wxWidgets/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.
Trying building and running this application is also a good way of checking
that wxWidgets is correctly installed on your system. And if you haven't
installed wxWidgets yet, please @ref overview_install "do it first".

Note that this simple example creates the UI entirely from C++ code which is
fine for a simple example, but more realistic examples will typically define
their UI at least partially in @ref overview_xrc "XRC resource files".

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 an "event" (such as mouse clicks, messages from the menu, or a
button) must declare an event table using the macro below.

Finally, reacting to such events is done via "event handlers" which are
just functions (or functors, including lambdas if you're using C++11)
taking the @c event parameter of the type corresponding to the event being
handled, e.g. wxCommandEvent for the events from simple controls such as
buttons, text fields and also menu items. In our example, we react to three
menu items: our custom "Hello", and the "Exit" and "About" items (any program
should normally implement the latter two). Notice that these handlers don't
need to be virtual or public.

@code
class MyFrame : public wxFrame
{
public:
    MyFrame();

private:
    void OnHello(wxCommandEvent& event);
    void OnExit(wxCommandEvent& event);
    void OnAbout(wxCommandEvent& event);
};
@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 "About" and "Exit", as
wxWidgets already predefines standard values such as wxID_ABOUT and wxID_EXIT.
You should use these whenever possible, as they can be handled in a special
way by a particular platform.

As in all programs, there must be a "main" function. Under wxWidgets, main is
implemented inside the ::wxIMPLEMENT_APP() macro, which creates an application
instance of the specified class and starts running the GUI event loop. It is
used simply as:

@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). Frames are created hidden by default, to allow the
creation of child windows before displaying them. We thus need to explicitly
show them. Finally, we return @true from this method to indicate successful
initialization:

@code
bool MyApp::OnInit()
{
    MyFrame *frame = new MyFrame();
    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 bound to the frame with respective calls.

@code
MyFrame::MyFrame()
        : wxFrame(NULL, wxID_ANY, "Hello World")
{
    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!");

    ... continued below ...
@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 standard accelerators correct for the current
platform, making our program behaviour more native. For this reason, you
should prefer reusing the standard ids (see @ref page_stockitems) where
possible.

We also have to connect our event handlers to the events we want to handle in
them. We do this by calling Bind() to send all the menu events (identified by
wxEVT_MENU event type) with the specified ID to the given function. The
parameters we pass to Bind() are

-# The event type, e.g. wxEVT_MENU, wxEVT_BUTTON, wxEVT_SIZE, or one
   of many other events used by wxWidgets.
-# A Pointer to the method to call, and the object to call it on. In
   this case, we just call our own function, and pass the `this` pointer
   for the object itself. We could instead call the method of another object,
   or a non-member function — in fact, any object that can be called with a
   wxCommandEvent, can be used here.
-# An optional identifier, allowing us to select just some events of wxEVT_MENU
   type, namely those from the menu item with the given ID, instead of handling
   all of them in the provided handler. This is mainly useful with menu items
   and rarely with other kinds of events.

@code
    ... continued from above ...

    Bind(wxEVT_MENU, &MyFrame::OnHello, this, ID_Hello);
    Bind(wxEVT_MENU, &MyFrame::OnAbout, this, wxID_ABOUT);
    Bind(wxEVT_MENU, &MyFrame::OnExit, this, wxID_EXIT);
}
@endcode

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 example",
                 "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

@note In C++11 programs, it can be convenient to use unnamed lambdas instead of
    functions for event handlers, especially when handling events from the
    controls as this allows to keep the code creating the control and handling
    its event together in the same place. Here, for example, we could replace
    the wxID_EXIT handler with just

@code
    Bind(wxEVT_MENU, [=](wxCommandEvent&) { Close(true); }, wxID_EXIT);
@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();

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

enum
{
    ID_Hello = 1
};

wxIMPLEMENT_APP(MyApp);

bool MyApp::OnInit()
{
    MyFrame *frame = new MyFrame();
    frame->Show(true);
    return true;
}

MyFrame::MyFrame()
    : wxFrame(NULL, wxID_ANY, "Hello World")
{
    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!");

    Bind(wxEVT_MENU, &MyFrame::OnHello, this, ID_Hello);
    Bind(wxEVT_MENU, &MyFrame::OnAbout, this, wxID_ABOUT);
    Bind(wxEVT_MENU, &MyFrame::OnExit, this, wxID_EXIT);
}

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

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

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

*/