wxWidget wxScrolledWindow cancel previous drawing - c++

Question

I'm trying to get a scrolling panel where I can draw picture like on MS Paint
The problem is that, when I draw something to try the application:
I draw one rectangle (or more than one)
I scroll the panel to the right, the rectangles remain to the left
I scroll the panel to the left, the rectangles are disappeared
I'm a newbie in wx and I think I'm missing something important
Using wxWidget in Codelite (C++), Windows 7 32 bit
That's the code:
editorshp.h
class EditorShp : public wxFrame {
protected:
wxClientDC *dcPannelloDisegno;
wxScrolledWindow *scrollwin;
wxPanel *pannello;
public:
EditorShp( wxWindow* parent, wxWindowID id = wxID_ANY, const wxString& title = _("Editor SHP"), const wxPoint& pos = wxDefaultPosition, const wxSize& size = wxSize( 800,500 ), long style = wxDEFAULT_FRAME_STYLE|wxTAB_TRAVERSAL );
void draw_temp(wxCommandEvent& WXUNUSED(event));
};
editorshp.cpp
PS: bDrawTemp is the button I use for drawing some rectangles (the first is at point 100,100; the second at 200,100 and so on)
EditorShp::EditorShp( wxWindow* parent, wxWindowID id, const wxString& title, const wxPoint& pos, const wxSize& size, long style ) : wxFrame( parent, id, title, pos, size, style ) {
scrollwin = new wxScrolledWindow(this, -1, wxPoint(0, 0), wxSize(700, 400));
scrollwin->SetScrollbars(1, 1, 1600, 1000, 0, 0);
pannello = new wxPanel(scrollwin, -1, wxPoint(0, 0), wxSize(700, 400), wxFULL_REPAINT_ON_RESIZE);
dcPannelloDisegno = new wxClientDC(pannello);
scrollwin->DoPrepareDC(*dcPannelloDisegno);
wxButton *bDrawTemp= new wxButton( this, wxID_ANY, _("Indietro"), wxPoint(0, 400), wxSize( 100, 24 ), 0 );
bDrawTemp->Connect(wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(EditorShp::disegnaTemp), NULL, this);
scrollwin->Show(true);
this->Show(true);
}
void EditorShp::draw_temp(wxCommandEvent& WXUNUSED(event)){
static int x = 100, y = 100;
dcPannelloDisegno->DrawRectangle(wxPoint(x, y), wxSize(10, 10));
//pannello->Refresh(); it cancels everything
pannello->Update();
x += 100;
x = x % 1600;
}
EDIT 1: Sorry it's not wxScrollingPanel but wxScrolledWindow

Answer 1

First, catch wxEVT_PAINT and do the drawing in its hadler, using a wxPaintDC (or one of its variants - wx[Auto]BufferedPaintDC).
Second, you are now attempting to paint on pannello, and for that you do not need to prepare the wxDC. However, from your description you want to paint on scrollwin, and in that case you might not even need pannello.
So, for drawing on a scrolled window, see scroll sample that comes with wxW. What you probably need is the implementation of MySimpleCanvas (described as a scrolled window which draws a simple rectangle) - to see it press F1 when running the sample.
As a general rule, the samples should be the first place to search for a functionality; there are a lot of them exemplified there.