Create User Interfaces with Glade

Mitch shows how to use gnome-python's libglade binding to build Python-based GUI applications with little manual coding.

Glade is a GUI builder for the Gtk+ toolkit. Glade makes it easy to create user interfaces interactively, and it can generate source code for those interfaces as well as stubs for user interface callbacks.

The libglade library allows programs to instantiate widget hierarchies defined in Glade project files easily. It includes a way to bind callbacks named in the project file to program-supplied callback routines.

James Henstridge maintains both libglade and the gnome-python package, which is a Python binding to the Gtk+ toolkit, the GNOME user interface libraries and libglade itself. Using libglade binding to build Python-based GUI applications can provide significant savings in development and maintenance costs.

All code examples in this article have been developed using Glade 0.5.11, gnome-python 1.0.53 and Python 2.1b1 running on Mandrake Linux 7.2.

Running Glade

When launched, Glade displays three top-level windows (see Figure 1). The application main window shows the contents of the current Glade project file as a list of top-level windows and dialogs defined in the project file. The Palette window shows the Gtk+ and GNOME widgets supported by Glade. When a widget is selected for editing, the Properties window displays the current values of that widget's properties.

Figure 1. Launching Glade

The Palette window partitions Glade's supported widgets into three groups. “GTK+ Basic” widgets are the most commonly used Gtk+ widgets. “GTK+ Additional” are less frequently used widgets such as rulers and calendars. “Gnome” widgets are taken from the GNOME UI library.

The Properties window displays widget properties in a four-page notebook. The Widget page displays the widget's name along with any properties that are specific to the widget's class. When the widget is placed inside a constraint-based container such as a GtkTable or GtkVBox, the Place page shows the properties that control the widget's placement within its container; otherwise the Place page is empty. The Basic page displays basic properties, such as width and height, possessed by all kinds of widgets. Finally, the Signals page lets you browse the set of Gtk+ signals that the selected widget can emit and lets you bind signal handler functions to those signals.

Creating Widget Hierarchies

The process of laying out a widget hierarchy within Glade is similar to that in environments such as Visual Basic. The root of every hierarchy is a top-level window or a dialog. Widgets can be placed within these top-level containers by first selecting, in the Glade Palette window, the type of widget to be created, then clicking on any cross-hatched region within the containers.

Defining Signal Handlers

The Signals page of the Glade Properties window lets you add application-specific behavior to your application. The top part of the page lists the signal handlers defined for the current widget. The controls in the bottom part of the page let you select one of the signals emitted by the widget and create a new handler for it.

To define a new signal handler, click on the ellipsis button to the right of the Signal entry field. A Select Signal dialog appears, listing all of the signals that this widget can emit. The signals are grouped by the Gtk+ widget class in which they are defined (see Figure 2).

Figure 2. Select Signal Dialog

Once you have selected a signal, click OK in the Select Signal dialog. The name of the selected signal appears in the Signal entry field of the Signals page. Glade also automatically fills in the Handler field, using the naming convention of “on_<widget>_<signal>”. You can change the name manually if Glade's naming conventions don't suit your needs.

The bottom portion of the Signals page provides additional entry fields where you can supply application-specific data, specify an object to receive the signal, and so on. I always leave these fields empty because they aren't needed when working with gnome-python.

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