The ConstructWidget method is used to creates the components of a widget instance. In the case of TixArrowButton, we want to create a new button subwidget, whose name is button, and use a bitmap to display an arrow on this button. Assuming the bitmap files are stored in the files up.xbm, down.xbm, left.xbm and right.xbm, the string substitution @$data(-direction).xbm will give us the appropriate bitmap depending on the current direction option of the widget instance.
proc tixArrowButton:ConstructWidget {w} {
upvar #0 $w data
tixChainMethod $w ConstructWidget
set data(w:button) [button $w.button -bitmap @$data(-direction).xbm]
pack $data(w:button) -expand yes -fill both
}
The tixArrowButton:ConstructWidget method shown above sets the variable data(w:button) to be the pathname of the button subwidget. As a convention of the Tix Intrinsics, we must declare a public subwidget swid by storing its pathname in the variable data(w:swid).