XE6 TListView column widths become zero if you read column.width

Question

There TListViewis a mistake.

Reading a column Widthmay cause listview to attempt to directly obtain the column width from the main Windows control LISTVIEW— before the Win32 control columns are initialized.

Because the columns were not initialized, the listview message LVM_GETCOLUMNWIDTHcrashes, returning zero . Value TListViewmeans width is zero and makes all columns null.

This error was introduced sometime after Delphi 5.

Playback Steps

Add a report style list view with three columns to the form:

Add an event handler OnResizeto the list:

procedure TForm1.ListView1Resize(Sender: TObject);
begin
    {
       Any column you attempt to read the width of 
       will **cause** the width to become zero
    }
    ListView1.Columns[0].Width;
//  ListView1.Columns[1].Width;
    ListView1.Columns[2].Width;
end;

Run it:

Error

TListColumn Windows LISTVIEW , , LISTVIEW Windows

:

function TListColumn.GetWidth: TWidth;
var
   IsStreaming: Boolean;
   LOwner: TCustomListView;
begin
   LOwner := TListColumns(Collection).Owner;
   IsStreaming := [csReading, csWriting, csLoading] * LOwner.ComponentState <> [];

   if (
         (FWidth = 0) 
         and (LOwner.HandleAllocated or not IsStreaming)
      ) 
      or
      (
         (not AutoSize) 
         and LOwner.HandleAllocated 
         and (LOwner.ViewStyle = vsReport) 
         and (FWidth <> LVSCW_AUTOSIZE) 
         and (LOwner.ValidHeaderHandle)
      ) then
   begin
      FWidth := ListView_GetColumnWidth(LOwner.Handle, FOrderTag);
   end;

   Result := FWidth;
end; 

, dfm:

ComCtrls.TListColumn.GetWidth: TWidth;
TForm1.ListView1Resize(Sender: TObject);
Windows.CreateWindowEx(...)
Controls.TWinControl.CreateWindowHandle(const Params: TCreateParams);
Controls.TWinControl.CreateWnd;
ComCtrls.TCustomListView.CreateWnd;

, TCustomListView.CreatWnd CreateWnd:

procedure TCustomListView.CreateWnd;
begin
   inherited CreateWnd; //triggers a call to OnResize, trying to read the column widths
   //...
   Columns.UpdateCols; //add the columns
   //...
end;

TListColumn.GetWidth , .

Delphi 5?

Delphi 5 TCustomListView:

procedure TCustomListView.CreateWnd;
begin
   inherited CreateWnd; //triggers a call to OnResize
   //...
   Columns.UpdateCols;
   //...
end;

Delphi 5 :

function TListColumn.GetWidth: TWidth;
begin
   if FWidth = 0 then
      FWidth := ListView_GetColumnWidth(TListColumns(Collection).Owner.Handle, Index);
   Result := FWidth;
end;

, .

TListColumn.GetWidth? ? , , VCL , .

, ? OnResize, TFixedListView; , , TFixedListViewColumn.

.

: Embarcadero ? , TListColumn.GetWidth, ? ComponentState . , :

FAreColumnsInitialized: Boolean;

, .

?

?

Visual Styles.

Windows WM_PARENTNOTIFY, " ". LISTVIEW header, listview . Delphi hwnd:

procedure TCustomListView.WMParentNotify(var Message: TWMParentNotify);
begin
  with Message do
    if (Event = WM_CREATE) and (FHeaderHandle = 0) then
    begin
      FHeaderHandle := ChildWnd;

      //...
    end;
  inherited;
end;

Windows WM_PARENTNOTIFY . , TListColumn , :

   if (
         (FWidth = 0) 
         and (LOwner.HandleAllocated or not IsStreaming)
      ) 
      or
      (
         (not AutoSize) 
         and LOwner.HandleAllocated 
         and (LOwner.ViewStyle = vsReport) 
         and (FWidth <> LVSCW_AUTOSIZE) 
         and (LOwner.ValidHeaderHandle) //<--- invalid
      ) then
   begin
      FWidth := ListView_GetColumnWidth(LOwner.Handle, FOrderTag);
   end;

Windows LISTVIEW, Windows, , WM_PARENTNOTIFY :

   if (
         (FWidth = 0) 
         and (LOwner.HandleAllocated or not IsStreaming)
      ) 
      or
      (
         (not AutoSize) 
         and LOwner.HandleAllocated 
         and (LOwner.ViewStyle = vsReport) 
         and (FWidth <> LVSCW_AUTOSIZE) 
         and (LOwner.ValidHeaderHandle) //<--- Valid!
      ) then
   begin
      FWidth := ListView_GetColumnWidth(LOwner.Handle, FOrderTag);
   end;

, , .

, VCL WM_PARENTNOTIY :

procedure TCustomListView.WMParentNotify(var Message: TWMParentNotify);
begin
  with Message do
    if (Event = WM_CREATE) and (FHeaderHandle = 0) then
    begin
      FHeaderHandle := ChildWnd;
      UpdateCols; //20140822 Ian Boyd  Fixed QC123456 where the columns aren't usable in time
      //...
    end;
  inherited;
end;

Chatter

Windows 2000, ListView , :

lvrept.c

BOOL_PTR NEAR ListView_CreateHeader(LV* plv)
{
   ...
   plv->hwndHdr = CreateWindowEx(0L, c_szHeaderClass, // WC_HEADER,
       NULL, dwStyle, 0, 0, 0, 0, plv->ci.hwnd, (HMENU)LVID_HEADER, GetWindowInstance(plv->ci.hwnd), NULL);

   if (plv->hwndHdr) 
   {
      NMLVHEADERCREATED nmhc;

      nmhc.hwndHdr = plv->hwndHdr;
      // some apps blow up if a notify is sent before the control is fully created.
      CCSendNotify(&plv->ci, LVN_HEADERCREATED, &nmhc.hdr);
      plv->hwndHdr = nmhc.hwndHdr;
   }
   ...
}