Re: Mutual object interdependency trouble

From: Tom de Neef (tdeneef_at_qolor.nl)
Date: 01/26/05 

Date: Wed, 26 Jan 2005 09:38:49 +0100

"Rune" <rune[insert current year]@runevision.com> schreef in bericht
news:xzCJd.85270$Vf.3768803@news000.worldonline.dk...
> I've encountered a problem in the design of a game I'm working on.
>
> I need certain information to be available practically everywhere.
>
> As a simplified example, I have these four resources:
>
> Creature
> Level
> Maze
> Images
>
> Each of them needs all the ones below. The Maze needs the Images (so it
> can
> draw itself); the Level needs the Maze (which it is build after) and the
> Images (so it can draw itself); and the Creature needs the Level and Maze
> (so it can navigate) and the Images (so it can draw itself). Creature
> objects also need data about other Creature objects.
>
> There is no interdependency here. The problem is that it's very cumbersome
> to manually have to hand over references to all the needed resources after
> new objects are created. It'd be nice if objects after being created could
> just fetch the needed references themselves.
>
> This led me to the idea of some kind of Keeper class that keep references
> to
> the various resources. Objects could then just be given a reference to
> this
> Keeper object, and then they could request all needed resources from the
> Keeper object.
>
> However, this creates the mutual interdependency. The Keeper class must
> know
> about the four other classes, and the other classes (except the Images
> class). must also know about the Keeper class.
>
> As we know, in Delphi classes are bundled in units. As I understand it, a
> unit is one big text file that contain one or more classes. Since my game
> is big, I have several units to keep a better overview and the four
> mentioned classes are all in different units.
>
> The problem is that it seems that in Delphi, if Unit A knows about Unit B,
> then Unit B can't know about Unit A. This means that the Keeper concept
> doesn't seem to work.
>
> I'm not sure what to do. I've had classes about programming patterns, but
> I'm used to Java, not Delphi, and I only have limited practical experience
> with complex projects...
>
> Anyone have any ideas?
>
> Rune
> --
>

In Delphi, the dependency between units is in the interface section. Unit A
can use unit B and B can use data from A, as long as there is no circular
reference via the interface sections. So, you could have:

unit A;
interface uses B;
implementation
...
end.

unit B;
interface
...
implementation uses A;
...
end;

This allows you a lot of freedom in separation code across units. The limits
are reached when you want to declare two types that depend on each other in
their declaration, such as:
type
  Ta = class(Tb);
  Tb = class
    c : Ta;
  end;

In such cases Ta and Tb need to be in the same unit. What you could still do
though, is change the declaration of Tb into:
type
  Tb = class
  c : Tobject;
  end;

Now, Ta can be in a unit with 'interface uses B' and Tb in unit B with
'implementation uses A'; But you have to introduce some extra code to
typecast the variable c.
Tom

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