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Прозрачный TListBox



Оформил: DeeCo

unit TransparentListBox;

 (*
  *
  *  Written by Walter Irion (CIS 114254, 2455) after the THotSpot
  *  sample component that Arne Sch?pers 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 Sch?pers. 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.





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