Ximba Radio: Developing a GTK+/Glade GUI to XM Satellite Radio
Ximba Radio required two primary windows, the Main Window and the Preferences Dialog, and a number of secondary pop-up windows. The Main Window's button bar was created with Glade's toolbar widget, and the buttons were added to that manually. GTK+ buttons can have text or images. Glade allows a choice of application images, stock buttons or stock icons. Stock buttons use the same icons as stock icons except that tooltips are not available. Because of this, I suggest using stock icons and leaving the stock button field blank.
Each button in the toolbar has a single callback function attached to the click signal. The callback function can have any name and, if desired, be passed the name of the widget itself as an argument. For callbacks attached to click signals, the latter is not necessary. In callbacks attached to realize signals, which I discuss in a moment, the widget name is passed to the function.
I added three GtkImage widgets in the Main Window. The first is a State icon positioned to the right of the Host name field. I set this to the Remove icon—Glade offers many stock icons—to show no connection to the dæmon. To show a connected state I used the Apply icon. In order to change the icon at runtime, I saved the widget ID of this GtkImage in a realize callback. During normal use, this icon also can be changed to the Off stock icon in order to show a connected but muted state. I examine the code for handling these changes in the next section.
The Favorites buttons are plus signs. Glade and GTK+ call these Add icons. These buttons have a single callback attached to their click signal. The callback adds the current artist or channel to the appropriate list of favorites. The menu bar at the top of the main window was created using Glade's built-in Menu Editor. The editor has many options, but for this project I used only the Label, Name and Handler options, the latter to define the function to be called when the menu item was selected.
A notebook widget provides access to the complete channel listings, as well as category, favorite and session-specific listings. All of these are provided through the CList widget. Glade fully supports this widget even though GTK+ prefers that new code use the newer and more complex Tree and List widget. I cover this controversial decision in more detail a little later.
Glade generates empty functions for callbacks, often referred to as stub functions. The stub functions make it possible to follow a simple process in prototype development: design the UI, generate code, write callbacks, test and repeat. I left most of the callback coding—aside from menu quit functions—until after the UI was complete. Later, I went back and filled in the callbacks. This methodology allowed me to experiment with the layout of the application before having to get involved too deeply with what that layout actually would do. Again, this is part of the whole goal of separating user-interface code from application code. By keeping these two pieces separate, I allow future changes to the UI to happen without serious impact on the core code. Callbacks are the glue between the UI and the application code because they map UI events to code that performs some action.
Callbacks have varying interfaces. Button click signals need callbacks that take the button widget ID and user data as input arguments. CList callbacks for the select-row signal, sent when a row is clicked on, get five arguments. Letting Glade generate them makes it possible to learn these varying interfaces quickly. In fact, because the API for callbacks is not well documented—at least documentation is not easy to find—letting Glade create these is the best way to learn callback syntax.
Filling in the callback code can be done directly in callbacks.c, but this C module will be dropped in the future when I move to libGlade. Instead, I usually pass parameters straight through to a similar function in utils.c that does the actual work. Despite this general rule, one important bit of code was put in callbacks.c: assigning widget IDs to global variables. Listing 1 shows how a global variable is used in a callback to save the ID of the Preferences dialog.
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