将进度条添加到WPF控件的背景中

How can I modify certain WPF controls (i.e. combobox, textbox, etc) to have a progressbar in the background of the control (which I can enable and disable on demand). I don't need it to actually show an accurate progress, rather I just need an indication that this control is waiting for something to finish. A spinning icon inside the control would be OK too. How can I modify certain WPF controls (i.e. com