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How to paint a TListBox with alternating background colors per row Turn on/off line numbers in source code. Switch to Orginial background IDE or DSP color Comment or reply to this aritlce/tip for discussion. Bookmark this article to my favorite article(s). Print this article
27-Aug-02
Category
VCL-General
Language
Delphi 2.x
Views
92
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Publisher:
DSP, Administrator
Reference URL:
DKB
			Author: Tomas Rutkauskas

I am trying to display a list box that has an alternating background color for each 
row. I realize I can do this by making the Listbox an owner draw list box and 
setting the background color for each line when it is drawn. The problem here is 
only the lines corresponding to existing items will be effected. Even if the 
listbox has no items in it, I still want it to be displayed with the alternating 
background colors.

Answer:

Solve 1:

It requires a combination of an OnDrawItem handler (or an overriden DrawItem 
method) and a handler for WM_ERASEBKGND. See example below. For some reason the 
WM_ERASEBKGND handler is not called when the listbox contains no items.

1   unit Unit1;
2   
3   interface
4   
5   uses
6     Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, 
7   StdCtrls;
8   
9   type
10    TListbox = class(Stdctrls.TListbox)
11    private
12      procedure wmEraseBkGnd(var msg: TWMEraseBkGnd); message WM_ERASEBKGND;
13    end;
14    TForm1 = class(TForm)
15      ListBox1: TListBox;
16      Button1: TButton;
17      procedure Button1Click(Sender: TObject);
18      procedure ListBox1DrawItem(Control: TWinControl; Index: Integer;
19        Rect: TRect; State: TOwnerDrawState);
20    private
21      { Private declarations }
22    public
23      { Public declarations }
24    end;
25  
26  var
27    Form1: TForm1;
28  
29  implementation
30  
31  {$R *.DFM}
32  
33  procedure TForm1.Button1Click(Sender: TObject);
34  var
35    i: Integer;
36  begin
37    for i := listbox1.items.count to listbox1.items.count + 5 do
38      listbox1.items.add(format('Item %d', [i]));
39  end;
40  
41  { TListbox }
42  const
43    colors: array[Boolean] of TColor = ($FFFFC0, $C0FFFF);
44  
45  procedure TListbox.wmEraseBkGnd(var msg: TWMEraseBkGnd);
46  var
47    cv: TCanvas;
48    h, max: Integer;
49    r: TRect;
50    b: Boolean;
51  begin
52    msg.result := 1;
53    h := Perform(LB_GETITEMHEIGHT, 0, 0);
54    if h = LB_ERR then
55      h := ItemHeight;
56    cv := TCanvas.Create;
57    try
58      cv.Handle := msg.DC;
59      r := Rect(0, 0, ClientWidth, h);
60      b := Odd(TopIndex) and (TopIndex >= 0);
61      max := ClientHeight;
62      cv.Brush.Style := bsSolid;
63      while r.Top < max do
64      begin
65        cv.Brush.Color := colors[b];
66        b := not b;
67        cv.FillRect(r);
68        OffsetRect(r, 0, h);
69      end;
70    finally
71      cv.Handle := 0;
72      cv.free;
73    end;
74  end;
75  
76  procedure TForm1.ListBox1DrawItem(Control: TWinControl; Index: Integer;
77    Rect: TRect; State: TOwnerDrawState);
78  var
79    cb, ct: TColor;
80  begin
81    if not (odSelected in State) then
82      with Control as TListbox do
83      begin
84        canvas.Brush.Color := colors[Odd(index)];
85        canvas.Brush.Style := bsSolid;
86      end;
87    Rect.Right := Control.ClientWidth;
88    with Control as TListbox do
89    begin
90      canvas.FillRect(Rect);
91      canvas.Brush.Style := bsClear;
92      canvas.TextRect(Rect, Rect.Left + 2, Rect.Top, Items[index]);
93    end;
94  end;
95  
96  end.


Solve 2:
97  
98  procedure TFrmAlignText.ListBoxDrawItem(Control: TWinControl;
99    Index: Integer; Rect: TRect; State: TOwnerDrawState);
100 var
101   horzOffset: integer;
102   vertOffset: integer;
103 begin
104   {ListBox.Style is set to lbOwnerDrawFixed.}
105   with ListBox.Canvas do
106   begin
107     {vertOffset added to Rect.Top causes the string to be vertically centered in 
108 the rectangle}
109     vertOffset := (((Rect.Bottom - Rect.Top) - TextExtent(ListBox.Items[Index]).CY) 
110 div 2);
111     {TextWidth('Mi') div 4 gives (roughly) half of an average character width}
112     horzOffset := TextWidth('Mi') div 4;
113     if not (odSelected in State) then
114     begin
115       if Odd(Index) then
116       begin
117         Brush.Color := clBtnFace;
118         Font.Color := clBtnText
119       end
120       else
121       begin
122         Font.Color := clFuchsia;
123       end;
124     end;
125     FillRect(Rect);
126     TextOut(Rect.Left + horzOffset, Rect.Top + vertOffset, ListBox.Items[Index]);
127   end;
128 end;


			
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