focus - Manage the input focus
$widget->focus
$widget->focusOption
$widget->focusNext
$widget->focusPrev
$widget->focusFollowsMouse
The focus methods are 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 method 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 methods focusNext and 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 methods can take any of the following forms:
focusNext is a utility method used for keyboard traversal, but can be useful in other contexts. It sets the focus to the ``next'' window after $widget 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 $widget are skipped, so that focusNext never returns a window in a different top-level from $widget.
After computing the next window, focusNext examines the window's -takefocus option to see whether it should be skipped. If so, 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 $widget.
focusPrev is similar to focusNext except that it sets the focus to the window just before $widget in the focus order.
When an internal window receives the input focus, Tk doesn't 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.
Note that for the Canvas widget, the call to focus has to be fully qualified. This is because there is already a focus method for the Canvas widget, which sets the focus on individual canvas tags.
$canvas->Tk::focus
events, focus, keyboard, top-level, window manager