Volume 6A: Motif Programming Manual

XFillRectangle (dpy, pixmap, gc, 0, 0, WIDTH, HEIGHT);

/* don't foreget to reset */

XSetForeground (dpy, gc, BlackPixelOfScreen (XtScreen (widget)));

}

/* Note: we don't have to use WIDTH and HEIGHT−−we could pull the

* exposed area out of the event structure, but only if the reason

* was XmCR_EXPOSE... make it simple for the demo; optimize as needed.

*/

XCopyArea (dpy, pixmap, event−>xany.window, gc,

0, 0, WIDTH, HEIGHT, 0, 0);

}

}

A frequent problem encountered in using the DrawingArea widget is the need to redraw after every Resize event.

When you enlarge the DrawingArea window, an Expose event is automatically generated since more of the window

becomes exposed. But, if you shrink the window, no Expose event is generated since no new part of the window is

being exposed.

The reason why no Expose event is generated when you shrink a DrawingArea widget is deep inside Xlib. The bit

gravity of a window indicates where new bits are placed automatically by X when a window is resized. If you resize a

window larger, then the data in the window remains in the top−left corner and the application gets a Resize event

and an Expose event. The Expose event just identifies the newly exposed area, not the entire window. If you make

the window smaller, all of the data in the window gets pushed to the top left; there is no newly exposed area, so there

is no Expose event.

The solution is to make the window forget about bit gravity, so every Resize event causes all of the bits to be

cleared. As a result, ...