How to draw some text when click a button in wxWidgets? - wxwidgets

Question

I want to draw some text when click a button. My code is:
#include <wx/wx.h>
enum
{
BUTTON_Hello = wxID_HIGHEST + 1
};
class myFrame : public wxFrame {
public:
wxButton* HelloWorld;
wxPanel* panel;
void OnPaint(wxCommandEvent& event) {
wxClientDC bdc =wxClientDC(this);
bdc.DrawText(wxString("Draw some text when button clicked."), wxPoint(300, 300));
Refresh();
};
myFrame(wxWindow* parent,
wxWindowID id,
const wxString& title,
const wxPoint& pos = wxDefaultPosition,
const wxSize& size = wxDefaultSize) :
wxFrame(parent,id,title, pos, size) {
panel =new wxPanel(this, wxID_ANY, wxPoint(0,0),wxSize(100,100));
//Connect(wxEVT_PAINT, wxPaintEventHandler(myFrame::OnPaint));
HelloWorld = new wxButton(panel, BUTTON_Hello, _T("Hello World"),
wxPoint(5,5), wxSize(100, 100));
HelloWorld->Bind(wxEVT_BUTTON, &myFrame::OnPaint, this);
};
};
class MyApp : public wxApp
{
bool OnInit() {
frame = new myFrame((wxFrame*)NULL, -1, wxT("Hello wxDC"),
wxPoint(50, 50), wxSize(800, 600));
frame->Show();
return true;
};
wxFrame* frame;
public:
};
IMPLEMENT_APP(MyApp)
I define the drawing function in a wxFrame and bind it to a wxButton using Bind(). The drawing function uses a wxClientDC. I have added Refersh() to force updating wxFrame. The wxButton belongs to a wxPanel which is a child of wxFrame.
However, when I click the button, nothing happens and no text is shown.

Answer 1

You must use wxPaintDC when drawing in your wxEVT_PAINT handler, not wxClientDC. While wxClientDC also "works", at least for now and at least under some platforms, this is definitely not the right way to do it.
When using wxPaintDC, Refresh() would work as expected and will result in a call to your paint handler during the next event loop iteration. Normally you don't need to call Update(), which immediately calls your handler, at all.

Answer 2

I solved this problem by myself.
As wxPanel is the only child of wxFrame, it will automatically cover the whole area of wxFrame. Now drawing on wxFrame is necessarily having no effect. So I have to draw on wxPanel:
void paintNow(wxCommandEvent& event) {
wxClientDC bdc =wxClientDC(panel);
bdc.DrawText(wxString("Draw some texts when button clicked."),
wxPoint(300, 300));
//panel->Refresh();
panel->Update();
};
Also I find that if I use Refresh() and Update() simultaneously, no text will be shown.
If I only use Refresh(), no text will be shown.