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Unit MyUnit;

procedure MyOnDrawItem //Für eine beliebige TListBox einer anderen Unit
procedure MachWas(var aListBox: TListBox);
//...
procedure MachWas(var aListBox: TListBox);
begin
  aListBox.OnDrawItem := MyOnDrawItem;
end;                    

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unit ListBoxDrawer;

interface

uses
  Vcl.StdCtrls, Vcl.Controls, System.Types, Vcl.Graphics, System.SysUtils;

type
  TListBoxDrawer = class
  private
    class var FInstance: TListBoxDrawer;
    procedure DoOnDrawItem(Control: TWinControl; Index: Integer; Rect: TRect; State: TOwnerDrawState);
  public
    class constructor Create;
    class destructor Destroy;
    class procedure RegisterListBox(const ATarget: TListBox);
    class procedure UnregisterListBox(const ATarget: TListBox);
  end;


implementation

{ TListBoxDrawer }

class constructor TListBoxDrawer.Create;
begin
  FInstance := TListBoxDrawer.Create;
end;

class destructor TListBoxDrawer.Destroy;
begin
  FInstance.Free;
end;

procedure TListBoxDrawer.DoOnDrawItem(Control: TWinControl; Index: Integer;
  Rect: TRect; State: TOwnerDrawState);
var
  ListBoxCanvas: TCanvas;
begin
  ListBoxCanvas := (Control as TListBox).Canvas;
  ListBoxCanvas.FillRect(Rect);
  ListBoxCanvas.TextOut(Rect.Left + 2, Rect.Top, 'Test ' + IntToStr(Index));
end;

class procedure TListBoxDrawer.RegisterListBox(const ATarget: TListBox);
begin
  ATarget.OnDrawItem := FInstance.DoOnDrawItem;
end;

class procedure TListBoxDrawer.UnregisterListBox(const ATarget: TListBox);
begin
  ATarget.OnDrawItem := nil;
end;

end.                    

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procedure TFormTest.FormCreate(Sender: TObject);
begin
  TListBoxDrawer.RegisterListBox(ListBox1);
end;                    

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procedure [...](var aListBox: TListBox);
begin
  TListBoxDrawer.RegisterListBox(aListBox);
  try
    //Hier folgt der Code, der Items in aListBox einliest
    //...
  finally
    TListBoxDrawer.UnRegisterListBox(aListBox);  //<- Hier wird TListBoxDrawer.DoOnDrawItem deaktiviert
  end;
end;                    

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unit ListBoxDrawer;
//...
type
  TListBoxDrawer = class
  private
    class var aLBx: TListBox;  //<- Hier eine private Variable
//...                    

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finalization
  if Assigned(TListBoxDrawer.aLBx) then
    TListBoxDrawer.UnregisterListBox(TListBoxDrawer.aLBx);