> Also, when I create my TCollection object, I pass the TCollectionItem
> class I wish to use, is it possible to have different Item classes in one
> collection? I was thinking of something like:
(snip)
Yes, it's eminently possible, and it's one of those wonderful OO things.
The secret lies in not using the TCollection.Add method at all. If you
examine the source for classes.pas, you'll find:
function TCollection.Add: TCollectionItem;
begin
Result := FItemClass.Create(Self);
end;
All the work is done in the constructor of the item class. I've enclosed a
small sample project to show you what I mean, but you basically don't have
to change anything. Just pass in the class of the BaseItemClass thing when
you create the collection, e.g.:
with TAnotherCollectionItem.Create(MyCollection) do
begin
// Initialise values etc here
end;
> I was thinking I could dupe the behaviour of TCollection.Add and pass in
> the class to create, but looking in the .dfm no classnames are mentioned
> for the items.
You can do that as well. Look at how I've done TZoo.AddAnimal. It's
essentially the same as TCollection.Add, but it parameterises the type of
the object. Yay.
(I'm not quite sure what you mean by "looking in the .dfm no classnames are
mentioned for the items"...?)
I love OO. It gives me a warm feeling inside.
- Matt
8<---
program Project1;
uses
Classes,
Dialogs;
{$R *.RES}
type
TAnimal = class;
TAnimalClass = class of TAnimal;
TZoo = class(TCollection)
procedure AgitateAnimals;
function AddAnimal(AnimalType: TAnimalClass): TAnimal;
end;
TAnimal = class(TCollectionItem)
public
procedure MakeNoise; virtual; abstract;
end;
TLion = class(TAnimal)
public
procedure MakeNoise; override;
end;
TSheep = class(TAnimal)
public
procedure MakeNoise; override;
end;
TCow = class(TAnimal)
public
procedure MakeNoise; override;
end;
procedure TZoo.AgitateAnimals;
var
i: Integer;
begin
for i := 0 to Count - 1 do
TAnimal(Items[i]).MakeNoise;
end;
function TZoo.AddAnimal(AnimalType: TAnimalClass): TAnimal;
begin
Result := AnimalType.Create(Self);
end;
procedure TLion.MakeNoise;
begin
ShowMessage('Roar!');
end;
procedure TSheep.MakeNoise;
begin
ShowMessage('Baa!');
end;
procedure TCow.MakeNoise;
begin
ShowMessage('Moo!');
end;
var
CityZoo: TZoo;
begin
// make a zoo
CityZoo := TZoo.Create(TAnimal);
// add some animals
TLion.Create(CityZoo);
TSheep.Create(CityZoo);
CityZoo.AddAnimal(TCow);
// now stir them up a bit...
CityZoo.AgitateAnimals;
CityZoo.Free;
end.
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