简单wxWidgets有关问题，怎么程序控制wxComboBox弹出列表框

#include "wx/wx.h"

class GUIFrame : public wxFrame
{
public:
    wxButton* m_button;
    // Virtual event handlers, overide them in your derived class
    wxComboBox* m_comboBox1;
    GUIFrame(wxWindow* parent, wxWindowID id = wxID_ANY, const wxString& title = wxT("wxWidgets Application Template"), const wxPoint& pos = wxDefaultPosition, const wxSize& size = wxSize(485, 244), long style = wxDEFAULT_FRAME_STYLE | wxTAB_TRAVERSAL);
    virtual void OnButton(wxCommandEvent& event);
    ~GUIFrame() {}
};

void GUIFrame::OnButton(wxCommandEvent& event)
{
    m_comboBox1->SetFocus();
}

GUIFrame::GUIFrame(wxWindow* parent, wxWindowID id, const wxString& title, const wxPoint& pos, const wxSize& size, long style) : wxFrame(parent, id, title, pos, size, style)
{
    wxBoxSizer* bSizer1 = new wxBoxSizer(wxHORIZONTAL);
    bSizer1->Add(0, 0, 1, wxEXPAND, 5);   //space
    const int wxID_BUTTON = 65530;
    const int wxID_COMBOBOX = 65531;
    m_button = new wxButton(this, wxID_BUTTON, wxT("Press Me..."));
    bSizer1->Add(m_button, 2, wxALL | wxALIGN_CENTER_VERTICAL, 5);

    m_comboBox1 = new wxComboBox(this, wxID_COMBOBOX);
    m_comboBox1->Append(wxT("1->Item1"));
    m_comboBox1->Append(wxT("2->Item2"));
    m_comboBox1->Append(wxT("3->Item3"));
    m_comboBox1->SetSelection(0);
    bSizer1->Add(m_comboBox1, 2, wxALL | wxALIGN_CENTER_VERTICAL, 5);
    bSizer1->Add(0, 0, 1, wxEXPAND, 5);   //space

    this->SetSizer(bSizer1);
    this->Layout();
    // Connect Events
    this->Connect(wxID_BUTTON, wxEVT_COMMAND_BUTTON_CLICKED, wxCommandEventHandler(GUIFrame::OnButton));
}

class comboBoxApp : public wxApp
{
public:
    virtual bool OnInit();
};

bool comboBoxApp::OnInit()
{
    wxFrame* frame = new GUIFrame(0L);
    frame->Show();

    return true;
}

IMPLEMENT_APP(comboBoxApp);