dburrows/ blog/ entry/ CWidget tutoral 1: 'Hello, world!'

Late last year, I spent some time disentangling aptitude's internal UI library and packaging it as a separate library, which I named cwidget. This article is the first in what will hopefully become a series describing the basic concepts and APIs of cwidget. It applies to the versions of cwidget currently available in Debian sid and lenny, 0.5.11-1 and 0.5.12-1. Autogenerated documentation of cwidget can be found here (but beware that as of this writing it is somewhat incomplete).


In this tutorial, I will walk through a simple program (hello.cc) that initializes cwidget, displays a simple message box, and then terminates the program when the user confirms the message. I assume a basic familiarity with C++.

Screenshot of the 'Hello, World' program

The first thing to do is to include the portions of cwidget that our program will use. Most importantly, we need the top level cwidget routines. These are the global routines that initialize cwidget, shut it down, and control its main loop (among other things).

#include <cwidget/toplevel.h>

cwidget comes with a collection of stock dialog boxes for things like displaying messages to the user. To access the routines that build these dialogs, we write:

#include <cwidget/dialogs.h>

All the symbols provided by the cwidget library are in the cwidget namespace. Using the full library name means that there's a reasonable chance that its names will not conflict with the names provided by other libraries; however, it's a real pain to type cwidget over and over. Since this is the only namespaced library we are using in this program, it's handy to define cw as an alias for cwidget:

namespace cw = cwidget;

Now we're ready to write the main routine. The first interaction of client code with the cwidget library is to initialize it. This will initialize the ncurses library and put the terminal into a mode suitable for a full-screen curses program.

cw::toplevel::init();

Next, we create a dialog box that will be displayed to the user. There are two things to note about this code:

  1. cwidget is Unicode-aware and the ok() routine expects the message string to be a wide-character string. In this case, that means that we need to pass in a wide-character string (indicated by typing L in front of the string).

  2. The second argument tells cwidget what to do when the ok button is pressed. The expression

      sigc::ptr_fun(cw::toplevel::exitmain);
    

    creates a slot using libsigc++. Briefly, a slot is a reference to a function or to a method of a class instance. When invoked, this particular slot will call cwidget::toplevel::exitmain(), which causes the main cwidget loop to exit. The slot is wrapped in cwidget::util::arg, a utility function that handles passing slots as optional arguments.

cw::widgets::widget_ref dialog =
    cw::dialogs::ok(L"Hello, world!",
                    cw::util::arg(sigc::ptr_fun(cw::toplevel::exitmain)));

Now that we have a widget, the next step is to arrange for it to appear on the terminal. In cwidget, control of the terminal is assigned to a single top-level widget: whatever it displays is what gets displayed on the terminal, and all user input is passed directly to it. In a real program this widget will typically manage a collection of sub-widgets and assign each one a screen region, but for now let's just display the dialog box we created:

cw::toplevel::settoplevel(dialog);

With everything initialized, we're ready to start the main loop. mainloop() will keep the display up-to-date and dispatch incoming events (for instance, keystrokes and mouse presses) until exitmain() is invoked.

cw::toplevel::mainloop();

When the user presses Enter or clicks on the Ok button in the dialog, exitmain() will be invoked by the binding that we set up earlier. Once this happens, we need to shut down cwidget in order to restore the terminal to its original state. If you skip this step, the terminal will be garbled and will not work properly when your program exits.

cw::toplevel::shutdown();

And that's it! To compile the program, change to the directory containing hello.cc and run:

$ g++ -o hello hello.cc $(pkg-config --cflags --libs cwidget)

Of course, you'll need to have the g++ and libcwidget-dev packages installed for this to work!