huì)有這個(gè)方法,通過Clang CodeGen生成,MRC都需要手動(dòng)release所以不需要
ARC下會(huì)遍歷當(dāng)前對(duì)象所有的實(shí)例變量通過objc_storeStrong() release掉
具體實(shí)現(xiàn)過程:https://blog.sunnyxx.com/2014/04/02/objc_dig_arc_dealloc/

void objc_removeAssociatedObjects(id object) objc-runtime 635行

void objc_removeAssociatedObjects(id object) 
{
    if (object && object->hasAssociatedObjects()) {
        _object_remove_assocations(object);
    }
}

void _object_remove_assocations(id object) objc-references 316行

關(guān)聯(lián)對(duì)象都存放在AssociationsHashMap中,以obj為key,以存放關(guān)聯(lián)對(duì)象的ObjectAssociationMap為value,然后拿到ObjectAssociationMap中的所有ObjcAssociation對(duì)象，然后此對(duì)象調(diào)用ReleaseValue(),繼而調(diào)用releaseValue绞呈，然后調(diào)用objc_release

void _object_remove_assocations(id object) {
    vector< ObjcAssociation,ObjcAllocator<ObjcAssociation> > elements;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        if (associations.size() == 0) return;
        disguised_ptr_t disguised_object = DISGUISE(object);
        AssociationsHashMap::iterator i = associations.find(disguised_object);
        if (i != associations.end()) {
            // copy all of the associations that need to be removed.
            ObjectAssociationMap *refs = i->second;
            for (ObjectAssociationMap::iterator j = refs->begin(), end = refs->end(); j != end; ++j) {
                elements.push_back(j->second);
            }
            // remove the secondary table.
            delete refs;
            associations.erase(i);
        }
    }
    // the calls to releaseValue() happen outside of the lock.
    for_each(elements.begin(), elements.end(), ReleaseValue());
}

struct ReleaseValue {
    void operator() (ObjcAssociation &association) {
        releaseValue(association.value(), association.policy());
    }
};
static void releaseValue(id value, uintptr_t policy) {
    if (policy & OBJC_ASSOCIATION_SETTER_RETAIN) {
        return objc_release(value);
    }
}

void objc_clear_deallocating(id obj) NSObject.mm 1705行

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

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

objc_object::clearDeallocating() objc-object.h 399行

inline void 
objc_object::clearDeallocating()
{
    if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
       //調(diào)用sidetable_clearDeallocating()把對(duì)象的weak指針置nil,把對(duì)象的計(jì)數(shù)引用移除
        sidetable_clearDeallocating();
    }
    else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
        /判斷是否有過弱引用   是否因?yàn)橛?jì)數(shù)太大有多個(gè)sidetable
        // Slow path for non-pointer isa with weak refs and/or side table data.
       //調(diào)用clearDeallocating_slow();內(nèi)部再分開判斷各自實(shí)現(xiàn)sidetable_clearDeallocating的內(nèi)容
        clearDeallocating_slow();
    }

    assert(!sidetable_present());
}

相信看了上面的源碼分析 對(duì)于下面這種圖 父類有屬性贸人，關(guān)聯(lián)對(duì)象，子類有屬性關(guān)聯(lián)對(duì)象佃声，此時(shí)創(chuàng)建個(gè)[SubClass new]然后釋放掉艺智，這幾個(gè)類的dealloc打印順序是什么應(yīng)該也清楚了，當(dāng)然屬性之間dealloc順序圾亏，關(guān)聯(lián)對(duì)象之間順序還需要注意力惯，同一個(gè)類多個(gè)屬性的dealloc順序是倒序，

• 屬性列表的順序：

分類屬性1召嘶，分類屬性2父晶，類擴(kuò)展屬性1，類擴(kuò)展屬性2弄跌，類屬性1甲喝，類屬性2

• 屬性的dealloc銷毀順序是：

類擴(kuò)展屬性2，類擴(kuò)展屬性1铛只，類屬性2埠胖，類屬性1，然后再銷毀父類的屬性

關(guān)聯(lián)對(duì)象添加的成員變量是先銷毀子類的成員變量淳玩，在銷毀父類的直撤，在同一個(gè)類里添加的順序不一定，因?yàn)榈讓邮谴鎯?chǔ)在無序map中蜕着，所以dealloc順序不一定

屏幕快照 2018-11-15 下午2.00.48.png

runtime源代碼下載地址
搜索objc4 選擇數(shù)字最大的即為最新的(并不是最下面的就是最新的谋竖，看數(shù)字大小)