Generic mechanism for instantiating distinct types in Delphi - oop

Question

I'm trying to use generics to 'genericize' a var that instantiates network transports of different types. I'm not sure if the "generic=no RTTI" rule would invalidate the approach or not, as I'm not yet up to speed with generics.
I've read this post:
What is the correct way to structure this generic object creation , which states the following in the question:
One other thing I would like to do if possible, is to change two
creations:
LAdapter := TSQLiteNativeConnectionAdapter.Create(LFilename)
LAdapter := TFireDacConnectionAdapter.Create(FDatabaseLink.FConnection as TFDConnection, FDatabaseLink.OwnedComponent)
to use an abstract "GetAdapterClass" type function in the parent
TModelDatabaseConnection and just declare the class of adapter in the
child to do something like:
LAdapter := GetAdapterClass.Create...
This is exactly what I would like to do as well. So if you can picture this:
type
TTransport<T> = class(TComponent)
private
...
function GetTransport: TTransport;
procedure SetTransport(AValue: TTransport);
...
public
...
property Transport: TTransport read GetTransport write SetTransport;
...
end;
TTCPIPTransport = class(TTransport<T>)
private
function GetSocket(Index: Integer): String;
procedure SetSocket(Index: Integer; AValue: String);
public
property Socket[Index: Integer]: String read GetSocket write SetSocket;
end;
TServiceTransport = class(TTransport<T>)
private
function GetServiceName: String;
procedure SetServiceName(AValue: String);
public
property ServiceName: String read GetServiceName write SetServiceName;
end;
TISAPITransport = class(TServiceTransport<T>);
THTTPSysTransport = class(TServiceTransport<T>)
private
function GetURL(Index: Integer): String;
procedure SetURL(Index: Integer; AValue: String);
public
property URL[Index: Integer]: read GetURL write SetURL;
end;
etc.
The idea is to create a base class that has all fields/properties/methods that are common to all transports, then have intermediate classes that contain fields/methods/properties that are common only to a certain subset of transports, then have the final version of each transport be specific to the type.
So when I call:
var
trans: TTransport<T> // or TTransport<TTCPIPTransport> etc.
begin
trans := TTransport<TTCPIPTransport>.Create(AOwner,....);
trans.Transport.Socket[0] := '127.0.0.1:8523';
OR
trans := TTransport<TISAPITransport>.Create(AOwner,...);
trans.Transport.ServiceName = 'Foo';
...
etc.
end;
or perhaps even more generic then that, but have each instance of trans - without typecasting - have properties/fields/methods that are specific to the subclass automagically show up.
This way I can have a config screen that allows an administrator to select the type of transport say in a combo box, the have that variable value set the type inside the <> in code, and one set of code handles creation of the object by it's type.
Is this possible using generics?

Answer 1

Here is my first (feeble) attempt at a class factory, never done this before. It works partially (generates the correct class) but isn't accessible as a distinct subclass of the base class without typecasting, which defeats the purpose. Please see inline comments
TWSTransport = class(TComponent)
...
public
constructor Create(AOwner: TComponent); virtual;
....
end;
TWSTransportClass = Class of TWSTransport;
TWSTCPIPTransportClass = class of TWSTCPIPTransport;
TWSHTTPSysTransport = class(TWSServiceTransport);
TWSServiceTransport = class(TWSTransport);
TWSTransportStringConversion = class(TWSTransport);
TWSTransportStreamFormat = class(TWSTransportStringConversion);
TTransportFactory = class(TClassList)
private
function GetTransport(Index: TWSTransportClass; AOwner: TkbmMWServer): TWSTransportClass;
public
procedure RegisterTransportClass(ATransportClass: TWSTransportClass);
property Transport[Index: TWSTransportClass; AOwner: TkbmMWServer]: TWSTransportClass read GetTransport;
end;
function TTransportFactory.GetTransport(Index: TWSTransportClass; AOwner: TkbmMWServer): TWSTransportClass;
begin
if IndexOf(Index) > -1 then
Result := TWSTransportClass(Items[IndexOf(Index)])
else
Result := TWSTransportClass(Index.Create(AOwner));
end;
procedure TTransportFactory.RegisterTransportClass(ATransportClass: TWSTransportClass);
var
index: Integer;
begin
// is the transport registered?
index := IndexOf(ATransportClass);
if index < 0 then
// the transport is not registered, add it to the list
Add(ATransportClass);
end;
initialization
factory := TTransportFactory.Create;
factory.RegisterTransportClass(TWSHTTPSysTransport);
factory.RegisterTransportClass(TWSISAPIRESTTransport);
factory.RegisterTransportClass(TWSTCPIPTransport);
finalization
FreeAndNil(factory);
end.
Here's how I tested it:
procedure TForm4.FormCreate(Sender: TObject);
var
//trans: TWSTCPIPTransport; // this doesn't work
trans: TWSTransport; // this works
begin
trans := factory.Transport[TWSTCPIPTransport,Self];
showmessage(trans.classname); // this shows the correct classname - TWSTCPIPTransport
trans.AddSocket('127.0.0.1:80'); // the compiler gives an error here because this call is specific to a subclass of TWSTransport, TWSTCPIPTransport.
end;
So I'm still missing something... anyone see the mistake?