TWinControl = class(TControl)

private

  procedure WMNCHitTest(var Message: TWMNCHitTest); message WM_NCHITTEST;

end;

procedure TWinControl.WMNCHitTest(var Message: TWMNCHitTest);

begin

  with Message do

    if (csDesigning in ComponentState) and (FParent <> nil) then

      Result := HTCLIENT

    else

      inherited;

end;

procedure TWinControl.WndProc(var Message: TMessage);

var

  Form: TCustomForm;

begin

  case Message.Msg of

    WM_SETFOCUS:

      begin

        Form := GetParentForm(Self);

        if (Form <> nil) and not Form.SetFocusedControl(Self) then Exit;

      end;

    WM_KILLFOCUS:

      if csFocusing in ControlState then Exit;

    WM_NCHITTEST:

      begin

        inherited WndProc(Message);

        if (Message.Result = HTTRANSPARENT) and (ControlAtPos(ScreenToClient(

          SmallPointToPoint(TWMNCHitTest(Message).Pos)), False) <> nil) then

          Message.Result := HTCLIENT;

        Exit;

      end;

    WM_MOUSEFIRST..WM_MOUSELAST:

      if IsControlMouseMsg(TWMMouse(Message)) then

      begin

        { Check HandleAllocated because IsControlMouseMsg might have freed the

          window if user code executed something like Parent := nil. }

        if (Message.Result = ) and HandleAllocated then

          DefWindowProc(Handle, Message.Msg, Message.wParam, Message.lParam);

        Exit;

      end;

    WM_KEYFIRST..WM_KEYLAST:

      if Dragging then Exit;

    WM_CANCELMODE:

      if (GetCapture = Handle) and (CaptureControl <> nil) and

        (CaptureControl.Parent = Self) then

        CaptureControl.Perform(WM_CANCELMODE, , );

  end;

  inherited WndProc(Message);

end;

THintWindow = class(TCustomControl)

private

  procedure WMNCHitTest(var Message: TWMNCHitTest); message WM_NCHITTEST;

end;

procedure THintWindow.WMNCHitTest(var Message: TWMNCHitTest);

begin

  Message.Result := HTTRANSPARENT;

end;

TScrollBox = class(TScrollingWinControl)

private

  procedure WMNCHitTest(var Message: TMessage); message WM_NCHITTEST;

end;

procedure TScrollBox.WMNCHitTest(var Message: TMessage);

begin

  DefaultHandler(Message); // TScrollBox和TScrollingWinControl都没有覆盖DefaultHandler函数，因此它会调用TWinControl.DefaultHandler

end;

procedure TCustomForm.ClientWndProc(var Message: TMessage);

  procedure Default;

  begin

    with Message do

      Result := CallWindowProc(FDefClientProc, ClientHandle, Msg, wParam, lParam);

  end;

  function MaximizedChildren: Boolean;

  var

    I: Integer;

  begin

    for I :=  to MDIChildCount -  do

      if MDIChildren[I].WindowState = wsMaximized then

      begin

        Result := True;

        Exit;

      end;

    Result := False;

  end;

var

  DC: HDC;

  PS: TPaintStruct;

  R: TRect;

begin

  with Message do

    case Msg of

      WM_NCHITTEST:

        begin

          Default;

          if Result = HTCLIENT then Result := HTTRANSPARENT;

        end;

      WM_ERASEBKGND:

        begin

          FillRect(TWMEraseBkGnd(Message).DC, ClientRect, Brush.Handle);

          { Erase the background at the location of an MDI client window }

          if (FormStyle = fsMDIForm) and (FClientHandle <> ) then

          begin

            Windows.GetClientRect(FClientHandle, R);

            FillRect(TWMEraseBkGnd(Message).DC, R, Brush.Handle);

          end;

          Result := ;

        end;

      $3F://!

        begin

          Default;

          if FFormStyle = fsMDIForm then

            ShowMDIClientEdge(FClientHandle, (MDIChildCount = ) or

              not MaximizedChildren);

        end;

      WM_PAINT:

        begin

          DC := TWMPaint(Message).DC;

          if DC =  then

            TWMPaint(Message).DC := BeginPaint(ClientHandle, PS);

          try

            if DC =  then

            begin

              GetWindowRect(FClientHandle, R);

              R.TopLeft := ScreenToClient(R.TopLeft);

              MoveWindowOrg(TWMPaint(Message).DC, -R.Left, -R.Top);

            end;

            PaintHandler(TWMPaint(Message));

          finally

            if DC =  then

              EndPaint(ClientHandle, PS);

          end;

        end;

    else

      Default;

    end;

end;

procedure TScreen.SetCursor(Value: TCursor);

var

  P: TPoint;

  Handle: HWND;

  Code: Longint;

begin

  if Value <> Cursor then

  begin

    FCursor := Value;

    if Value = crDefault then

    begin

      { Reset the cursor to the default by sending a WM_SETCURSOR to the

        window under the cursor }

      GetCursorPos(P);

      Handle := WindowFromPoint(P);

      if (Handle <> ) and

        (GetWindowThreadProcessId(Handle, nil) = GetCurrentThreadId) then

      begin

        Code := SendMessage(Handle, WM_NCHITTEST, , LongInt(PointToSmallPoint(P)));

        SendMessage(Handle, WM_SETCURSOR, Handle, MakeLong(Code, WM_MOUSEMOVE));

        Exit;

      end;

    end;

    Windows.SetCursor(Cursors[Value]);

  end;

  Inc(FCursorCount);

end;

procedure TCustomCombo.ComboWndProc(var Message: TMessage; ComboWnd: HWnd;

  ComboProc: Pointer);

var

  Point: TPoint;

  Form: TCustomForm;

begin

  try

    with Message do

    begin

      case Msg of

        WM_SETFOCUS:

          begin

            Form := GetParentForm(Self);

            if (Form <> nil) and not Form.SetFocusedControl(Self) then Exit;

          end;

        WM_KILLFOCUS:

          if csFocusing in ControlState then Exit;

        WM_NCHITTEST:

          if csDesigning in ComponentState then

          begin

            Result := HTTRANSPARENT;

            Exit;

          end;

        CN_KEYDOWN, CN_CHAR, CN_SYSKEYDOWN, CN_SYSCHAR:

          begin

            WndProc(Message);

            Exit;

          end;

      end;

      Result := CallWindowProc(ComboProc, ComboWnd, Msg, WParam, LParam);

      if (Msg = WM_LBUTTONDBLCLK) and (csDoubleClicks in ControlStyle) then

        DblClick;

    end;

  except

    Application.HandleException(Self);

  end;

end;