LCL Interface Redesign Idea

From Lazarus wiki
Revision as of 09:13, 4 April 2007 by Marc (talk | contribs) (This is current Idea for the structure of the interface and how it relates to handles)

Why would we want this ?

In the present situation, the handle returned by TWSWinControl.CreateHandle is in case of win32 the winapi handle of the widget and in case of gtk a PGtkWidget. For the QT and Carbon widgetset a private internal object is returned. Using an internal object has some advantages. One of them is that we can use a true object hierarchy, with clear inheritence inside the widgetset. However, many code using the LCL expects the handle being castable. Returning an object would break this.

NOTE: In case of gtk, we don't have to return the internal object in the case of gtk, we still can use the existing way of retrieving the widgetinfo

This is current Idea for the structure of the interface and how it relates to handles

Overview:

  • Fork a branch of lazarus/
  • Replace TWinControl.Handle with TWinControl.LCLHandle (there should exist no Handle yet)
  • Try to get the LCL compiled and maybe the IDE so it doesn't use the (inefficient) .Handle anymore
  • Introduce .Handle again, now as WSclass call
  • Merge the branches

The change to .Handle will not break existing code.

TWSxxWinControl.CreateHandle Returns a TLCLHandle

TLCLHandle = IInterface; // interfaces as com objects

TxxPrivate = class(TPrivate, TLCLHandle);

TGtkPrivateButton = class(TGtkBin, TLCLHandle);

All TWSxx. methods should be moved to the private class. for instance:

 class procedure TWSGtkWinControl.SetText(const AWinControl; const AText: String);
 var
   PrivWinControl: TGtkPrivateWinControl;
 begin
   PrivWinControl := TGtkPrivateWinControl(AWinControl.LCLHandle);
   PrivWinControl.SetText(AText);
 end;

NOTE: I'm not to sure if we should expose the TLCLHandle as Interface. This will cause an overhead of adding exception frames, addref and releaseref calls. We can do it internally and use TLCLHandle = PtrUInt (MWE).

Other possibilities

For accessing widgetset native handles or IDs from the LCL, in the current situation, ppl have to cast the handle. In the case of gtk, to retrieve the XID, one needs the following cast:

 PGdkPrivateWindow(PGtkWidget(Handle)^.window)^.XID

When interfaces are used, one can imagine the following code

{$IFDEF GtkHasX}
TGtkPrivateWidget = class(TGtkPrivate, ILCLHandle, IX11Handle);
(AWinControl.LCLHandle as IX11Handle).XID

or maybe better:

if Supports(AWinControl.LCLHandle, IX11Handle, X11Handle) then
  ... DoSomething with X11Handle

Which leads to cleaner constructs, without the need of internal knowledge of the handle.

Note that for this we need COM interfaces and not CORBA, however we don't need reference counting, so AddRef and ReleseRef can be implemented by just returning -1