Programming with GTK

[Steffen Gutmann <muibase yahoo com>]

Consider the following macros:

#define WindowObject gtk_widget_new(GTK_TYPE_WINDOW
#define VBoxObject gtk_widget_new(GTK_TYPE_VBOX
#define HBoxObject gtk_widget_new(GTK_TYPE_HBOX
#define LabelObject gtk_widget_new(GTK_TYPE_LABEL
#define ButtonObject gtk_widget_new(GTK_TYPE_BUTTON
... // add one for each widget type
#define End NULL)

... allows to write code like this:

    GtkWidget *win, *label, *ok, *cancel;

    win = WindowObject,
        "child", VBoxObject,
            "child", label = LabelObject,
                "label", _("Hello world!"),
                "wrap", FALSE.
                End,
            "child", HBoxObject,
                "child", ok = ButtonObject,
                    "label", _("_Ok"),
                    "use-underline", TRUE,
                    End,
                "child", cancel = ButtonObject,
                    "label", _("_Cancel"),
                    "use-underline", TRUE,
                    End,
                End,
            End
        End;

The same code, using "traditional" gtk coding would be:

    GtkWidget *win, *label, *ok, *cancel;
    GtkWidget *top, *buttons;

    ok = gtk_button_new();
    gtk_button_set_label(GTK_BUTTON(ok), _("_Ok"));
    gtk_button_set_use_underline((GTK_BUTTON(ok), TRUE);

    cancel = gtk_button_new();
    gtk_button_set_label(GTK_BUTTON(cancel), _("_Cancel"));
    gtk_button_set_use_underline(GTK_BUTTON(cancel), TRUE);

    buttons = gtk_hox_new();
    gtk_container_add(GTK_CONTAINER(buttons), ok);
    gtk_container_add(GTK_CONTAINER(buttons), cancel);

    label = gtk_label_new(_("Hello world!");
    gtk_label_set_wrap(GTK_LABEL(label), FALSE);

    top = gtk_vbox_new();
    gtk_container_add(GTK_CONTAINER(top), label);
    gtk_container_add(GTK_CONTAINER(top), buttons);

    win = gtk_window_new();
    gtk_container_add(GTK_CONTAINER(win), top);

What do people think about this style of writing GTK programs?

Obviously there are pros and cons.  I start with the latter

- no type checking.  Since the whole creation of the object tree is one
nested function call with variable number of arguments, the compiler
cannot check the types of arguments.  I actually had a crash once when
I forgot to place a comma between two string constants, an attribute
followed by its value.  I am wondering if there any extensions in
current C compilers that would enable type checking on vargars calls
(similar to how printf format strings and arguments are type-checked
nowadays)?

- no syntax checking of attributes.  Since the code uses strings like
"label" there is no check (at compile-time) possible when one
accidentally misspells an attribute.  A solution would be to use
defines (e.g. GTKA_LABEL instead of "label").

- formatting of the calling tree has to be done by manually.  In case
the structure changes, e.g. inserting another VBox somewhere, other
lines might have to be indented accordingly.  

And the pros:

+ reads much better (in my opinion)

+ source code looks more compact

+ it is possible to use attributes that are not present in all versions
of GTK.  E.g. "xalign" for GtkEntry was introduced in 2.4, thus by
checking the version of GTK at runtime, one can use or not use this
attribute:

if (gtk_minor_version >= 4) {
    gtk_widget_set(entry, "xalign", 0.5);   // center text
}

I believe that this programming style would make GTK more attractive
for  developers as it looks very simple and straight forward, without
the need for various casts (GTK_WHATEVER(widget)) as in the
"traditional" version.  However, the current implementation does not
support it really.  For example if one also wants to specify packing
attributes (e.g. make the label expandable but not the button box) then
this is not currently possible.  It could be done by adding new
attributes to the Box widget API though (actually I did this by writing
my own GtkGroup class).

Well, just some thoughts.  I have coded a major application using this
technique and simply wanted to share my experience with you.

Best,

    Steffen

PS: if this sounds familiar to you, I first saw this idea proposed in
the MUI programming toolkit in the early 90's.  SunOS's OpenWindows
also had a similar approach).

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