Creating-an-equivalent-to-the-missing-TListViewOnColumnDblClick


            Title: Creating an equivalent to the missing TListView.OnColumnDblClick
Question: How to subclass your header-control (using a TListView), to receive a OnColumnDblClick-equivalent notification ?
Answer:
 This requires a bit of work. MS did not see fit to send a notification to the TListView when the user double-clicks on the header. But the header control class does have the CS_DBLCLKS style, so it does get WM_LBUTTONDBLCLK messages, it just does not do anything with them. 
 To get at these messages requires API-style subclassing of the header control. How? See below.
uses ..., Commctrl;
...
const
 UM_LISTVIEW_COLUMN_DBLCLICK = WM_USER + 1982;
....
{ the HeaderProc function should look something like this: }
Function
 HeaderProc( wnd: HWND; msg: Cardinal; wparam: WPARAM; lparam: LPARAM): Longint; stdcall;
var
 hti: THDHitTestInfo;
Begin
 Result := CallWindowProc( Pointer( GetWindowLong( wnd, GWL_USERDATA )),
 wnd, msg, wparam, lparam );
 If msg = WM_LBUTTONDBLCLK Then Begin
 FillChar( hti, sizeof(hti), 0 );
 hti.Point := SmallPointToPoint( TSmallPoint( lparam ));
 If SendMessage( wnd, HDM_HITTEST, 0, Longint(@hti)) = 0 Then
 If hti.Flags = HHT_ONHEADER 
 Then PostMessage(MainForm.Handle,UM_LISTVIEW_COLUMN_DBLCLICK,hti.Item,0);
 { Change MainForm to whatever you need }
 End;
End;
 
procedure TMainForm.FormCreate(Sender: TObject);
var
 wnd: HWND;
 oldProc: Integer;
begin
{beginning of workaround for missing TListView.OnColumnDblClick}
 wnd:= GetWindow( aListView.handle, GW_CHILD ); { If wnd 0 then begin
 If (GetClassLong( wnd, GCL_STYLE ) and CS_DBLCLKS) 0 then begin
 oldproc := GetWIndowLong( wnd, GWL_WNDPROC );
 If GetWindowLong( wnd, GWL_USERDATA ) 0 Then
 raise Exception.Create( 'Cannot sublcass ListView header, USERDATA already in use');
 SetWIndowLong( wnd, GWL_USERDATA, oldproc );
 SetWindowLong( wnd, GWL_WNDPROC, integer(@HeaderProc));
 end;
 end
 else
 ShowMessage('ListView component in vsReport state is missing !!!');
 {...}
 {Do some more wonderful things}
end;
 
and then don't forget to declare a custom message handler for UM_LISTVIEW_COLUMN_DBLCLICK (this will be your OnColumnDblClick equivalent).
 Bogdan Grigorescu - BogdanG@gmail.com
 BG Remote Programming Group
VCL Delphi
