Re: Determine if an item in a collection has changed

Dmitriy-

Thanks for the reply.

How do others handle writing APIs for their application? Do people simply
not use APIs? Rather, does everyone slog Recordsets around?

While I would have thought what I am trying to do is pretty common, I
haven't seen any postings or articles about this. The closest is Rocky
Lhotka's VB6 Business Objects. While he's done a good job with it, I still
feel that there is too much ADO exposed to the client.

Thoughts?

Craig

"Dmitriy Antonov" <antonovdima@xxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:e3ywx4i6FHA.1248@xxxxxxxxxxxxxxxxxxxxxxx
> Yes, callbacks is an option. But you should be aware of one important
> nuance: this will require your Person objects to hold a reference to its
> parent, which by necessity holds a reference to each of its children (via
> internal Collection object). This introduces circular reference, which
> will hold all objects in memory even if you release all top-level
> references to any of these objects. Memory leak is guaranteed (and often
> very serious one).
>
> I know two methods to solve this problem.
>
> 1. The easiest, but not recommended. Add public method to your Persons
> collection (something like CleanRefs). Within this method you should
> remove all references from children to parent (generally you use simple
> Friend method like ReleaseParent or directly Set Parent=Nothing, assuming
> Parent is a property in Person class).
> Why I don't recommend this method - because it requires explicit call to
> that method just before destroying reference to Persons object from the
> last holder (and only from the last one). It opens the door for human
> mistakes, which are not immediately visible.
>
> 2. More complex. Instead of references to parent objects, all children
> hold its pointer (obtained using ObjPtr). You can resolve the object from
> that pointer using CopyMemory. E.g.
>
> Public Function ObjectFromPtr(lPtr As Long) As Object
> If lPtr <= 0 Then Exit Function
> Dim obj As Object
> CopyMemory ByVal ObjPtr(obj), lPtr, 4&
> Set ObjectFromPtr = obj
> CopyMemory ByVal ObjPtr(obj), 0&, 4&
> Set obj = Nothing
> End Function
>
> Be careful here, if pointer points to already destroyed object then GPF or
> something equally bad is guaranteed.
>
> Since it is possible that some of the children objects can be held
> somewhere outside collection it is still recommended to assign zero to all
> those pointers in collection's Terminate event, but there is no need to
> call anything explicitly and thus all objects will be successfully
> destroyed after the last reference is released.
>
> Dmitriy.
>
>
> "Craig Buchanan" <someone@xxxxxxxxxxxxx> wrote in message
> news:%23HOCegi6FHA.3544@xxxxxxxxxxxxxxxxxxxxxxx
>> Dmitriy-
>>
>> Thanks for the response.
>>
>> Let's say that i have two classes: Persons and Person.
>>
>> The Persons class is a Collection class. It has two relevant methods:
>> Add and Save. The Add requires a Person reference. The Save persists
>> all objects in the collection that have changed (either by a batch
>> update/insert or calling each Person instances Save method).
>>
>> The Person class is a custom object. It has two relevant, Friend
>> members: IsDirty property and Save method. If I make a change to the
>> Person or create a new person, IsDirty is true. The Save method manages
>> the insert/update.
>>
>> I could look at the state of the Person instance in the Add method and
>> set internal state accordingly. However, I'm more concerned about
>> changes to an item already in the collection. For instance:
>>
>> ...
>> Dim PP as New Persons
>> PP.Load 'load from db
>>
>> Dim P as Person: Set P = PP("#" & 10)
>> With P
>> .FirstName="Craig"
>> ...
>> End With
>> Set P = Nothing
>>
>> Set P = New Person
>> With P
>> ...
>> End With
>>
>> PP.Add P
>> Set P = nothing
>>
>> PP.Save
>>
>> ...
>>
>> Obviously, in PP.Save, i could enumerate each Person instance and call
>> the .Save method if the instance needed persistance, but i'd rather be
>> smart about it.
>>
>> As i'm not creating an explicit WithEvents reference for each Person in
>> the Persons class, raising an event is out. I guess that leaves me with
>> classbacks.
>>
>> Am I understanding the situation correctly?
>>
>> Craig
>>
>> "Dmitriy Antonov" <antonovdima@xxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
>> news:eALBbp05FHA.4036@xxxxxxxxxxxxxxxxxxxxxxx
>>>
>>> "Craig Buchanan" <someone@xxxxxxxxxxxxx> wrote in message
>>> news:%23dZ2o0z5FHA.3188@xxxxxxxxxxxxxxxxxxxxxxx
>>>> It's there an easy way to determine if one or more objects in a
>>>> collection has changed, without enumerating the collection and checking
>>>> each object individually? Is this a case for call backs?
>>>>
>>>> Thanks for the help.
>>>>
>>>
>>> If you mean an object of type Collection then it depends, what kind of
>>> items it contains. There is no way to notify about such changes if it
>>> contains items of primitive datatypes (yes you still can enumerate them
>>> and compare with some previous state as long as you preserve that state
>>> in some way). If it contains Objects created from classes, where such
>>> functionality is implemented than you can use that functionality. If you
>>> are talking about objects, which you create (or going to create) then
>>> you have many different options of doing this. You can add some
>>> property, like IsDirty or Changed, into your class and/or add event,
>>> which will be raised from the object of that class, when its state is
>>> changed.
>>>
>>> Callbacks? Well, you can use callbacks, if you know why. And yes you can
>>> use callbacks to notify other objects about changes in the state within
>>> items of your collection. Whether you need to do it depends on specifics
>>> of your application. Callback is just a mechanism, where some object
>>> holds a reference to another object and calls predefined method(s) of
>>> the latter one under certain conditions.
>>>
>>> Dmitriy.
>>>
>>
>>
>
>


.

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