研究一下TForm.WMPaint过程

procedure TCustomForm.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if not IsIconic(Handle) then inherited 
  else
  begin
    Message.Msg := WM_ICONERASEBKGND;
    DefaultHandler(Message);
  end;
end;

procedure TWinControl.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  with ThemeServices do
  if ThemesEnabled and Assigned(Parent) and (csParentBackground in FControlStyle) then
    begin
      { Get the parent to draw its background into the control‘s background. }
      DrawParentBackground(Handle, Message.DC, nil, False);
    end
    else
    begin
      { Only erase background if we‘re not doublebuffering or painting to memory. }
      if not FDoubleBuffered or
         (TMessage(Message).wParam = TMessage(Message).lParam) then
        FillRect(Message.DC, ClientRect, FBrush.Handle); // Brush的颜色事先读取好了
    end;

  Message.Result := 1;
end;

procedure TCustomForm.WMPaint(var Message: TWMPaint);
var
  DC: HDC;
  PS: TPaintStruct;
begin
  if not IsIconic(Handle) then
  begin
    ControlState := ControlState + [csCustomPaint];
    inherited;
    ControlState := ControlState - [csCustomPaint];
  end
  else
  begin
    DC := BeginPaint(Handle, PS);
    DrawIcon(DC, 0, 0, GetIconHandle);
    EndPaint(Handle, PS);
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  //
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  tag := 100;
end;