🚀 go-pugleaf

"OD" <OD[.@.]e-naxos.com> a �crit dans le message de news:
mn.cca47d57c87d5b9d.18651@0e-naxos.com...

> once again with a problem of modeling under C# more than a syntax
> problem.
> here is the problem :
>
> Class A is showing a property (or accessors, no matter) for a
> collection of object of class P.
>
> users of class A can add and suppress instances of P
>
> Now, I've got a class B that is referencing some instances of P stores
> in A.
>
> Well. Here is the question : what is the best pattern to be sure that
> when the user is deleting an object from the list stores in A, the
> object is no more referenced by the instance of class B ?

You could use the Observer pattern, but in C#, you would simply declare an
event on A or P and add a handler/delegate on B that adds its delegate to
the event on A/P. Since .NET events are multicast, you have the Observer
pattern for free :-))

Try the following untested but possibly correct code :

public delegate void OnRemovePHandler(P item);

public class P
{
  public void OnRemove()
  {
    if (Remove != null)
      Remove(this);
  }

  public event OnRemovePHandler Remove;
}

public class A
{
  ArrayList internalList = new ArrayList;

  public AddP(P item)
  {
    internalList.Add(item);
  }

  public RemoveP(P item)
  {
    internalList.Remove(item);
    item.OnRemove();
  }

  public P this[int index]
  {
    get
    {
      return internalList[index];
    }
  }
}

public class B
{
  private P pRef;

  public P PRef
  {
    get { return pRef; }
    set
    {
      if (pRef != value)
      {
        if(pRef != null)
          pRef.Remove -= new OnRemovePHandler(HandleRemoveP);

        pRef = value;

        if (pRef != null)
          pRef.Remove += new OnRemovePHandler(HandleRemoveP);
      }
    }
  }

  private void HandleRemoveP(P item)
  {
    if (pRef == item)
      PRef = null;
  }
}

Joanna

--
Joanna Carter (TeamB)

Consultant Software Engineer
TeamBUG support for UK-BUG
TeamMM support for ModelMaker