1. dealloc

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

2. _objc_rootDealloc()

_objc_rootDealloc(id obj)
{
    ASSERT(obj);

    obj->rootDealloc();
}

3. obj->rootDealloc()

objc_object::rootDealloc()
{
      //isTaggedPointer 类型  直接返回
    if (isTaggedPointer()) return;  // fixme necessary?

    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&       //弱引用
                 !isa.has_assoc  &&                    //关联对象
                 !isa.has_cxx_dtor  &&                //C++
                 !isa.has_sidetable_rc))               //散列表（引用计数表）
    {
        assert(!sidetable_present());
        free(this);
    } 
    else {
        object_dispose((id)this);
    }
}

4. object_dispose((id)this);

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

    objc_destructInstance(obj);    
    free(obj);

    return nil;
}

5. objc_destructInstance(obj)

/***********************************************************************
* 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();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj);                     //C++
        if (assoc) _object_remove_assocations(obj);         //移除关联对象
        obj->clearDeallocating();
    }

    return obj;
}

6. object_cxxDestruct

/***********************************************************************
* 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());
}

7. _object_remove_assocations

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

    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.get());
        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();
    }
}

8. clearDeallocating()

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());
}

9. sidetable_clearDeallocating()

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();
}

10. clearDeallocating_slow

// 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();
}