TControl的显示函数（5个非虚函数，4个虚函数）和三个例子的执行过程（包括SetParent的例子）

function TControl.GetDeviceContext(var WindowHandle: HWnd): HDC;
begin
  if Parent = nil then
    raise EInvalidOperation.CreateFmt(SParentRequired, [Name]);
  Result := Parent.GetDeviceContext(WindowHandle);
  SetViewportOrgEx(Result, Left, Top, nil);
  IntersectClipRect(Result, 0, 0, Width, Height);
end;

procedure TControl.InvalidateControl(IsVisible, IsOpaque: Boolean);
var
  Rect: TRect;

  function BackgroundClipped: Boolean;
  var
    R: TRect;
    List: TList;
    I: Integer;
    C: TControl;
  begin
    Result := True;
    List := FParent.FControls;
    I := List.IndexOf(Self);
    while I > 0 do
    begin
      Dec(I);
      C := List[I];
      with C do
        if C.Visible and (csOpaque in ControlStyle) then
        begin
          IntersectRect(R, Rect, BoundsRect);
          if EqualRect(R, Rect) then Exit;
        end;
    end;
    Result := False;
  end;

begin
  if (IsVisible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and (Parent <> nil) and
    Parent.HandleAllocated then
  begin
    Rect := BoundsRect;
    InvalidateRect(Parent.Handle, @Rect, not (IsOpaque or
      (csOpaque in Parent.ControlStyle) or BackgroundClipped));
  end;
end;

procedure TControl.Invalidate;
begin
  InvalidateControl(Visible, csOpaque in ControlStyle);
end;

procedure TControl.Hide;
begin
  Visible := False;
end;

procedure TControl.Show;
begin
  if Parent <> nil then Parent.ShowControl(Self);
  if not (csDesigning in ComponentState) or
    (csNoDesignVisible in ControlStyle) then Visible := True;
end;

procedure TControl.Update;
begin
  if Parent <> nil then Parent.Update; // 图形控件没法刷新自己，让父控件去刷新
end;

procedure TControl.Refresh;
begin
  Repaint;
end;

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);
      finally
        ReleaseDC(Parent.Handle, DC);
      end;
    end else
    begin
      Invalidate;
      Update;
    end;
end;