Author: Tomas Rutkauskas

How to modify the color of a TCheckBox

Answer:

I would do the drawing in the CN_DRAWITEM message handler. Below is the code of 
such a checkbox:

1   { ... }
2   type
3     TMyCheckBox = class(TCheckBox)
4     protected
5       procedure CNDrawItem(var message: TWMDrawItem); message CN_DRAWITEM;
6       procedure CMEnabledchanged(var message: TMessage); message CM_ENABLEDCHANGED;
7       procedure CreateParams(var Params: TCreateParams); override;
8       procedure CreateWnd; override;
9       procedure SetChecked(Value: Boolean); override;
10      procedure SetButtonStyle;
11    public
12      constructor Create(AOwner: TComponent); override;
13    end;
14  
15    { ... }
16  
17  constructor TMyCheckBox.Create(AOwner: TComponent);
18  begin
19    inherited Create(AOwner);
20    ControlStyle := ControlStyle - [csDoubleClicks];
21  end;
22  
23  procedure TMyCheckBox.CNDrawItem(var message: TWMDrawItem);
24  var
25    XCanvas: TCanvas;
26    XCaptionRect, XGlyphRect: TRect;
27  
28    procedure xxDrawBitMap(ACanvas: TCanvas);
29    const
30      xx_h = 13;
31      xx_w = 13;
32    var
33      xxGlyph: TBitmap;
34      xxX, xxY, xxStepY, xxStepX: integer;
35    begin
36      xxGlyph := TBitmap.Create;
37      try
38        xxGlyph.Handle := LoadBitmap(0, PChar(OBM_CHECKBOXES));
39        xxY := XGlyphRect.Top + (XGlyphRect.Bottom - XGlyphRect.Top - xx_h) div 2;
40        xxX := 2;
41        xxStepX := 0;
42        xxStepY := 0;
43        if Enabled then
44        begin
45          case State of
46            cbChecked:
47              xxStepX := xxStepX + xx_w;
48            cbGrayed:
49              xxStepX := xxStepX + xx_w * 3;
50          end;
51        end
52        else if State = cbChecked then
53          xxStepX := xxStepX + xx_w * 3
54        else
55          xxStepX := xxStepX + xx_w * 2;
56        ACanvas.CopyRect(Rect(xxX, xxY, xxX + xx_w, xxY + xx_h), xxGlyph.Canvas,
57          Rect(xxStepX, xxStepY, xx_w + xxStepX, xx_h + xxStepY));
58      finally
59        xxGlyph.Free;
60      end;
61    end;
62  
63    procedure xxDrawCaption;
64    var
65      xXFormat: longint;
66    begin
67      xXFormat := DT_VCENTER + DT_SINGLELINE + DT_LEFT;
68      xXFormat := DrawTextBiDiModeFlags(xXFormat);
69      DrawText(message.DrawItemStruct.hDC, PChar(Caption),
70        length(Caption), XCaptionRect, xXFormat);
71    end;
72  
73  begin
74    XGlyphRect := message.DrawItemStruct.rcItem;
75    XGlyphRect.Right := 20;
76    XCaptionRect := message.DrawItemStruct.rcItem;
77    XCaptionRect.Left := XGlyphRect.Right;
78    XCanvas := TCanvas.Create;
79    try
80      XCanvas.Handle := message.DrawItemStruct.hDC;
81      XCanvas.Brush.Style := bsClear;
82      xxDrawBitMap(XCanvas);
83      xxDrawCaption;
84    finally
85      XCanvas.Free;
86    end;
87  end;
88  
89  procedure TMyCheckBox.CMEnabledchanged(var message: TMessage);
90  begin
91    inherited;
92    Invalidate;
93  end;
94  
95  procedure TMyCheckBox.CreateParams(var Params: TCreateParams);
96  begin
97    inherited CreateParams(Params);
98    Params.ExStyle := Params.ExStyle or WS_EX_Transparent;
99  end;
100 
101 procedure TMyCheckBox.CreateWnd;
102 begin
103   inherited CreateWnd;
104   SetButtonStyle;
105 end;
106 
107 procedure TMyCheckBox.SetChecked(Value: Boolean);
108 begin
109   inherited SetChecked(Value);
110   Invalidate;
111 end;
112 
113 procedure TMyCheckBox.SetButtonStyle;
114 const
115   BS_MASK = $000F;
116 var
117   Style: Word;
118 begin
119   if HandleAllocated then
120   begin
121     Style := BS_CHECKBOX or BS_OWNERDRAW;
122     if GetWindowLong(Handle, GWL_STYLE) and BS_MASK <> Style then
123       SendMessage(Handle, BM_SETSTYLE, Style, 1);
124   end;
125 end;