1 files changed, 169 insertions, 0 deletions
diff --git a/native/src/widget/operation/focusable.rs b/native/src/widget/operation/focusable.rs
new file mode 100644
index 00000000..f17bf178
--- /dev/null
+++ b/native/src/widget/operation/focusable.rs
@@ -0,0 +1,169 @@
+//! Operate on widgets that can be focused.
+use crate::widget::operation::{Operation, Outcome};
+use crate::widget::Id;
+
+/// The internal state of a widget that can be focused.
+pub trait Focusable {
+    /// Returns whether the widget is focused or not.
+    fn is_focused(&self) -> bool;
+
+    /// Focuses the widget.
+    fn focus(&mut self);
+
+    /// Unfocuses the widget.
+    fn unfocus(&mut self);
+}
+
+/// A summary of the focusable widgets present on a widget tree.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct Count {
+    /// The index of the current focused widget, if any.
+    focused: Option<usize>,
+
+    /// The total amount of focusable widgets.
+    total: usize,
+}
+
+/// Produces an [`Operation`] that focuses the widget with the given [`Id`].
+pub fn focus<T>(target: Id) -> impl Operation<T> {
+    struct Focus {
+        target: Id,
+    }
+
+    impl<T> Operation<T> for Focus {
+        fn focusable(&mut self, state: &mut dyn Focusable, id: Option<&Id>) {
+            match id {
+                Some(id) if id == &self.target => {
+                    state.focus();
+                }
+                _ => {
+                    state.unfocus();
+                }
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    Focus { target }
+}
+
+/// Produces an [`Operation`] that generates a [`Count`] and chains it with the
+/// provided function to build a new [`Operation`].
+pub fn count<T, O>(f: fn(Count) -> O) -> impl Operation<T>
+where
+    O: Operation<T> + 'static,
+{
+    struct CountFocusable<O> {
+        count: Count,
+        next: fn(Count) -> O,
+    }
+
+    impl<T, O> Operation<T> for CountFocusable<O>
+    where
+        O: Operation<T> + 'static,
+    {
+        fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
+            if state.is_focused() {
+                self.count.focused = Some(self.count.total);
+            }
+
+            self.count.total += 1;
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+
+        fn finish(&self) -> Outcome<T> {
+            Outcome::Chain(Box::new((self.next)(self.count)))
+        }
+    }
+
+    CountFocusable {
+        count: Count::default(),
+        next: f,
+    }
+}
+
+/// Produces an [`Operation`] that searches for the current focused widget, and
+/// - if found, focuses the previous focusable widget.
+/// - if not found, focuses the last focusable widget.
+pub fn focus_previous<T>() -> impl Operation<T> {
+    struct FocusPrevious {
+        count: Count,
+        current: usize,
+    }
+
+    impl<T> Operation<T> for FocusPrevious {
+        fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
+            if self.count.total == 0 {
+                return;
+            }
+
+            match self.count.focused {
+                None if self.current == self.count.total - 1 => state.focus(),
+                Some(0) if self.current == 0 => state.unfocus(),
+                Some(0) => {}
+                Some(focused) if focused == self.current => state.unfocus(),
+                Some(focused) if focused - 1 == self.current => state.focus(),
+                _ => {}
+            }
+
+            self.current += 1;
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    count(|count| FocusPrevious { count, current: 0 })
+}
+
+/// Produces an [`Operation`] that searches for the current focused widget, and
+/// - if found, focuses the next focusable widget.
+/// - if not found, focuses the first focusable widget.
+pub fn focus_next<T>() -> impl Operation<T> {
+    struct FocusNext {
+        count: Count,
+        current: usize,
+    }
+
+    impl<T> Operation<T> for FocusNext {
+        fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
+            match self.count.focused {
+                None if self.current == 0 => state.focus(),
+                Some(focused) if focused == self.current => state.unfocus(),
+                Some(focused) if focused + 1 == self.current => state.focus(),
+                _ => {}
+            }
+
+            self.current += 1;
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    count(|count| FocusNext { count, current: 0 })
+}