TCustomListBox.WndProc Method

Handles Windows messages.

Pascal

procedure WndProc(var Message: TMessage); override;

C++

virtual __fastcall WndProc(TMessage Message);

Description

WndProc overrides the inherited WndProc to provide specialized responses to messages that allows the list box to handle auto drag mode itself. Then it calls the inherited WndProc to handle all other messages.

When overriding the WndProc method, be sure to call the inherited WndProc at the end to dispatch any other messages.

C++ Examples:

Copy Code

/*
This example shows how to use the WndProc method and the
WindowProc property to subclass a custom control’s window
procedure.  This example subclasses the window procedure of
a TListBox descendant to respond to a user-defined message
called WM_STYLEMESSAGE. The subclassed window procedure can
be turned on or off by pressing a radio button.
*/
class TMyListBoxDescendant : public TListBox
{
__published:    // IDE-managed Components
    void __fastcall SubClassWndProc(Messages::TMessage &Message);
    void __fastcall ToggleSubClass(bool On);
    void __fastcall OnDrawItemProc(
      TWinControl *Control, int Index, const TRect &Rect,
      TOwnerDrawState State);
private:    // User declarations
public:     // User declarations
    __fastcall TMyListBoxDescendant(TComponent* Owner);
};

TForm1 *Form1;
TMyListBoxDescendant *MyListBoxDescendant1;
Graphics::TBitmap *bitmap0;

const WM_STYLEMESSAGE = WM_USER + 2000;

__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
}

__fastcall TMyListBoxDescendant::TMyListBoxDescendant(TComponent* Owner)
    : TListBox(Owner)
{
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  PostMessage(
    MyListBoxDescendant1->Handle,
    WM_STYLEMESSAGE,
    Integer(lbOwnerDrawFixed),
    0);
}

void __fastcall TForm1::Button2Click(TObject *Sender)
{
  PostMessage(
    MyListBoxDescendant1->Handle,
    WM_STYLEMESSAGE,
    Integer(lbStandard),
    0);
}

void __fastcall TForm1::SubClassRadioGroup1Click(TObject *Sender)
{
  MyListBoxDescendant1->ToggleSubClass(SubClassRadioGroup1->ItemIndex == 0);
}

void __fastcall TMyListBoxDescendant::SubClassWndProc(Messages::TMessage &Message)
{
  if (Message.Msg == WM_STYLEMESSAGE)
    Style = (TListBoxStyle)Message.WParam;
  else
    WndProc(Message);
}

void __fastcall TMyListBoxDescendant::ToggleSubClass(bool On)
{
  if (On)
    WindowProc = SubClassWndProc;
  else
    WindowProc = WndProc;
}

void __fastcall TMyListBoxDescendant::OnDrawItemProc(TWinControl *Control, int Index,
      const TRect &Rect, TOwnerDrawState State)
{
  Graphics::TBitmap *bitmap; // temporary variable for item’s bitmap
  int Offset = 2;   // default text offset width
  // note that we draw on the listbox’s canvas, not on the form
  TCanvas *canvas = ((TListBox *)Control)->Canvas;
  canvas->FillRect(Rect); // clear the rectangle
  bitmap = (Graphics::TBitmap *)((TListBox *)Control)->Items->Objects[Index];
  if (bitmap)
  {
    canvas->BrushCopy(
      Bounds(Rect.Left + Offset, Rect.Top, bitmap->Width, bitmap->Height),
      bitmap, Bounds(0, 0, bitmap->Width, bitmap->Height), clRed); // render bitmap
    Offset += bitmap->Width + 4;   // add four pixels between bitmap and text
  }
  // display the text
  canvas->TextOut(Rect.Left + Offset, Rect.Top, ((TListBox *)Control)->Items->Strings[Index]);
}

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  MyListBoxDescendant1 = new TMyListBoxDescendant(Form1);
  MyListBoxDescendant1->Visible = True;
  MyListBoxDescendant1->Parent = Form1;
  MyListBoxDescendant1->Visible = True;
  MyListBoxDescendant1->Left =
    SubClassRadioGroup1->Left + SubClassRadioGroup1->Width + 30;;
  MyListBoxDescendant1->Top = SubClassRadioGroup1->Top;
  MyListBoxDescendant1->Height = SubClassRadioGroup1->Height;
  MyListBoxDescendant1->OnDrawItem =
    MyListBoxDescendant1->OnDrawItemProc;

  bitmap0 = new Graphics::TBitmap;
  ImageList1->GetBitmap(0, bitmap0);
  MyListBoxDescendant1->Items->AddObject("Butterfly", bitmap0);

  SubClassRadioGroup1->Items->Add("SubClassWndProc");
  SubClassRadioGroup1->Items->Add("WndProc");
  SubClassRadioGroup1->ItemIndex = 2;
}

Delphi Examples:

Copy Code

{
This example shows how to use the WndProc method and the
WindowProc property to subclass a custom control’s window
procedure.  This example subclasses the window procedure of
a TListBox descendant to respond to a user-defined message
called WM_STYLEMESSAGE. The subclassed window procedure can
be turned on or off by pressing a radio button.
}

type
  TMyListBoxDescendant = class(TlistBox)
    procedure SubClassWndProc(var Message: TMessage);
    procedure ToggleSubClass(On: Boolean);
    procedure OnDrawItemProc(
      Control: TWinControl;
      Index: Integer;
      Rect:TRect;
      State: TOwnerDrawState);
  end;
  TForm1 = class(TForm)
    SubClassRadioGroup1: TRadioGroup;
    Button1: TButton;
    ImageList1: TImageList;
    Button2: TButton;
    procedure SubClassRadioGroup1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  MyListBoxDescendant1: TMyListBoxDescendant;
  bitmap0: TBitmap;

implementation

{$R *.dfm}

const WM_STYLEMESSAGE = WM_USER + 2000;

procedure TForm1.Button1Click(Sender: TObject);
begin
  PostMessage(
    MyListBoxDescendant1.Handle,
    WM_STYLEMESSAGE,
    Integer(lbOwnerDrawFixed),
    0);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  PostMessage(
    MyListBoxDescendant1.Handle,
    WM_STYLEMESSAGE,
    Integer(lbStandard),
    0);
end;

procedure TMyListBoxDescendant.SubClassWndProc(var Message: TMessage);
begin
  if (Message.Msg = WM_STYLEMESSAGE) then
    Style:= TListBoxStyle(Message.WParam)
  else
    WndProc(Message);
end;

procedure TMyListBoxDescendant.ToggleSubClass(On: Boolean);
begin
  if On then
    WindowProc := SubClassWndProc
  else
    WindowProc := WndProc;
end;

procedure TForm1.SubClassRadioGroup1Click(Sender: TObject);
begin
  MyListBoxDescendant1.ToggleSubClass(
    SubClassRadioGroup1.ItemIndex = 0);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  MyListBoxDescendant1:= TMyListBoxDescendant.Create(self);
  MyListBoxDescendant1.Visible:= True;
  MyListBoxDescendant1.Parent:= Form1;
  MyListBoxDescendant1.Visible:= True;
  MyListBoxDescendant1.Left:=
    SubClassRadioGroup1.Left + SubClassRadioGroup1.Width + 30;;
  MyListBoxDescendant1.Top:= SubClassRadioGroup1.Top;
  MyListBoxDescendant1.Height:= SubClassRadioGroup1.Height;
  MyListBoxDescendant1.OnDrawItem:=
    MyListBoxDescendant1.OnDrawItemProc;

  bitmap0 := TBitmap.Create;
  ImageList1.GetBitmap(0, bitmap0);
  MyListBoxDescendant1.Items.AddObject('Butterfly', bitmap0);

  SubClassRadioGroup1.Items.Add('SubClassWndProc');
  SubClassRadioGroup1.Items.Add('WndProc');
  SubClassRadioGroup1.ItemIndex := 2;
end;

procedure TMyListBoxDescendant.OnDrawItemProc(
  Control: TWinControl;
  Index: Integer;
  Rect:TRect;
  State: TOwnerDrawState);
var
  Bitmap: TBitmap;      { temporary variable for the item’s bitmap }
  Offset: Integer;      { text offset width }
begin
  { draw on control canvas, not on the form }
  with (Control as TListBox).Canvas do
  begin
    FillRect(Rect);       { clear the rectangle }
    Offset := 2;          { provide default offset }
    Bitmap := TBitmap((Control as TListBox).Items.Objects[Index]);  { get the bitmap }
    if Bitmap <> nil then
    begin
      BrushCopy(
        Bounds(Rect.Left + Offset, Rect.Top, Bitmap.Width, Bitmap.Height),
        Bitmap,
        Bounds(0, 0, Bitmap.Width, Bitmap.Height),
        clRed);  {render bitmap}
      Offset := Bitmap.width + 6;    { add four pixels between bitmap and text}
    end;
    TextOut(Rect.Left + Offset, Rect.Top, (Control as TListBox).Items[Index])  { display the text }
  end;
end;