TCustomControl绘制自己和图形子控件共四步，TWinControl关键属性方法速记

  TCustomControl = class(TWinControl)
  private
    FCanvas: TCanvas;
    procedure WMPaint(var Message: TWMPaint); message WM_PAINT;
  protected
    procedure Paint; virtual;
    procedure PaintWindow(DC: HDC); override;
    property Canvas: TCanvas read FCanvas; // 到这步才有Canvas
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  end;

constructor TCustomControl.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FCanvas := TControlCanvas.Create;
  TControlCanvas(FCanvas).Control := Self;
end;

destructor TCustomControl.Destroy;
begin
  FCanvas.Free;
  inherited Destroy;
end;

procedure TCustomControl.WMPaint(var Message: TWMPaint); // 第一步，接受到自绘消息
begin
  Include(FControlState, csCustomPaint); // 进入自绘状态
  inherited; // 第二步，调用VCL框架提供的同名管理函数（TWinControl.WMPaint）来重绘所有子控件和自己。有个问题：如果每个TCustomControl都要求父控件重绘一遍，会不会效率很低？
  Exclude(FControlState, csCustomPaint); // 结束自绘状态
end;

procedure TCustomControl.PaintWindow(DC: HDC); // 第三步，等待VCL框架的管理函数（TWinControl.WMPaint-->TWinControl.PaintHandler-->当前函数）来调用自己（只能是控件重绘自己时产生的调用，至于当前控件的WinControl子控件的重绘属由更高层次TWinControl.UpdateShowing来管理，而不归这些低层次的函数来管理）
begin
  FCanvas.Lock;
  try
    FCanvas.Handle := DC;
    try
      TControlCanvas(FCanvas).UpdateTextFlags;
      Paint; // 第四步，进一步调用自己的Paint虚函数
    finally
      FCanvas.Handle := 0; // 当场释放资源
    end;
  finally
    FCanvas.Unlock;
  end;
end;

procedure TCustomControl.Paint; // 提前准备：程序员覆盖这个方法，真正调用程序员的绘制函数（默认为空，等待程序员来绘制）
begin
end;

新发现一个函数（尽管不起作用）：

procedure TWinControl.PaintWindow(DC: HDC);
var
  Message: TMessage;
begin
  Message.Msg := WM_PAINT;
  Message.WParam := DC;
  Message.LParam := 0;
  Message.Result := 0;
  DefaultHandler(Message);
end;

 TWinControl用来处理鼠标和实际重绘，而处理鼠标全部在TControl里。此外还申请句柄，处理自己绘图，与父类的交互。虽然TControl也有绘图，但最后还是要让父类TWinControl来画。

  TWinControl = class(TControl)
    constructor Create(AOwner: TComponent); override;
    constructor CreateParented(ParentWindow: HWnd);
    class function CreateParentedControl(ParentWindow: HWND): TWinControl;
    destructor Destroy; override;
    procedure Broadcast(var Message);

    function GetHandle: HWND;
    procedure SetParentWindow(Value: HWND);
    function GetControlCount: Integer;
    function GetControl(Index: Integer): TControl;
    procedure Insert(AControl: TControl);
    procedure SetZOrderPosition(Position: Integer);
    function HandleAllocated: Boolean;
    procedure HandleNeeded;
    procedure InsertControl(AControl: TControl);

    procedure CreateHandle; virtual;
    procedure CreateParams(var Params: TCreateParams); virtual;
    procedure CreateWindowHandle(const Params: TCreateParams); virtual;
    procedure CreateWnd; virtual;
    procedure RecreateWnd;
    procedure DestroyHandle; virtual;
    procedure DestroyWindowHandle; virtual;
    procedure DestroyWnd; virtual;
    function GetDeviceContext(var WindowHandle: HWND): HDC; overload; override;
    function GetParentHandle: HWND;
    function GetTopParentHandle: HWnd;

    procedure SetParent(AParent: TWinControl); override;
    procedure SetParentBackground(Value: Boolean); virtual;
    procedure SetParentDoubleBuffered(Value: Boolean); virtual;
    procedure SetZOrder(TopMost: Boolean); override;
    procedure RemoveControl(AControl: TControl);procedure UpdateControlState;
    procedure CreateSubClass(var Params: TCreateParams; ControlClassName: PChar); // 超类化

    procedure WndProc(var Message: TMessage); override;
    property DefWndProc: Pointer read FDefWndProc write FDefWndProc;
    property WindowHandle: HWnd read FHandle write FHandle;
    procedure DefaultHandler(var Message); override;
    procedure MainWndProc(var Message: TMessage);
    function PreProcessMessage(var Msg: TMsg): Boolean; dynamic;

    procedure Repaint; override; // 就两句：调用 类函数Invalidate; 和 类函数Update（今天怎么觉得思路格外清楚啊）; // 让程序员有机会总揽全局，既使控件失效，又立即重绘，因为一般情况下只需要让控件失效即可，然后系统空闲时发信息让控件重绘，程序员不需要管理和调用后者。
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
    procedure NotifyControls(Msg: Word);
    procedure PaintControls(DC: HDC; First: TControl); // 给FControls发Perform(WM_PAINT, DC, 0); 把图像子控件全部重绘了一遍。对所有的FWinControls 调用WinAPI FrameRect 用指定的画刷为指定的矩形画边框
 UpdateShowing; // 只处理是否Visible
    procedure Invalidate; override; // 内容就一句 Perform(CM_INVALIDATE, 0, 0); 另外观察一下TControl.Invalidate;函数 也是就一句：InvalidateControl(Visible, csOpaque in ControlStyle); 
    procedure PaintTo(Canvas: TCanvas; X, Y: Integer); overload;
    procedure Update; override; // 调用WinAPI函数 if HandleAllocated then UpdateWindow(WindowHandle); // 会产生WM_PAINT消息。让程序员有机会强行自绘控件。
    总结：自绘过程一共有四个函数实际干活，第一步是收到系统消息调用WMPaint，第二步把消息转交给PaintHandler，第三步首先绘制自己，即调用自己的PaintWindow，第四步调用PaintControls绘制所有子控件（会给所有子控件挨个发送WM_PAINT）。
    备注：对于一个单独的控件自己而言，重绘其实就两步，第一步让控件失效，第二步等待系统给Win控件发送WM_PAINT后真正重绘，但是也可由程序员强迫立即重绘（UpdateWindow）。具体重绘过程由VCL控制，包含了自动重绘图形子控件和调用程序员的事件

    procedure AssignTo(Dest: TPersistent); override;
    function GetClientRect: TRect; override;
    function IsDoubleBufferedStored: Boolean;
    procedure SetDoubleBuffered(Value: Boolean);

    procedure DoEnter; dynamic;
    procedure DoExit; dynamic;
    function DoKeyDown(var Message: TWMKey): Boolean;
    function DoKeyPress(var Message: TWMKey): Boolean;
    function DoKeyUp(var Message: TWMKey): Boolean;

    procedure WMInputLangChange(var Message: TMessage); message WM_INPUTLANGCHANGE; // 输入法改变
    procedure WMPaint(var Message: TWMPaint); message WM_PAINT; // 这里只是收到消息，收到以后怎么做不一定要放在这里。实际上：如果不双缓冲就调用 PaintHandler，否则立刻重绘（并且还考虑Windows主题的影响）。
    procedure WMCommand(var Message: TWMCommand); message WM_COMMAND; // 菜单、工具栏命令
    procedure WMNotify(var Message: TWMNotify); message WM_NOTIFY; // 通知消息
    procedure WMSysColorChange(var Message: TWMSysColorChange); message WM_SYSCOLORCHANGE; // 系统颜色改变
    procedure WMHScroll(var Message: TWMHScroll); message WM_HSCROLL; 
    procedure WMVScroll(var Message: TWMVScroll); message WM_VSCROLL;
    procedure WMCompareItem(var Message: TWMCompareItem); message WM_COMPAREITEM;
    procedure WMDeleteItem(var Message: TWMDeleteItem); message WM_DELETEITEM;
    procedure WMDrawItem(var Message: TWMDrawItem); message WM_DRAWITEM;
    procedure WMMeasureItem(var Message: TWMMeasureItem); message WM_MEASUREITEM;
    procedure WMEraseBkgnd(var Message: TWmEraseBkgnd); message WM_ERASEBKGND; // 画背景色，间接的发送 WM_ERASEBKGND消息，不实际干活
    procedure WMWindowPosChanged(var Message: TWMWindowPosChanged); message WM_WINDOWPOSCHANGED; // 位置改变
    procedure WMWindowPosChanging(var Message: TWMWindowPosChanging); message WM_WINDOWPOSCHANGING;
    procedure WMSize(var Message: TWMSize); message WM_SIZE;
    procedure WMMove(var Message: TWMMove); message WM_MOVE;
    procedure WMSetCursor(var Message: TWMSetCursor); message WM_SETCURSOR; // 设置光标
    procedure WMKeyDown(var Message: TWMKeyDown); message WM_KEYDOWN; // 按键方法
    procedure WMSysKeyDown(var Message: TWMSysKeyDown); message WM_SYSKEYDOWN;
    procedure WMKeyUp(var Message: TWMKeyUp); message WM_KEYUP;
    procedure WMSysKeyUp(var Message: TWMSysKeyUp); message WM_SYSKEYUP; // 系统按键
    procedure WMChar(var Message: TWMChar); message WM_CHAR;
    procedure WMSysCommand(var Message: TWMSysCommand); message WM_SYSCOMMAND; // 系统命令
    procedure WMCharToItem(var Message: TWMCharToItem); message WM_CHARTOITEM;
    procedure WMParentNotify(var Message: TWMParentNotify); message WM_PARENTNOTIFY; // 父控件通知
    procedure WMVKeyToItem(var Message: TWMVKeyToItem); message WM_VKEYTOITEM;
    procedure WMDestroy(var Message: TWMDestroy); message WM_DESTROY; // 销毁
    procedure WMMouseActivate(var Message: TWMMouseActivate); message WM_MOUSEACTIVATE; 
    procedure WMNCCalcSize(var Message: TWMNCCalcSize); message WM_NCCALCSIZE; // 非客户区重新计算
    procedure WMNCDestroy(var Message: TWMNCDestroy); message WM_NCDESTROY; // 非客户区销毁
    procedure WMNCHitTest(var Message: TWMNCHitTest); message WM_NCHITTEST; 
    procedure WMNCPaint(var Message: TWMNCPaint); message WM_NCPAINT; // 非客户区绘图
    procedure WMQueryNewPalette(var Message: TMessage); message WM_QUERYNEWPALETTE;
    procedure WMPaletteChanged(var Message: TMessage); message WM_PALETTECHANGED;
    procedure WMWinIniChange(var Message: TMessage); message WM_WININICHANGE; 
    procedure WMFontChange(var Message: TMessage); message WM_FONTCHANGE; // 字体被改变
    procedure WMTimeChange(var Message: TMessage); message WM_TIMECHANGE; // 时间被改变
    procedure WMSetFocus(var Message: TWMSetFocus); message WM_SETFOCUS; 
    procedure WMKillFocus(var Message: TWMKillFocus); message WM_KILLFOCUS;
    procedure WMIMEStartComp(var Message: TMessage); message WM_IME_STARTCOMPOSITION;
    procedure WMIMEEndComp(var Message: TMessage); message WM_IME_ENDCOMPOSITION;
    procedure WMContextMenu(var Message: TWMContextMenu); message WM_CONTEXTMENU;
    procedure WMGesture(var Message: TMessage); message WM_GESTURE;
    procedure WMGestureNotify(var Message: TWMGestureNotify); message WM_GESTURENOTIFY;
    procedure WMTabletQuerySystemGestureStatus(var Message: TMessage); message WM_TABLET_QUERYSYSTEMGESTURESTATUS;
    procedure WMPrintClient(var Message: TWMPrintClient); message WM_PRINTCLIENT;

    procedure CMInputLangChange(var Message: TMessage); message CM_INPUTLANGCHANGE;
    procedure CMChanged(var Message: TCMChanged); message CM_CHANGED;
    procedure CMChildKey(var Message: TCMChildKey); message CM_CHILDKEY;
    procedure CMDialogKey(var Message: TCMDialogKey); message CM_DIALOGKEY;
    procedure CMDialogChar(var Message: TCMDialogChar); message CM_DIALOGCHAR;
    procedure CMVisibleChanged(var Message: TMessage); message CM_VISIBLECHANGED;
    procedure CMEnabledChanged(var Message: TMessage); message CM_ENABLEDCHANGED;
    procedure CMColorChanged(var Message: TMessage); message CM_COLORCHANGED;
    procedure CMFontChanged(var Message: TMessage); message CM_FONTCHANGED;
    procedure CMBorderChanged(var Message: TMessage); message CM_BORDERCHANGED;
    procedure CMCursorChanged(var Message: TMessage); message CM_CURSORCHANGED;
    procedure CMCtl3DChanged(var Message: TMessage); message CM_CTL3DCHANGED;
    procedure CMParentCtl3DChanged(var Message: TMessage); message CM_PARENTCTL3DCHANGED;
    procedure CMParentDoubleBufferedChanged(var Message: TMessage); message CM_PARENTDOUBLEBUFFEREDCHANGED;
    procedure CMShowingChanged(var Message: TMessage); message CM_SHOWINGCHANGED;
    procedure CMShowHintChanged(var Message: TMessage); message CM_SHOWHINTCHANGED;
    procedure CMEnter(var Message: TCMEnter); message CM_ENTER;
    procedure CMExit(var Message: TCMExit); message CM_EXIT;
    procedure CMDesignHitTest(var Message: TCMDesignHitTest); message CM_DESIGNHITTEST;
    procedure CMSysColorChange(var Message: TMessage); message CM_SYSCOLORCHANGE;
    procedure CMSysFontChanged(var Message: TMessage); message CM_SYSFONTCHANGED;
    procedure CMWinIniChange(var Message: TWMWinIniChange); message CM_WININICHANGE;
    procedure CMFontChange(var Message: TMessage); message CM_FONTCHANGE;
    procedure CMTimeChange(var Message: TMessage); message CM_TIMECHANGE;
    procedure CMDrag(var Message: TCMDrag); message CM_DRAG;

    procedure CNKeyDown(var Message: TWMKeyDown); message CN_KEYDOWN;
    procedure CNKeyUp(var Message: TWMKeyUp); message CN_KEYUP;
    procedure CNChar(var Message: TWMChar); message CN_CHAR;
    procedure CNSysKeyDown(var Message: TWMKeyDown); message CN_SYSKEYDOWN;
    procedure CNSysChar(var Message: TWMChar); message CN_SYSCHAR;

    procedure CMRecreateWnd(var Message: TMessage); message CM_RECREATEWND;
    procedure CMInvalidate(var Message: TMessage); message CM_INVALIDATE;
    procedure CMBiDiModeChanged(var Message: TMessage); message CM_BIDIMODECHANGED;
    procedure CMDoubleBufferedChanged(var Message: TMessage); message CM_DOUBLEBUFFEREDCHANGED;
    procedure CMTextChanged(var Message: TMessage); message CM_TEXTCHANGED;
    procedure CMTabletOptionsChanged(var Message: TMessage); message CM_TABLETOPTIONSCHANGED;
    procedure CMFocusChanged(var Message: TCMFocusChanged); message CM_FOCUSCHANGED;
    procedure CMControlListChange(var Message: TMessage); message CM_CONTROLLISTCHANGE;
    procedure CMControlListChanging(var Message: TMessage); message CM_CONTROLLISTCHANGING;
    procedure CMDockClient(var Message: TCMDockClient); message CM_DOCKCLIENT;
    procedure CMUnDockClient(var Message: TCMUnDockClient); message CM_UNDOCKCLIENT;
    procedure CMFloat(var Message: TCMFloat); message CM_FLOAT;

    property Brush: TBrush read FBrush;
    property Controls[Index: Integer]: TControl read GetControl;
    property ControlCount: Integer read GetControlCount;
    property Handle: HWND read GetHandle;
    property ParentDoubleBuffered: Boolean read FParentDoubleBuffered write SetParentDoubleBuffered default True;
    property ParentWindow: HWND read FParentWindow write SetParentWindow;
    property Showing: Boolean read FShowing;

贴一下TWinControl的几个关键的自绘代码：

procedure TWinControl.Repaint;
begin
  Invalidate;
  Update;
end;

procedure TWinControl.Invalidate;
begin
  Perform(CM_INVALIDATE, 0, 0);
end;

procedure TWinControl.CMInvalidate(var Message: TMessage);
var
  I: Integer;
begin
  if HandleAllocated then
  begin
    if Parent <> nil then Parent.Perform(CM_INVALIDATE, 1, 0);
    if Message.WParam = 0 then
    begin
      InvalidateRect(FHandle, nil, not (csOpaque in ControlStyle));
      { Invalidate child windows which use the parentbackground when themed }
      if ThemeServices.ThemesEnabled then
        for I := 0 to ControlCount - 1 do
          if csParentBackground in Controls[I].ControlStyle then
            Controls[I].Invalidate;
    end;
  end;
end;

procedure TWinControl.Update;
begin
  if HandleAllocated then UpdateWindow(FHandle);
end;

 TWinControl的属性：

    FDefWndProc: Pointer;
    FHandle: HWnd;
    FObjectInstance: Pointer;
    FParentWindow: HWND;

    FBrush: TBrush;
    FControls: TList;
    FWinControls: TList;
    FAlignControlList: TList;
    FParentDoubleBuffered: Boolean;
    FDoubleBuffered: Boolean;
    FDesignSize: TPoint;

    FMouseControl: TControl;
    FTouchControl: TControl;

    FOnEnter: TNotifyEvent;
    FOnExit: TNotifyEvent;
    FOnKeyDown: TKeyEvent;
    FOnKeyUp: TKeyEvent;
    FOnKeyPress: TKeyPressEvent;

 再来窗口参数：

  TCreateParams = record
    Caption: PChar;
    Style: DWORD;
    ExStyle: DWORD;
    X, Y: Integer;
    Width, Height: Integer;
    WndParent: HWnd;
    Param: Pointer;
    WindowClass: TWndClass;
    WinClassName: array[0..63] of Char;
  end;
 

PaintControls的定义，一共2处调用：

procedure TControl.Repaint;
var
  DC: HDC;
begin
  if (Visible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and (Parent <> nil) and
    Parent.HandleAllocated then
    if csOpaque in ControlStyle then
    begin
      DC := GetDC(Parent.Handle);
      try
        IntersectClipRect(DC, Left, Top, Left + Width, Top + Height);
        Parent.PaintControls(DC, Self); // 写清楚了，要求父控件更新当前图形控件的显示（而不是重绘全部子控件）
      finally
        ReleaseDC(Parent.Handle, DC);
      end;
    end else
    begin
      Invalidate;
      Update;
    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 // 不含子控件的话，直接重绘控件自己
    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); // 如果根据所有图形子控件的区域计算以后，发现真的有无效区域，才进行重绘
      RestoreDC(DC, SaveIndex);
    end;
    PaintControls(DC, nil); // 当重绘全部图形子控件的时候，第二个参数就传入nil
  finally
    if Message.DC = 0 then EndPaint(Handle, PS);
  end;
end;

procedure TWinControl.PaintControls(DC: HDC; First: TControl);
var
  I, Count, SaveIndex: Integer;
  FrameBrush: HBRUSH;
begin
  if DockSite and UseDockManager and (DockManager <> nil) then
    DockManager.PaintSite(DC);
  if FControls <> nil then // 就像这个函数名字的含义一样，真的是只重绘所有图形子控件，而不重绘它的WinControl子控件（只重绘它们的边框）
  begin
    I := 0;
    if First <> nil then
    begin
      I := FControls.IndexOf(First);
      if I < 0 then I := 0;
    end;
    Count := FControls.Count;
    while I < Count do
    begin
      with TControl(FControls[I]) do
        if (Visible or (csDesigning in ComponentState) and
          not (csNoDesignVisible in ControlStyle)) and
          RectVisible(DC, Rect(Left, Top, Left + Width, Top + Height)) then
        begin
          if csPaintCopy in Self.ControlState then
            Include(FControlState, csPaintCopy);
          SaveIndex := SaveDC(DC);
          MoveWindowOrg(DC, Left, Top);
          IntersectClipRect(DC, 0, 0, Width, Height);
          Perform(WM_PAINT, DC, 0);
          RestoreDC(DC, SaveIndex);
          Exclude(FControlState, csPaintCopy);
        end;
      Inc(I);
    end;
  end;
  if FWinControls <> nil then
    for I := 0 to FWinControls.Count - 1 do
      with TWinControl(FWinControls[I]) do
        if FCtl3D and (csFramed in ControlStyle) and
          (Visible or (csDesigning in ComponentState) and
          not (csNoDesignVisible in ControlStyle)) then
        begin // 只重绘WinControl子控件的边框，而不是重绘整个控件
          FrameBrush := CreateSolidBrush(ColorToRGB(clBtnShadow));
          FrameRect(DC, Rect(Left - 1, Top - 1, Left + Width, Top + Height),
            FrameBrush);
          DeleteObject(FrameBrush);
          FrameBrush := CreateSolidBrush(ColorToRGB(clBtnHighlight));
          FrameRect(DC, Rect(Left, Top, Left + Width + 1, Top + Height + 1),
            FrameBrush);
          DeleteObject(FrameBrush);
        end;
end;

 另外，整个Controls.pas里一共有7处取DC（搜索DC :=），其实重要的地方只有三处（TWinControl.PaintHandler和TControl.Repaint和TWinControl.WMPaint），总体方针是DC现用现取：

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


procedure TControl.Repaint;
var
  DC: HDC;
begin
  if (Visible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and (Parent <> nil) and
    Parent.HandleAllocated then
    if csOpaque in ControlStyle then
    begin
      DC := GetDC(Parent.Handle);


function TControl.PaletteChanged(Foreground: Boolean): Boolean;
var
  OldPalette, Palette: HPALETTE;
  WindowHandle: HWnd;
  DC: HDC;
begin
  Result := False;
  if not Visible then Exit;
  Palette := GetPalette;
  if Palette <> 0 then
  begin
    DC := GetDeviceContext(WindowHandle);



procedure TControl.DefaultDockImage(DragDockObject: TDragDockObject;
  Erase: Boolean);
var
  DesktopWindow: HWND;
  DC: HDC;
  OldBrush: HBrush;
  DrawRect: TRect;
  PenSize: Integer;
begin
  with DragDockObject do
  begin
    PenSize := FrameWidth;
    if Erase then DrawRect := FEraseDockRect
    else DrawRect := FDockRect;
  end;
  DesktopWindow := GetDesktopWindow;
  DC := GetDCEx(DesktopWindow, 0, DCX_CACHE or DCX_LOCKWINDOWUPDATE);


procedure TWinControl.WMNCPaint(var Message: TMessage);
var
  DC: HDC;
  RC, RW, SaveRW: TRect;
  EdgeSize: Integer;
  WinStyle: Longint;
begin
  { Get window DC that is clipped to the non-client area }
  if (BevelKind <> bkNone) or (BorderWidth > 0) then
  begin
    DC := GetWindowDC(Handle);
    try
      Windows.GetClientRect(Handle, RC);


procedure THintWindow.WMNCPaint(var Message: TMessage);
var
  DC: HDC;
begin
  DC := GetWindowDC(Handle);
  try
    NCPaint(DC);
  finally
    ReleaseDC(Handle, DC);
  end;
end;

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
    else
      PaintHandler(Message);
  end
  else
  begin
    DC := GetDC(0); // 取桌面的DC
    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;

 再研究Canvas，待续。。。