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Manual Reference Pages  -  TK::FOCUS (3)

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Tk::focus - Manage the input focus









The <B>focusB> 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 <B>focusFollowsMouseB> 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 <B>focusNextB> and <B>focusPrevB> 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 <B>focusB> methods can take any of the following forms:
$widget-><B>focusCurrentB> Returns the focus window on the display containing the $widget, or an empty string if no window in this application has the focus on that display.
$widget-><B>focusB> If the application currently has the input focus on $widget’s display, this command resets the input focus for $widget’s display to $widget and returns an empty string. If the application doesn’t currently have the input focus on $widget’s display, $widget will be remembered as the focus for its top-level; the next time the focus arrives at the top-level, Tk will redirect it to $widget.
$widget-><B>focusForceB> Sets the focus of $widget’s display to $widget, even if the application doesn’t currently have the input focus for the display. This command should be used sparingly, if at all. In normal usage, an application should not claim the focus for itself; instead, it should wait for the window manager to give it the focus.
$widget-><B>focusLastforB> Returns the name of the most recent window to have the input focus among all the windows in the same top-level as $widget. If no window in that top-level has ever had the input focus, or if the most recent focus window has been deleted, then the top-level is returned. The return value is the window that will receive the input focus the next time the window manager gives the focus to the top-level.
$widget-><B>focusPrevB> <B>focusNextB> 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 <B>focusNextB> never returns a window in a different top-level from $widget.

After computing the next window, <B>focusNextB> examines the window’s <B>-takefocusB> option to see whether it should be skipped. If so, <B>focusNextB> 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.

<B>focusPrevB> is similar to <B>focusNextB> except that it sets the focus to the window just before $widget in the focus order.

$widget-><B>focusFollowsMouseB> <B>focusFollowsMouseB> changes the focus model for the application to an implicit one where the window under the mouse gets the focus. After this procedure is called, whenever the mouse enters a window Tk will automatically give it the input focus. The <B>focusB> command may be used to move the focus to a window other than the one under the mouse, but as soon as the mouse moves into a new window the focus will jump to that window. Note: at present there is no built-in support for returning the application to an explicit focus model; to do this you’ll have to write a script that deletes the bindings created by <B>focusFollowsMouseB>.


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 <B>CanvasB> widget, the call to <B>focusB> has to be fully qualified. This is because there is already a focus method for the <B>CanvasB> widget, which sets the focus on individual canvas tags.



events, focus, keyboard, top-level, window manager
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