Utility procedure for managing the input focus.
http://purl.org/tcl/home/man/tcl8.4/TkCmd/focusNext.htm
SYNOPSIS
tk_focusNext window
DESCRIPTION
tk_focusNext is a utility procedure used for keyboard traversal. It returns the "next" window after window in focus order. The focus order is determined by the stacking order of windows and the structure of the window hierarchy. Among siblings, the focus order is the same as the stacking order, with the lowest window being first. If a window has children, the window is visited first, followed by its children (recursively), followed by its next sibling. Top-level windows other than window are skipped, so that tk_focusNext never returns a window in a different top-level from window.
After computing the next window, tk_focusNext examines the window's -takefocus option to see whether it should be skipped. If so, tk_focusNext continues on to the next window in the focus order, until it eventually finds a window that will accept the focus or returns back to window.
HaO: The focus order follows Stacking Order while the clipping order is controled by packing order (first packed, first seen). To achieve different clipping orders and focus orders, one may create and pack widgets in different order (or use the raise or lower commands).
Examples: First, two buttons obscure an entry widget, if place is limited (resize window). Tab order is backward as desired.
pack [button .b1 -text Button1] [button .b2 -text Button2] -side right pack [entry .e -width 30] -side left
To change tab-order, one may raise the .b2 and the .e widget:
pack [button .b1 -text Button1] [button .b2 -text Button2] -side right pack [entry .e -width 30] -side left raise .b1 .b2 raise .e .b1
A (IMHO) more natural way to achieve the same configuration, one may create widgets in focus order and pack widgets in clipping order:
entry .e -width 30 button .b1 -text Button1 button .b2 -text Button2 pack .b2 .b1 -side right pack .e -side left
To assign the functionality of the Tab key to other keys (ex. Return key), one may use two alternatives:
bind .w <Return> {focus [tk_focusNext %W]}
bind .w <Return> {event generate %W <Tab>}The first only switches focus. The second performs additional tasks, like selecting the current entry if it gains focus to an entry box.
MG By default tk_focusNext fires as part of the <<NextWindow>> event binding (which is bount to <Tab>), and does:
tk::TabToWindow [tk_focusNext %W]