inline Class

objc_object::changeIsa(Class newCls)

{

    // This is almost always true but there are

    // enough edge cases that we can't assert it.

    // assert(newCls->isFuture()  ||

    //        newCls->isInitializing()  ||  newCls->isInitialized());

    assert(!isTaggedPointer()); 

    isa_t oldisa;

    isa_t newisa;+构建新对象体 isa_t

    bool sideTableLocked = false;

    bool transcribeToSideTable = false;

    do {

        transcribeToSideTable = false;

        oldisa = LoadExclusive(&isa.bits);

        if ((oldisa.bits ==   ||  oldisa.nonpointer)  &&

            !newCls->isFuture()  &&  newCls->canAllocNonpointer())

        {

            // 0 -> nonpointer

            // nonpointer -> nonpointer

#if SUPPORT_INDEXED_ISA

            if (oldisa.bits == ) newisa.bits = ISA_INDEX_MAGIC_VALUE;

            else newisa = oldisa;

            // isa.magic is part of ISA_MAGIC_VALUE

            // isa.nonpointer is part of ISA_MAGIC_VALUE

            newisa.has_cxx_dtor = newCls->hasCxxDtor();

            assert(newCls->classArrayIndex() > );

            newisa.indexcls = (uintptr_t)newCls->classArrayIndex();

#else
　　　　　　　　
　　　　　　　　

            if (oldisa.bits == ) newisa.bits = ISA_MAGIC_VALUE;

            else newisa = oldisa;

            // isa.magic is part of ISA_MAGIC_VALUE

            // isa.nonpointer is part of ISA_MAGIC_VALUE

            newisa.has_cxx_dtor = newCls->hasCxxDtor();

            newisa.shiftcls = (uintptr_t)newCls >> ;+改变类型的指针，存储的时候清除低位没用的3位

#endif } else if (oldisa.nonpointer) { // nonpointer -> raw pointer // Need to copy retain count et al to side table. // Acquire side table lock before setting isa to // prevent races such as concurrent -release.
　　　　　　　　+如果是TargetPoint对象，标记为需要转移其他数据

            if (!sideTableLocked) sidetable_lock();

            sideTableLocked = true;

            transcribeToSideTable = true;

            newisa.cls = newCls;

        }

        else {

            // raw pointer -> raw pointer

            newisa.cls = newCls;

        }
　　　　　　+上面在为新对象体转移数据

    } while (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits));+将数据覆盖对象体，内部原子的比较并交换

    if (transcribeToSideTable) {

        // Copy oldisa's retain count et al to side table.

        // oldisa.has_assoc: nothing to do

        // oldisa.has_cxx_dtor: nothing to do
　　　　　　+被标记为转移其他数据，转移引用计数器-1标识，转移是否正在dealloc的标识，转移所引用标识

        sidetable_moveExtraRC_nolock(oldisa.extra_rc,

                                     oldisa.deallocating,

                                     oldisa.weakly_referenced);

    }

    if (sideTableLocked) sidetable_unlock();

    if (oldisa.nonpointer) {

#if SUPPORT_INDEXED_ISA

        return classForIndex(oldisa.indexcls);

#else

        return (Class)((uintptr_t)oldisa.shiftcls << );

#endif

    }

    else {

        return oldisa.cls;

    }

}