Is there a bug in the Delphi list view control when using custom drawing?

Question

QC#101189

I'm trying to custom draw a progress bar in a Delphi TListView as suggested by NGLN's answer to another SO question. This works fine, apart from the interaction with hot tracking when drawn using the new explorer theme introduced in Vista.

The hot tracking painting and the Delphi custom drawing events appear to interfere with each other. For example, the sort of output I am seeing looks like this:

The text in Column 1 should read Item 3 but is obliterated. It looks like a bug in the Delphi wrapper to the list view control, but it could equally be that I'm doing something wrong!

Although I've been developing this in XE2, the same behaviour occurs in 2010 and, presumably, XE.

Here's the code to reproduce this behaviour:

Pascal file

unit Unit1;

interface

uses
  Windows, Classes, Controls, Forms, CommCtrl, ComCtrls;

type
  TForm1 = class(TForm)
    ListView: TListView;
    procedure FormCreate(Sender: TObject);
    procedure ListViewCustomDrawSubItem(Sender: TCustomListView;
      Item: TListItem; SubItem: Integer; State: TCustomDrawState;
      var DefaultDraw: Boolean);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  ListView.RowSelect := True;
  ListView.Items.Add.Caption := 'Item 1';
  ListView.Items.Add.Caption := 'Item 2';
  ListView.Items.Add.Caption := 'Item 3';
end;

procedure TForm1.ListViewCustomDrawSubItem(Sender: TCustomListView;
  Item: TListItem; SubItem: Integer; State: TCustomDrawState;
  var DefaultDraw: Boolean);
var
  R: TRect;
begin
  DefaultDraw := False;
  ListView_GetSubItemRect(Sender.Handle, Item.Index, SubItem, LVIR_BOUNDS, @R);
  Sender.Canvas.MoveTo(R.Left, R.Top);
  Sender.Canvas.LineTo(R.Right-1, R.Bottom-1);
end;

end.

Form file

object Form1: TForm1
  Caption = 'Custom Draw List View Bug'
  ClientHeight = 290
  ClientWidth = 554
  OnCreate = FormCreate
  object ListView: TListView
    Align = alClient
    Columns = <
      item
        Caption = 'Column 1'
        Width = 250
      end
      item
        Caption = 'Column 2'
        Width = 250
      end>
    ViewStyle = vsReport
    OnCustomDrawSubItem = ListViewCustomDrawSubItem
  end
end

Answer 1

This is a workaround for the defective behavior rather than being an answer to the question if there's a bug in the VCL, and a few thoughts.

The workaround is to set the background mode of the device context assigned by the common control for item painting cyle to transparent after carrying out custom drawing:

procedure TForm1.ListViewCustomDrawSubItem(Sender: TCustomListView;
  Item: TListItem; SubItem: Integer; State: TCustomDrawState;
  var DefaultDraw: Boolean);
var
  R: TRect;
begin
  if not [CustomDrawing] then  // <- If we're not gonna do anything do not
    Exit;                      //    fiddle with the DC in any way

  DefaultDraw := False;
  ListView_GetSubItemRect(Sender.Handle, Item.Index, SubItem, LVIR_BOUNDS, @R);
  Sender.Canvas.MoveTo(R.Left, R.Top);
  Sender.Canvas.LineTo(R.Right-1, R.Bottom-1);

  SetBkMode(Sender.Canvas.Handle, TRANSPARENT); // <- will effect the next [sub]item
end; 

In an [sub]item paint cycle, the painting is always done in a top-down fashion, items having a lower index are sent NM_CUSTOMDRAW notification prior to ones with higher indexes. When the mouse is moved from one row to another, two rows need to be re-drawn - the one loosing the hot state, and the one gaining it. It would seem, when custom drawing is in-effect, drawing the row that's loosing the hot-state leaves the DC in an undesirable state. This is not a problem when moving the mouse upwards, because that item gets drawn last.

Custom drawing ListView and TreeView controls are different than custom drawing other controls and somewhat complicated (see: Custom Draw With List-View and Tree-View Controls). But you have full control over the entire process. The code in the NM_CUSTOMDRAW case of TCustomListView.CNNotify in 'comctrls.pas' of the VCL is equally complicated. But despite being provided a bunch of custom drawing handlers (half of them being advanced), you have no control over what the VCL does. For instance you can't return the CDRF_xxx you'd like or you can't set the clrTextBk you want. My biased opinion is that, there's a bug/design issue in the Delphi list view control, but I have nothing more concrete than an intuition as in finding a workaround.