Painting TPaintBox during Drag&Drop with DragImage - delphi

Question

In my application (Delphi 2007) I want to drag items from a ListView to a PaintBox and highlight corresponding areas in the PaintBox's OnPaint handler. However I always get ugly artefacts. Do you have any advice how I can get rid of them?
Test project: Just create a new VCL application and replace the code in Unit1.pas with the following. Then start the app and drag list items over the rectangle in the PaintBox.
unit Unit1;
interface
uses
Windows,
Messages,
SysUtils,
Variants,
Classes,
Graphics,
Controls,
Forms,
Dialogs,
ExtCtrls,
ComCtrls,
ImgList;
type
TForm1 = class(TForm)
private
PaintBox1: TPaintBox;
ListView1: TListView;
ImageList1: TImageList;
FRectIsHot: Boolean;
function GetSensitiveRect: TRect;
procedure PaintBox1DragOver(Sender, Source: TObject; X, Y: Integer;
State: TDragState; var Accept: Boolean);
procedure PaintBox1Paint(Sender: TObject);
public
constructor Create(AOwner: TComponent); override;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
uses
TypInfo;
const
IconIDs: array[TMsgDlgType] of PChar = (IDI_EXCLAMATION, IDI_HAND,
IDI_ASTERISK, IDI_QUESTION, nil);
{ TForm1 }
constructor TForm1.Create(AOwner: TComponent);
var
Panel1: TPanel;
mt: TMsgDlgType;
Icon: TIcon;
li: TListItem;
begin
inherited Create(AOwner);
Width := 600;
Height := 400;
ImageList1 := TImageList.Create(Self);
ImageList1.Name := 'ImageList1';
ImageList1.Height := 32;
ImageList1.Width := 32;
ListView1 := TListView.Create(Self);
ListView1.Name := 'ListView1';
ListView1.Align := alLeft;
ListView1.DragMode := dmAutomatic;
ListView1.LargeImages := ImageList1;
Panel1 := TPanel.Create(Self);
Panel1.Name := 'Panel1';
Panel1.Caption := 'Drag list items here';
Panel1.Align := alClient;
PaintBox1 := TPaintBox.Create(Self);
PaintBox1.Name := 'PaintBox1';
PaintBox1.Align := alClient;
PaintBox1.ControlStyle := PaintBox1.ControlStyle + [csDisplayDragImage];
PaintBox1.OnDragOver := PaintBox1DragOver;
PaintBox1.OnPaint := PaintBox1Paint;
PaintBox1.Parent := Panel1;
ListView1.Parent := Self;
Panel1.Parent := Self;
Icon := TIcon.Create;
try
for mt := Low(TMsgDlgType) to High(TMsgDlgType) do
if Assigned(IconIDs[mt]) then
begin
li := ListView1.Items.Add;
li.Caption := GetEnumName(TypeInfo(TMsgDlgType), Ord(mt));
Icon.Handle := LoadIcon(0, IconIDs[mt]);
li.ImageIndex := ImageList1.AddIcon(Icon);
end;
finally
Icon.Free;
end;
end;
function TForm1.GetSensitiveRect: TRect;
begin
Result := PaintBox1.ClientRect;
InflateRect(Result, -PaintBox1.Width div 4, -PaintBox1.Height div 4);
end;
procedure TForm1.PaintBox1Paint(Sender: TObject);
var
r: TRect;
begin
r := GetSensitiveRect;
if FRectIsHot then
begin
PaintBox1.Canvas.Pen.Width := 5;
PaintBox1.Canvas.Brush.Style := bsSolid;
PaintBox1.Canvas.Brush.Color := clAqua;
end
else
begin
PaintBox1.Canvas.Pen.Width := 1;
PaintBox1.Canvas.Brush.Style := bsClear;
end;
PaintBox1.Canvas.Rectangle(r.Left, r.Top, r.Right, r.Bottom);
end;
procedure TForm1.PaintBox1DragOver(Sender, Source: TObject; X,
Y: Integer; State: TDragState; var Accept: Boolean);
var
r: TRect;
MustRepaint: Boolean;
begin
MustRepaint := False;
if State = dsDragEnter then
begin
FRectIsHot := False;
MustRepaint := True;
end
else
begin
r := GetSensitiveRect;
Accept := PtInRect(r, Point(X, Y));
if Accept <> FRectIsHot then
begin
FRectIsHot := Accept;
MustRepaint := True;
end;
end;
if MustRepaint then
PaintBox1.Invalidate;
end;
end.
Edit: Here is a picture of the glitch:DragImage artefact http://img269.imageshack.us/img269/6535/15778780.png
I expect to see the complete blue rectangle with thick border. However beneath the drag image one can see the un-highlighted rectangle.
Edit 2: This site talks about "Painting Issues":
The ImageList SDK notes that when
drawing the drag image you can get
issues with updates or screen painting
unless you use the ImageList_DragLeave
API function to hide the drag image
whilst the painting occurs (which is
what the HideDragImage method in the
class does). Unfortunately, if you
don't own the control that's being
painted doing this isn't really an
option.
I don't have the problem mentioned in the last sentence. Nevertheless I wasn't able to find the right place and the right imagelist (it's not ImageList1 in my test project - probably ListView1.GetDragImages) to call ImageList_DragLeave.

Answer 1

The key is to hide the drag image before the paint box is redrawn, and to show it again after that. If you replace this code in your question:
procedure TForm1.PaintBox1DragOver(Sender, Source: TObject; X,
Y: Integer; State: TDragState; var Accept: Boolean);
var
r: TRect;
MustRepaint: Boolean;
begin
MustRepaint := False;
if State = dsDragEnter then
begin
FRectIsHot := False;
MustRepaint := True;
end
else
begin
r := GetSensitiveRect;
Accept := PtInRect(r, Point(X, Y));
if Accept <> FRectIsHot then
begin
FRectIsHot := Accept;
MustRepaint := True;
end;
end;
if MustRepaint then
PaintBox1.Invalidate;
end;
with this
procedure TForm1.PaintBox1DragOver(Sender, Source: TObject; X,
Y: Integer; State: TDragState; var Accept: Boolean);
var
r: TRect;
begin
if State = dsDragEnter then
begin
FRectIsHot := False;
PaintBox1.Invalidate;
end
else
begin
r := GetSensitiveRect;
Accept := PtInRect(r, Point(X, Y));
if Accept <> FRectIsHot then
begin
FRectIsHot := Accept;
ImageList_DragShowNolock(False);
try
PaintBox1.Refresh;
finally
ImageList_DragShowNolock(True);
end;
end;
end;
end;
it should work. Well, it does for me with Delphi 2007 on Windows XP 64 bit.
And kudos for the demonstration code in your question, excellent way to let us see the problem.

Answer 2

Tested on XP, Delphi 2010 - I get the artifacts, so it's XP related and not fixed in D2010
Edit:
Upon further investigation - if you drag an icon so that the mouse only just enters the box (but the icon doesn't) then the box is drawn correctly, it's only when the icon enters the paintbox that the artifacts occur.
I added code so that if state was dsDragMove then it would force a repaint and this worked, but suffered from flicker