wxMessageBox off the main thread lost result code.

[wxWidgets.git] / docs / doxygen / overviews / helloworld.h

/////////////////////////////////////////////////////////////////////////////
// Name:        helloworld.h
// Purpose:     topic overview
// Author:      wxWidgets team
// Licence:     wxWindows licence
/////////////////////////////////////////////////////////////////////////////

/**

@page overview_helloworld Hello World Example

@tableofcontents

This page shows a very simple wxWidgets program that can be used as a skeleton
for your own code. While it does nothing very useful, it introduces a couple of
important concepts and explains how to write a working wxWidgets application.

First, you have to include wxWidgets' header files, of course. This can be done
on a file by file basis (such as @c wx/window.h) or using one global include
(@c wx/wx.h) which includes most of the commonly needed headers (although not
all of them as there are simply too many wxWidgets headers to pull in all of
them). For the platforms with support for precompiled headers, as indicated by
@c WX_PRECOMP, this global header is already included by @c wx/wxprec.h so we
only include it for the other ones:

@code
// wxWidgets "Hello world" Program

// For compilers that support precompilation, includes "wx/wx.h".
#include <wx/wxprec.h>

#ifndef WX_PRECOMP
    #include <wx/wx.h>
#endif
@endcode

Practically every app should define a new class derived from wxApp. By
overriding wxApp's OnInit() virtual method the program can be initialized, e.g.
by creating a new main window.

@code
class MyApp: public wxApp
{
public:
    virtual bool OnInit();
};
@endcode

The main window is created by deriving a class from wxFrame and giving it a
menu and a status bar in its constructor. Also, any class that wishes to
respond to any "event" (such as mouse clicks or messages from the menu or a
button) must declare an event table using the macro below.

Finally, the way to react to such events must be done in "handlers". In our
sample, we react to three menu items, one for our custom menu command and two
for the standard "Exit" and "About" commands (any program should normally
implement the latter two). Notice that these handlers don't need to be neither
virtual nor public.

@code
class MyFrame: public wxFrame
{
public:
    MyFrame(const wxString& title, const wxPoint& pos, const wxSize& size);

private:
    void OnHello(wxCommandEvent& event);
    void OnExit(wxCommandEvent& event);
    void OnAbout(wxCommandEvent& event);

    wxDECLARE_EVENT_TABLE();
};
@endcode

In order to be able to react to a menu command, it must be given a unique
identifier which can be defined as a const variable or an enum element. The
latter is often used because typically many such constants will be needed:

@code
enum
{
    ID_Hello = 1
};
@endcode

Notice that you don't need to define identifiers for the "About" and "Exit". We
then proceed to actually implement an event table in which the events are
routed to their respective handler functions in the class MyFrame.

There are predefined macros for routing all common events, ranging from the
selection of a list box entry to a resize event when a user resizes a window on
the screen. If @c wxID_ANY is given as the ID, the given handler will be
invoked for any event of the specified type, so that you could add just one
entry in the event table for all menu commands or all button commands etc.

The origin of the event can still be distinguished in the event handler as the
(only) parameter in an event handler is a reference to a wxEvent object, which
holds various information about the event (such as the ID of and a pointer to
the class, which emitted the event).

@code
wxBEGIN_EVENT_TABLE(MyFrame, wxFrame)
    EVT_MENU(ID_Hello,   MyFrame::OnHello)
    EVT_MENU(wxID_EXIT,  MyFrame::OnExit)
    EVT_MENU(wxID_ABOUT, MyFrame::OnAbout)
wxEND_EVENT_TABLE()
@endcode

As in all programs there must be a "main" function. Under wxWidgets main is
implemented using this macro, which creates an application instance and starts
the program.

@code
wxIMPLEMENT_APP(MyApp)
@endcode

As mentioned above, wxApp::OnInit() is called upon startup and should be used
to initialize the program, maybe showing a "splash screen" and creating the
main window (or several). The frame should get a title bar text ("Hello World")
and a position and start-up size. One frame can also be declared to be the top
window. Returning @true indicates a successful initialization.

@code
bool MyApp::OnInit()
{
    MyFrame *frame = new MyFrame( "Hello World", wxPoint(50, 50), wxSize(450, 340) );
    frame->Show( true );
    return true;
}
@endcode

In the constructor of the main window (or later on) we create a menu with our
menu items as well as a status bar to be shown at the bottom of the main
window. Both have to be associated with the frame with respective calls.

@code
MyFrame::MyFrame(const wxString& title, const wxPoint& pos, const wxSize& size)
        : wxFrame(NULL, wxID_ANY, title, pos, size)
{
    wxMenu *menuFile = new wxMenu;
    menuFile->Append(ID_Hello, "&Hello...\tCtrl-H",
                     "Help string shown in status bar for this menu item");
    menuFile->AppendSeparator();
    menuFile->Append(wxID_EXIT);

    wxMenu *menuHelp = new wxMenu;
    menuHelp->Append(wxID_ABOUT);

    wxMenuBar *menuBar = new wxMenuBar;
    menuBar->Append( menuFile, "&File" );
    menuBar->Append( menuHelp, "&Help" );

    SetMenuBar( menuBar );

    CreateStatusBar();
    SetStatusText( "Welcome to wxWidgets!" );
}
@endcode

Notice that we don't need to specify the labels for the standard menu items
@c wxID_ABOUT and @c wxID_EXIT, they will be given standard (even correctly
translated) labels and also standard accelerators correct for the current
platform making your program behaviour more native. For this reason you should
prefer reusing the standard ids (see @ref page_stockitems) if possible.

Here are the standard event handlers implementations. MyFrame::OnExit() closes
the main window by calling Close(). The parameter @true indicates that other
windows have no veto power such as after asking "Do you really want to close?".
If there is no other main window left, the application will quit.

@code
void MyFrame::OnExit(wxCommandEvent& event)
{
    Close( true );
}
@endcode

MyFrame::OnAbout() will display a small window with some text in it. In this
case a typical "About" window with information about the program.

@code
void MyFrame::OnAbout(wxCommandEvent& event)
{
    wxMessageBox( "This is a wxWidgets' Hello world sample",
                  "About Hello World", wxOK | wxICON_INFORMATION );
}
@endcode

The implementation of custom menu command handler may perform whatever task
your program needs to do, in this case we will simply show a message from it as
befits a hello world example:

@code
void MyFrame::OnHello(wxCommandEvent& event)
{
    wxLogMessage("Hello world from wxWidgets!");
}
@endcode

Here is the entire program that can be copied and pasted:

@code
// wxWidgets "Hello world" Program

// For compilers that support precompilation, includes "wx/wx.h".
#include <wx/wxprec.h>

#ifndef WX_PRECOMP
    #include <wx/wx.h>
#endif

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

class MyFrame: public wxFrame
{
public:
    MyFrame(const wxString& title, const wxPoint& pos, const wxSize& size);

private:
    void OnHello(wxCommandEvent& event);
    void OnExit(wxCommandEvent& event);
    void OnAbout(wxCommandEvent& event);

    wxDECLARE_EVENT_TABLE();
};

enum
{
    ID_Hello = 1
};

wxBEGIN_EVENT_TABLE(MyFrame, wxFrame)
    EVT_MENU(ID_Hello,   MyFrame::OnHello)
    EVT_MENU(wxID_EXIT,  MyFrame::OnExit)
    EVT_MENU(wxID_ABOUT, MyFrame::OnAbout)
wxEND_EVENT_TABLE()

wxIMPLEMENT_APP(MyApp);

bool MyApp::OnInit()
{
    MyFrame *frame = new MyFrame( "Hello World", wxPoint(50, 50), wxSize(450, 340) );
    frame->Show( true );
    return true;
}

MyFrame::MyFrame(const wxString& title, const wxPoint& pos, const wxSize& size)
        : wxFrame(NULL, wxID_ANY, title, pos, size)
{
    wxMenu *menuFile = new wxMenu;
    menuFile->Append(ID_Hello, "&Hello...\tCtrl-H",
                     "Help string shown in status bar for this menu item");
    menuFile->AppendSeparator();
    menuFile->Append(wxID_EXIT);

    wxMenu *menuHelp = new wxMenu;
    menuHelp->Append(wxID_ABOUT);

    wxMenuBar *menuBar = new wxMenuBar;
    menuBar->Append( menuFile, "&File" );
    menuBar->Append( menuHelp, "&Help" );

    SetMenuBar( menuBar );

    CreateStatusBar();
    SetStatusText( "Welcome to wxWidgets!" );
}

void MyFrame::OnExit(wxCommandEvent& event)
{
    Close( true );
}

void MyFrame::OnAbout(wxCommandEvent& event)
{
    wxMessageBox( "This is a wxWidgets' Hello world sample",
                  "About Hello World", wxOK | wxICON_INFORMATION );
}

void MyFrame::OnHello(wxCommandEvent& event)
{
    wxLogMessage("Hello world from wxWidgets!");
}
@endcode

*/