Programming with XView
Let's extend this sample program by adding two buttons to it. One button will insert a fixed text string into the text sub-window, and the other button will erase the text sub-window. To do this, we will need a place to put them: namely, a control panel which is implemented in the PANEL package. The buttons themselves are panel items called PANEL_BUTTONs. We associate a subroutine, or “callback” routine, with a button, to be called when the user clicks on the button. The callback routines will manipulate the text sub-window. The second version of the sample.c program is shown in Listing 2.
Let's start with the new code inside our main. We created a panel inside the frame, positioned in the upper-left corner (x=0, y=0), extending to the right edge, 30 pixels tall and borderless.
Next, we added two buttons to the panel (not the frame), each with different button labels and different callback routines. For this example, XView handles the chore of positioning the buttons within the panel. If we wanted, we could use the XV_X and XV_Y attributes to position the buttons within the control panel.
Note that a hierarchy (see Figure 1) is forming. The frame is a parent of the panel and the text sub-window. The panel is a parent of the two buttons. When we resize the frame, its child components resize with it. The user interface could become quite sophisticated (i.e., complex) but still remain manageable, because of the relations that form among the frames, panels and other components. Our callback routines are invoked by other routines deep within XView and are passed the component and the event that produced this call (in this simple case, the button and the mouse-button-up event).
For the insert string callback routine, we use the global handle to the text sub-window and call an auxiliary routine to insert the literal text into the text pane. For the clear_window callback routine, we use another helper function to reset the text sub-window, which erases all the text from its pane. Although we use xv_set and xv_get to manipulate the attributes of the XView components, some components have a nice set of helper functions to make our job easier. The text sub-window is one such component.
Although this sample program doesn't give you an earth-shattering application, it does show you the core features of XView:
xv_init and xv_main_loop for setup and event handling
xv_create, xv_set, xv_get for component attributes
callback functions for event handling
This article has demonstrated the simplicity, elegance and beauty of XView. Perhaps you will be inspired to look into it further.
Mike Hall is a senior consultant with BALR Corporation, a Chicago-area computer consulting firm. His last assignment used XView and display PostScript on Suns. He can be reached at firstname.lastname@example.org.
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