XGetDeviceFocus(display, device, focus_return, revert_to_return, time_return)
Depending on the focus argument, the following occurs:
If focus is None , all device events are discarded until a new focus window is set, and the revert_to argument is ignored.
If focus is a window, it becomes the device's focus window. If a generated device event would normally be reported to this window or one of its inferiors, the event is reported as usual. Otherwise, the event is reported relative to the focus window.
If focus is PointerRoot, the focus window is dynamically taken to be the root window of whatever screen the pointer is on at each event from the specified device. In this case, the revert_to argument is ignored.
If focus is FollowKeyboard, the focus window is dynamically taken to be the window to which the X keyboard focus is set at each input event.
The specified focus window must be viewable at the time XSetDeviceFocus is called, or a BadMatch error results. If the focus window later becomes not viewable, the X server evaluates the revert_to argument to determine the new focus window as follows:
If revert_to is RevertToParent, the focus reverts to the parent (or the closest viewable ancestor), and the new revert_to value is taken to be RevertToNone.
If revert_to is RevertToPointerRoot, RevertToFollowKeyboard, or RevertToNone, the focus reverts to PointerRoot, FollowKeyboard, or None, respectively.
When the focus reverts, the X server generates DeviceFocusIn and DeviceFocusOut events, but the last-focus-change time is not affected.
Input extension devices are not required to support the ability to be focused. Attempting to set the focus of a device that does not support this request will result in a BadMatch error. Whether or not given device can support this request can be determined by the information returned by XOpenDevice. For those devices that support focus, XOpenDevice will return an XInputClassInfo structure with the input_class field equal to the constant FocusClass (defined in the file XI.h).
XSetDeviceFocus can generate BadDevice, BadMatch, BadValue, and BadWindow errors.
The XGetDeviceFocus request returns the focus window and the current focus state.
Not all input extension devices can be focused. Attempting to query the focus state of a device that can't be focused results in a BadMatch error. A device that can be focused returns information for input Class Focus when an XOpenDevice request is made.
XGetDeviceFocus can generate BadDevice, and BadMatch errors.
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