func (callback HookFun) Connect(xu *xgbutil.XUtil) {
xu.HooksLck.Lock()
defer xu.HooksLck.Unlock()
// COW
newHooks := make([]xgbutil.CallbackHook, len(xu.Hooks))
copy(newHooks, xu.Hooks)
newHooks = append(newHooks, callback)
xu.Hooks = newHooks
}