focus focus window focus option ?arg arg ...?
The focus command is used to manage the Tk input focus. At any given time, one window on each display is designated as the focus window; any key press or key release events for the display are sent to that window. It is normally up to the window manager to redirect the focus among the top-level windows of a display. For example, some window managers automatically set the input focus to a top-level window whenever the mouse enters it; others redirect the input focus only when the user clicks on a window. Usually the window manager will set the focus only to top-level windows, leaving it up to the application to redirect the focus among the children of the top-level.
Tk remembers one focus window for each top-level (the most recent descendant of that top-level to receive the focus); when the window manager gives the focus to a top-level, Tk automatically redirects it to the remembered window. Within a top-level Tk uses an explicit focus model by default. Moving the mouse within a top-level does not normally change the focus; the focus changes only when a widget decides explicitly to claim the focus (e.g., because of a button click), or when the user types a key such as Tab that moves the focus.
The Tcl procedure tk_focusFollowsMouse may be invoked to create an implicit focus model: it reconfigures Tk so that the focus is set to a window whenever the mouse enters it. The Tcl procedures tk_focusNext and tk_focusPrev implement a focus order among the windows of a top-level; they are used in the default bindings for Tab and Shift-Tab, among other things.
The focus command can take any of the following forms:
When an internal window receives the input focus, Tk does not actually set the X focus to that window; as far as X is concerned, the focus will stay on the top-level window containing the window with the focus. However, Tk generates FocusIn and FocusOut events just as if the X focus were on the internal window. This approach gets around a number of problems that would occur if the X focus were actually moved; the fact that the X focus is on the top-level is invisible unless you use C code to query the X server directly.
To make a window that only participates in the focus traversal ring when a variable is set, add the following bindings to the widgets before and after it in that focus ring:
button .before -text "Before"
button .middle -text "Middle"
button .after -text "After"
checkbutton .flag -variable traverseToMiddle -takefocus 0
pack .flag -side left
pack .before .middle .after
bind .before <Tab> {
if {!$traverseToMiddle} {
focus .after
break
}
}
bind .after <Shift-Tab> {
if {!$traverseToMiddle} {
focus .before
break
}
}
focus .before