继承自TWinControl的控件不能在设计期间接受子控件，用代码设置子控件却可以（它的自绘是直接改写PaintWindow虚函数，而不是覆盖Paint函数——对TWinControl.WMPaint又有新解了）

unit MyWinControl;interfaceuses
SysUtils, Classes, Controls, Windows;type
TMyWinControl = class(TWinControl)
protected
procedure PaintWindow(DC: HDC); override;
public
constructor Create(AOwner: TComponent); override;
end;procedure Register;implementationprocedure Register;
begin
RegisterComponents('Samples', [TMyWinControl]);
end;constructor TMyWinControl.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
ControlState := ControlState + [csCustomPaint]; // 强行加上了自绘条件，改变了自绘的走向，使用这种方法，控件不需要处理WM_PAINT消息（不需要WMPaint函数和Paint函数）
// 必须通知 WMPaint 需要画自己
end;procedure TMyWinControl.PaintWindow(DC: HDC);
var
Rect: TRect;
begin
Windows.GetClientRect(Handle, Rect);
FillRect(DC, Rect, COLOR_BTNSHADOW + );
SetBkMode(DC, TRANSPARENT);
DrawText(DC, 'Hello, TMyWinControl', -, Rect, DT_SINGLELINE or DT_VCENTER or DT_CENTER);
end;end.

procedure TForm1.Button1Click(Sender: TObject);
begin
button2.Parent:=MyWinControl1;
end;procedure TForm1.Button3Click(Sender: TObject);
begin
MyWinControl1.Destroy;
end;

procedure TWinControl.WMPaint(var Message: TWMPaint);
var
DC, MemDC: HDC;
MemBitmap, OldBitmap: HBITMAP;
PS: TPaintStruct;
begin
if not FDoubleBuffered or (Message.DC <> ) then
begin
if not (csCustomPaint in ControlState) and (ControlCount = ) then
inherited // 优先找子类的WM_PAINT消息函数，找不到就调用DefaultHandler函数，这么说，所有TWinControl（非TCustomControl）都处理了WM_PAINT消息？
else
PaintHandler(Message); // 要求自绘，所以走这里
end
else
begin
DC := GetDC();
MemBitmap := CreateCompatibleBitmap(DC, ClientRect.Right, ClientRect.Bottom);
ReleaseDC(, DC);
MemDC := CreateCompatibleDC();
OldBitmap := SelectObject(MemDC, MemBitmap);
try
DC := BeginPaint(Handle, PS);
Perform(WM_ERASEBKGND, MemDC, MemDC);
Message.DC := MemDC;
WMPaint(Message);
Message.DC := ;
BitBlt(DC, , , ClientRect.Right, ClientRect.Bottom, MemDC, , , SRCCOPY);
EndPaint(Handle, PS);
finally
SelectObject(MemDC, OldBitmap);
DeleteDC(MemDC);
DeleteObject(MemBitmap);
end;
end;
end;

procedure TWinControl.PaintHandler(var Message: TWMPaint);
var
I, Clip, SaveIndex: Integer;
DC: HDC;
PS: TPaintStruct;
begin
DC := Message.DC;
if DC = then DC := BeginPaint(Handle, PS);
try
if FControls = nil then PaintWindow(DC) else // 一般情况下，这样执行就结束了
begin
SaveIndex := SaveDC(DC);
Clip := SimpleRegion;
for I := to FControls.Count - do
with TControl(FControls[I]) do
if (Visible or (csDesigning in ComponentState) and
not (csNoDesignVisible in ControlStyle)) and
(csOpaque in ControlStyle) then
begin
Clip := ExcludeClipRect(DC, Left, Top, Left + Width, Top + Height);
if Clip = NullRegion then Break;
end;
if Clip <> NullRegion then PaintWindow(DC);
RestoreDC(DC, SaveIndex);
end;
PaintControls(DC, nil);
finally
if Message.DC = then EndPaint(Handle, PS);
end;
end;procedure TWinControl.PaintWindow(DC: HDC); // 这个函数等于没用（它是virtual函数），必须覆盖
var
Message: TMessage;
begin
Message.Msg := WM_PAINT;
Message.WParam := DC;
Message.LParam := ;
Message.Result := ;
DefaultHandler(Message);
end;