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）

这个控件直接继承自TWinControl，因此不是改写Paint;函数，而是直接改写PaintWindow虚函数，它在VCL框架里被直接调用，直接就把自己画好了（不用走给控件Perform(WM_Paint)的路线了），很有意思。

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unit MyWinControl;

interface

uses
SysUtils, Classes, Controls, Windows;

type
TMyWinControl = class(TWinControl)
protected
procedure PaintWindow(DC: HDC); override;
public
constructor Create(AOwner: TComponent); override;
end;

procedure Register;

implementation

procedure 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 + 1);
SetBkMode(DC, TRANSPARENT);
DrawText(DC, 'Hello, TMyWinControl', -1, Rect, DT_SINGLELINE or DT_VCENTER or DT_CENTER);
end;

end.

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但代码指定子控件则可以，而且还能跟随父控件一起销毁：

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

procedure TForm1.Button3Click(Sender: TObject);
begin
MyWinControl1.Destroy;
end;

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不使用Paint()的原因也比较清楚（TWinControl根本没有Paint函数）：

procedure TWinControl.WMPaint(var Message: TWMPaint);
var
DC, MemDC: HDC;
MemBitmap, OldBitmap: HBITMAP;
PS: TPaintStruct;
begin
if not FDoubleBuffered or (Message.DC <> 0) then
begin
if not (csCustomPaint in ControlState) and (ControlCount = 0) 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(0);
MemBitmap := CreateCompatibleBitmap(DC, ClientRect.Right, ClientRect.Bottom);
ReleaseDC(0, DC);
MemDC := CreateCompatibleDC(0);
OldBitmap := SelectObject(MemDC, MemBitmap);
try
DC := BeginPaint(Handle, PS);
Perform(WM_ERASEBKGND, MemDC, MemDC);
Message.DC := MemDC;
WMPaint(Message);
Message.DC := 0;
BitBlt(DC, 0, 0, ClientRect.Right, ClientRect.Bottom, MemDC, 0, 0, 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 = 0 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 := 0 to FControls.Count - 1 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 = 0 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 := 0;
Message.Result := 0;
DefaultHandler(Message);
end;