Delphi XE4 discovers a switch of elements between TListGroups

Question

I have been using Delphi XE4 for a week now and I just opened a new TListGroups function. Easy to set TlistItem GroupID to switch / move between groups with ease. Now I would like to have a procedure that calls as soon as TlistItem DELETES a group and / or as soon as TlistItem ENTERS a Group. I am looking for something like this:

 procedure TForm1.ListView1On(Before/After)ListGroupEnter(Sender: TObject; Item: TListItem); begin // Do some stuff here like... // AllowedToDraw := TRUE/FALSE // I/O Code... // etc... end; procedure TForm1.ListView1On(Before/After)ListGroupLeave(Sender: TObject; Item: TListItem); begin // Do some stuff here like... // This might be replaceable with OnDeletion... // I/O Code... end; 

The main problem here is finding a switch between different groups if you set the GroupID property. So, is there anything to detect the switch as soon as you change / set the GroupID property?

Answer 1

The set property TListItem.GroupID sends the LVM_SETITEM message to TListView's HWND. You can subclass TListView and intercept this message before the TListView process processes it, for example:

 const APPWM_LISTVIEW_CHECKGROUP = WM_APP + 100; var PrevLVWndProc: TWndMethod = nil; procedure TForm1.FormCreate(Sender: TObject); begin PrevLVWndProc := ListView1.WindowProc; ListView1.WindowProc := LVWndProc; end; procedure TForm1.FormDestroy(Sender: TObject); begin ListView1.WindowProc := PrevLVWndProc; end; procedure TForm1.LVWndProc(var Message: TMessage); var LVItem: PLVItem; ListItem: TListItem; begin case Message.Msg of LVM_SETITEM: begin LVItem := PLVItem(Message.LParam); if (LVItem^.mask and LVIF_GROUPID) <> 0 then begin ListItem := ListView1.Items[LVItem.iItem]; if ListItem.GroupID <> LVItem.iGroupID then begin if ListItem.GroupID >= 0 then ListView1GroupLeave(ListView1, ListItem); PrevLVWndProc(Message); // this gives the TListItem time to actually update itself PostMessage(ListView1.Handle, APPWM_LISTVIEW_CHECKGROUP, 0, LVItem.iItem); Exit; end; end; end; APPWM_LISTVIEW_CHECKGROUP: begin ListItem := ListView1.Items[Message.LParam]; if ListItem.GroupID >= 0 then ListView1GroupEnter(ListView1, ListItem); Exit; end; end; PrevLVWndProc(Message); end; procedure TForm1.ListView1GroupLeave(Sender: TObject; Item: TListItem); begin // Item is about to leave from its current GroupID ... end; procedure TForm1.ListView1GroupEnter(Sender: TObject; Item: TListItem); begin // Item has entered into its current GroupID ... end;