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"Focus Events"

Tk's model of the input focus is different than X's model, and the focus window set with the focus command is not usually the same as the X focus window. Tk never explicitly changes the official X focus window. It waits for the window manager to direct the X input focus to and from the application's top-level windows, and it intercepts FocusIn and FocusOut events coming from the X server to detect these changes. All of the focus events received from X are discarded by Tk; they never reach the application. Instead, Tk generates a different stream of FocusIn and FocusOut for the application. This means that FocusIn and and FocusOut events seen by the application will not obey the conventions described in the documentation for Xlib.

Tk applications receive two kinds of FocusIn and FocusOut events, which can be distinguished by their detail fields. Events with a detail of NotifyAncestor are directed to the current focus window when it becomes active or inactive. A window is the active focus whenever two conditions are simultaneously true: (a) the window is the focus window for its application, and (b) some top-level window in the application has received the X focus. When this happens Tk generates a FocusIn event for the focus window with detail NotifyAncestor. When a window loses the active focus (either because the window manager removed the focus from the application or because the focus window changed within the application) then it receives a FocusOut event with detail NotifyAncestor.

The events described above are directed to the application's focus window regardless of which top-level window within the application has received the focus. The second kind of focus event is provided for applications that need to know which particular top-level window has the X focus. Tk generates FocusIn and FocusOut events with detail NotifyVirtual for top-level windows whenever they receive or lose the X focus. These events are generated regardless of which window in the application has the Tk input focus. They do not imply that keystrokes will be directed to the window that receives the event; they simply indicate which top-level window is active as far as the window manager is concerned. If a top-level window is also the application's focus window, then it will receive both NotifyVirtual and NotifyAncestor events when it receives or loses the X focus.

Tk does not generate the hierarchical chains of FocusIn and FocusOut events described in the Xlib documentation (e.g. a window can get a FocusIn or FocusOut event without all of its ancestors getting events too). Furthermore, the mode field in focus events is always NotifyNormal and the only values ever present in the detail field are NotifyAncestor and NotifyVirtual.


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