Transparente Listbox

[toms]

Hi,

Habe mal eine TransparentListBox Komponente gefunden.
So wie sie jetzt ist funktioniert sie aber nur für nicht-scrollende Listboxen.

Delphi-Quellcode:

unit TransparentListBox;

(*
 *
 *  Written by Walter Irion (CIS 114254, 2455) after the THotSpot
 *  sample component that Arne Schäpers presented in the German
 *  c't magazine (issue 6/1996, pp. 286 ff.).
 *
 *  TTransparentListBox is far from being a universal solution:
 *  it does not prevent Windows' scrolling mechanism from
 *  shifting the background along with scrolled listbox lines.
 *  Moreover, the scroll bar remains hidden until the keyboard
 *  is used to change the selection, and the scroll buttons
 *  become visible only when clicked.
 *
 *  To break it short: TTransparentListBox is only suitable
 *  for non-scrolling lists.
 *
 *  In fact it must be possible to write a listbox component
 *  that handles scrolling correctly. But my essays to intercept
 *  EM_LINESCROLL messages were fruitles, even though I tried
 *  subclassing via WndProc.
 *
 *  A solution for transparent TEdit and TMemo controls is
 *  introduced in issue 9/1996 of the c't magazine, again
 *  by Arne Schäpers. But these are outright monsters with
 *  wrapper windows to receive notification messages as well
 *  as so-called pane windows that cover the actual control's
 *  client area and display its content.
 *
 *  Previous issues of the c't magazine can be ordered from:
 *
 *    c't-Kopierservice
 *    Helstorfer Str. 7
 *    30625 Hannover, Germany
 *
 *  They expect a crossed cheque amounting to DM 14,00
 *  to be included with your order, but I don't know about
 *  international orders.
 *
 *)

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;

type
  TTransparentListBox = class(TListBox)
  private
    { Private declarations }
  protected
    { Protected declarations }
    procedure CreateParams(var Params: TCreateParams); override;
    procedure WMEraseBkgnd(var Msg: TWMEraseBkgnd); message WM_ERASEBKGND;
    procedure DrawItem(Index: Integer; Rect: TRect; State: TOwnerDrawState);
      override;
  public
    { Public declarations }
    constructor Create(AOwner: TComponent); override;
    procedure SetBounds(ALeft, ATop, AWidth, AHeight: Integer); override;
  published
    { Published declarations }
    property Style default lbOwnerDrawFixed;
    property Ctl3D default False;
    property BorderStyle default bsNone;
  end;

procedure Register;

implementation

constructor TTransparentListBox.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  Ctl3D := False;
  BorderStyle := bsNone;
  Style := lbOwnerDrawFixed; // changing it to lbStandard results
                              // in loss of transparency
end;

procedure TTransparentListBox.CreateParams(var Params: TCreateParams);
begin
  inherited CreateParams(Params);
  Params.ExStyle := Params.ExStyle or WS_EX_TRANSPARENT;
end;

procedure TTransparentListBox.WMEraseBkgnd(var Msg: TWMEraseBkgnd);
begin
  Msg.Result := 1; // Prevent background from getting erased
end;

procedure TTransparentListBox.SetBounds(ALeft, ATop, AWidth, AHeight: Integer);
var
  tlbVisible: Boolean;
begin
  tlbVisible := (Parent <> nil) and IsWindowVisible(Handle); // Check for visibility
  if tlbVisible then ShowWindow(Handle, SW_HIDE); // Hide-Move-Show strategy ...
  inherited SetBounds(ALeft, ATop, AWidth, AHeight); // ... to prevent background ...
  if tlbVisible then ShowWindow(Handle, SW_SHOW); // ... from getting copied
end;

procedure TTransparentListBox.DrawItem(Index: Integer; Rect: TRect; State:
  TOwnerDrawState);
var
  FoundStyle: TBrushStyle;
  R: TRect;

begin

  FoundStyle := Canvas.Brush.Style; // Remember the brush style

  R := Rect; // Adapt coordinates of drawing rect ...
  MapWindowPoints(Handle, Parent.Handle, R, 2); // ... to parent's coordinate system
  InvalidateRect(Parent.Handle, @R, True); // Tell parent to redraw the item position
  Parent.Update; // Trigger instant redraw (required)

  if not (odSelected in State) then begin // If an unselected line is being handled
    Canvas.Brush.Style := bsClear; //   use a transparent background
  end else begin // otherwise, if the line needs to be highlighted,
    Canvas.Brush.Style := bsSolid; //   some colour to the brush is essential
  end;

  inherited DrawItem(Index, Rect, State); // Do the regular drawing and give component users ...
                                          // ... a chance to provide an OnDrawItem handler

  Canvas.Brush.Style := FoundStyle; // Boy-scout rule No. 1: leave site as you found it

end;

procedure Register;
begin
  RegisterComponents('Samples', [TTransparentListBox]);
end;

end.