從源碼角度看蘋果是如何實現 dealloc 的

大多數與生命週期有關的方法咱們都已經講完了，接下來來看最後一個方法，dealloc。

dealloc

dealloc 方法的實現以下：

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

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

    obj->rootDealloc();
}

inline void
objc_object::rootDealloc()
{
    if (isTaggedPointer()) return;  // fixme necessary?

    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    { // 若是 isa 不是經過指針實現的，不是弱引用，沒有關聯對象，沒有 C++ 析構函數，沒有在 sidetable 中存儲引用計數，直接 free
        assert(!sidetable_present());
        free(this);
    } 
    else {
        object_dispose((id)this);
    }
}
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object_dispose 的實現以下所示：

/*********************************************************************** * object_dispose * fixme * Locking: none **********************************************************************/
id object_dispose(id obj) {
    if (!obj) return nil;

    objc_destructInstance(obj);    
    free(obj);

    return nil;
}

/*********************************************************************** * objc_destructInstance * Destroys an instance without freeing memory. * Calls C++ destructors. * Calls ARC ivar cleanup. * Removes associative references. * Returns `obj`. Does nothing if `obj` is nil. **********************************************************************/
void *objc_destructInstance(id obj) {
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor(); // 獲取是否有 C++ 析構函數
        bool assoc = obj->hasAssociatedObjects(); // 獲取是否有關聯對象

        // This order is important.
        if (cxx) object_cxxDestruct(obj); // 若是有析構函數，調用 object_cxxDestruct
        if (assoc) _object_remove_assocations(obj); // 若是有關聯對象，調用 _object_remove_assocations
        obj->clearDeallocating();
    }

    return obj;
}
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hasCxxDtor & object_cxxDestruct

hasCxxDtor 實現以下：

inline bool
objc_object::hasCxxDtor()
{
    assert(!isTaggedPointer());
    if (isa.nonpointer) return isa.has_cxx_dtor;
    else return isa.cls->hasCxxDtor();
}
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object_cxxDestruct 實現以下：

/*********************************************************************** * object_cxxDestruct. * Call C++ destructors on obj, if any. * Uses methodListLock and cacheUpdateLock. The caller must hold neither. **********************************************************************/
void object_cxxDestruct(id obj) {
    if (!obj) return;
    if (obj->isTaggedPointer()) return;
    object_cxxDestructFromClass(obj, obj->ISA());
}

/*********************************************************************** * object_cxxDestructFromClass. * Call C++ destructors on obj, starting with cls's * dtor method (if any) followed by superclasses' dtors (if any), * stopping at cls's dtor (if any). * Uses methodListLock and cacheUpdateLock. The caller must hold neither. **********************************************************************/
static void object_cxxDestructFromClass(id obj, Class cls) {
    void (*dtor)(id);

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

    for ( ; cls; cls = cls->superclass) { // 遞歸調用 obj 及其父類的析構函數
        if (!cls->hasCxxDtor()) return; 
        dtor = (void(*)(id))
            lookupMethodInClassAndLoadCache(cls, SEL_cxx_destruct); // 查找方法
        if (dtor != (void(*)(id))_objc_msgForward_impcache) {
            if (PrintCxxCtors) {
                _objc_inform("CXX: calling C++ destructors for class %s", 
                             cls->nameForLogging());
            }
            (*dtor)(obj);
        }
    }
}
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hasAssociatedObjects & _object_remove_assocations

hasAssociatedObjects 實現以下：

inline bool
objc_object::hasAssociatedObjects()
{
    if (isTaggedPointer()) return true;
    if (isa.nonpointer) return isa.has_assoc;
    return true;
}
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_object_remove_assocations 實現以下，簡而言之就是查找對象關聯的全部對象，並依次調用 ReleaseValue。

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());
    }
};
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clearDeallocating

clearDeallocating 方法的實現以下：

inline void 
objc_object::clearDeallocating()
{
    if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
        sidetable_clearDeallocating();
    }
    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());
}
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sidetable_clearDeallocating

sidetable_clearDeallocating 方法會在 isa 是指針的實現中被調用，其實現以下：

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();
}
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clearDeallocating_slow

clearDeallocating_slow 方法會在 isa 是非指針的實現中被調用，其實現以下：

// 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();
}
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