Programming with the XForms Library, Part 2: Writing an Application

We learn to write an application with XForms by simulating a game with 2 players and 2 actions.
The xgtsim Program

Let's just plunge right in. We'll get an example up and running right away, and then use the rest of this article to explain how it works. The program is called xgtsim and the C source code can be found in Listing 1.1 Although you're welcome to type it in, it's also available on the web site for this series (see, Listing 1). It should compile with the command:

gcc -lX11 -lforms -lm xgtsim.c -o xgtsim

From within the X Window System, you should be able to run the program by typing ./xgtsim in an xterm window. If you have problems, you may want to go back and review last month's article on installing XForms. With all possible windows open, the running program should look something like Figure 2.

Figure 2. xgtsim with Windows Open

If you want to play around with the program before continuing with the rest of this article, one useful exercise would be to set up a prisoner's dilemma. Just use the payoff editor to set values the same as they appear in the primer, and then try some different player strategies. In particular, try and figure out what happens when two Tit-for-Tat strategies come up against each other. Does it matter what initial strategies they play?

The Flow of the Program

Last month, we saw that the basic steps to designing an XForms program are as follows:

  1. Include forms.h to access the XForms routines

  2. Call fl_initialize() as soon as possible

  3. Set up your graphical interface by creating forms

  4. Assign actions to relevant objects by setting callbacks

  5. Show one or more forms

  6. Turn control over to fl_do_forms()

We use this approach in xgtsim. Like all C programs, execution begins in the main() routine, which is at the very end of the source code. First we call fl_initialize() to set up XForms, and allow it to parse command line arguments. Next, we call set_defaults() which seeds the random number generator, and sets some default values for our payoffs and player design variables: payoffs[][][], state_actions[][] and state_transitions[][][].

A call to create_forms() is then made, which sets up all of our windows, graphic elements and callbacks. We'll go into more detail shortly, but let's look at how this works for the simplest case: quitting the program. Within the create_forms() code, we use main_window (a variable of type FL_FORM) to create a window which will be shown when the program starts up. This window has four buttons on it, called Players, Payoffs, Run and Quit. Notice that the Quit button is set to call the function quit_xgtsim() with the command:

fl_set_object_callback(obj, quit_xgtsim, 1);

This means whenever the mouse is clicked on the button labeled Quit, the quit routine will be called. This function, in turn, simply calls fl_finish() and then exits.

To return to the flow of the main() function, after setting up all of our windows, buttons and so on with create_forms(), we then make our main_window appear with a call to fl_show_form(). Then we turn control over to the user by calling fl_do_forms().

It's crucial to understand that setting up our forms in create_forms() does more than just decide how graphics should be laid out on the screen. By setting callbacks to link button pushes and data inputs with specific actions, we've actually set up the whole flow of the program. When the user pushes the Payoffs button, it is XForms (via fl_do_forms()) which calls the relevant routine to make the Payoffs window appear, and to handle subsequent interaction with that window. In fact, if we've set all our callbacks correctly, execution never returns to main(). The fl_do_forms() routine returns only if the user activates an object which does not have a callback associated with it.

Some Details

Since create_forms() is so important, lets look at it in more detail. We use and re-use a generic pointer called *obj, which is of type FL_OBJECT, to create many of our graphical elements. This can be a little confusing, but we'll clarify things as we go.

The first form created in create_forms() is main_window. This is a global pointer variable which we declare early in the source code. We tell XForms it is a window which should be 290 pixels wide and 50 pixels high with the assignment:

main_window = fl_bgn_form(FL_NO_BOX, 290, 50);

In the following nine lines of code, we create four buttons which will be used to pop up windows for user interaction and to quit the program. Each time we need a new graphical element, we just use obj, which saves memory and keeps things simple. Just remember that whenever we reassign obj, all subsequent functions passing obj as a value will affect the most recent assignment. The Players, Payoffs and Run buttons are all linked by a callback to a routine called display_forms(), but they are set to call that routine with the values 1, 2 and 3 respectively. The display_forms() routine, in turn, uses these values to decide which window to display. After creating the Quit button, we tell XForms we're done adding elements to this form by calling fl_end_form().

We then go on to create the player_window, payoff_window and run_window. These all follow the same general pattern; declare the dimensions of the window with fl_bgn_form(), add as many objects as we want (assigning callbacks as we go), and then finish with fl_end_form(). We'll look at the run_window in detail, since it's the simplest. Once you have it figured out, you'll probably want to look over the other two on your own.

Since we want visual feedback from the game, we create two charts in the run_window. We make these into line charts by specifying FL_LINE_CHART, and we set the dimensions by including 4 integer values. The first two values represent where the upper left corner of our chart should appear, with 0,0 being the very top left corner of the form the object is being created on. The next values describe the width and height of the object. Finally, we supply a string to give the chart a label:

column_chart = fl_add_chart(FL_LINE_CHART, 10, 30,
        190, 90, "Column Players");

You may be wondering why we assign this function call to a variable called column_chart instead of using our generic obj variable. This is done because column_chart is declared as a global variable, which is accessible to all the routines in xgtsim. In particular, when the game is actually being run, the play_the_game() routine uses this global variable to add values to the chart we just created—look for the function fl_add_chart_value().

With the label "Column Players" assigned to our chart, the default behavior is for it to appear below the chart. We move it to the top left corner with the call to fl_set_object_lalign(). Then we limit the number of items which can be displayed with fl_set_chart_maxnumb(). We then create an almost identical chart to display information about Row Players.

In addition to chart feedback, we create two browsers to display numerical data. This is accomplished with calls to fl_add_browser(). Browsers are very useful objects in XForms, and they can be used in many different ways. Our implementation here is very simple, but you can learn more about them in the XForms documentation.

To allow the user to set the number of iterations the game should run, we create a counter, and set lots of options. First we align the label to appear on the left, then set the precision to 0. This just means we want our counter to hold integer data, since you can't really perform half an iteration. A standard counter appears on the screen with two sets of arrows. Whenever they are pushed, they change the value of the numerical data the counter is holding. We set bounds on this data with a call to fl_set_counter_bounds(), and then make one set of buttons change the value by 1 and the other set change the value by 100 by setting the counter step rates. We also set the starting value in the counter to a default value (stored in numb_iterations), and then record a callback. Whenever the counter object is changed, the routine set_iterations() is called which sets the variable numb_iterations.

The run window also contains two buttons, one to start the game running and one to stop it. Notice that we create these two buttons in exactly the same place on the form, so that they are on top of each other. Before finishing, though, we hide the stop_button to ensure the go_button is visible. When the go_button is pushed, it calls play_the_game(), which hides the go_button and makes the stop_button appear. The ability to call fl_hide_object() and fl_show_object() makes form design in XForms very flexible, since you can design windows where objects appear and disappear according to any number of conditions. When an object is hidden, it is impossible for the user to activate it.

Once fl_end_form() is called, we are nearly done with this window. Immediately afterwards though, we call:

fl_set_form_atclose(run_window, close_forms, 0);

This tells XForms what routine to run when the window manager sends the close window signal. On most window managers, this signal is sent when the user clicks on a close icon in the title bar of the window in question. This is like a callback, but the format is slightly different. In a normal callback, the declaration is of the form:

fl_set_object_callback(the_object, the_function,
The function pointed to by the_function must accept two arguments, an FL_OBJECT pointer and a long, as in:
the_function(FL_OBJECT *obj, long an_argument);
This function must return void. When the window close signal is sent, however, it applies to an entire window/form, not a particular object on that form. So the first argument to fl_set_form_atclose() must be a pointer of type FL_FORM, as in:
close_forms(FL_FORM *form, void *an_argument);
This function must return an integer, and in particular, it should return FL_OK if you want the window to actually close, and FL_IGNORE if you want the window to remain visible.

Having looked at main() and create_forms(), the rest of the source code is fairly easy to follow. The most complicated part is how the player_window uses the row_or_column variable to edit both types of players on a single form. The general idea is as follows. The global variables state_actions[][] and state_transitions[][][] hold all the data on the current state of both types of players, i.e., Column Players and Row Players. On the Player window, there are two buttons allowing the user to choose which type of player they want to edit. Whenever these are pushed, the Player window must be updated to reflect the state of these variables. This is done by the set_row_or_column() routine, which reads values from state_actions[][] and state_transitions[][][] into the relevant objects on the Player window, which are action_choices[] and transition_inputs[][].

With the window updated to reflect the current state of the relevant set of players, the user can now edit these values. This is accomplished via the set_player_values() function, which is called whenever any of these on-screen objects are changed. We do not bother trying to figure out which object is changed, but simply read all values on the Player window into state_actions[][] and state_transitions[][][].

The only remaining subtleties in the program are the use of the abort_flag variable and the call to fl_check_forms() in the iteration loop of play_the_game(). When the Go button is activated in the Run window, play_the_game() is called. One of the first things done in that routine is to set abort_flag to zero. Players are matched, payoffs made and the charts on the Run window are updated. At the bottom of the iteration loop, we check to see if abort_flag has been changed from 0 to 1, and if it has, we stop the run. You may be confused as to how this flag's value could have possibly changed within this algorithm.

The key lies in the call to fl_check_forms(). This is a non-blocking routine that works just like fl_do_forms(), except that it exits immediately if no objects were activated. This exacts a small performance penalty, since the program is effectively monitoring all its objects while the game is running, but it is well worth it. Since we set a callback to the Stop button to change abort_flag to one (via stop_the_game()), clicking on the Stop button will cause the current game to be aborted.

This has the added benefit of allowing us to modify all our data while the simulation is running. For example, we can alter payoff values and immediately see how this changes the unfolding game via the visual feedback in the Run window. Similarly, we can change player strategies on the fly, and watch how this affects their performance. This probing and runtime editing of parameters is often very difficult to achieve with standard C running on a console, but with a few global variables, a little sensible design and a call to fl_check_forms(), XForms makes it almost trivial.


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