feat(widget): add dropdown::multi list widget

src/widget/dropdown/multi/model.rs

@@ -0,0 +1,102 @@
+//! A [`Model`] for a multi menu dropdown widget.
+
+/// Create a [`Model`] for a multi-list dropdown.
+pub fn model<S, Item>() -> Model<S, Item> {
+    Model {
+        lists: Vec::new(),
+        selected: None,
+    }
+}
+
+/// Create a [`List`] for a multi-list dropdown widget.
+pub fn list<S, Item>(description: Option<S>, options: Vec<(S, Item)>) -> List<S, Item> {
+    List {
+        description,
+        options,
+    }
+}
+
+/// A model for managing the options in a multi-list dropdown.
+///
+/// ```no_run
+/// #[derive(Copy, Clone, Eq, PartialEq)]
+/// enum MenuOption {
+///     Option1,
+///     Option2,
+///     Option3,
+///     Option4,
+///     Option5,
+///     Option6
+/// }
+/// use cosmic::widget::dropdown;
+///
+/// let mut model = dropdown::multi::model();
+///
+/// model.insert(dropdown::multi::list(Some("List A"), vec![
+///     ("Option 1", MenuOption::Option1),
+///     ("Option 2", MenuOption::Option2),
+///     ("Option 3", MenuOption::Option3)
+/// ]));
+///
+/// model.insert(dropdown::multi::list(Some("List B"), vec![
+///     ("Option 4", MenuOption::Option4),
+///     ("Option 5", MenuOption::Option5),
+///     ("Option 6", MenuOption::Option6)
+/// ]));
+///
+/// model.clear();
+/// ```
+#[must_use]
+pub struct Model<S, Item> {
+    pub lists: Vec<List<S, Item>>,
+    pub selected: Option<Item>,
+}
+
+impl<S, Item: PartialEq> Model<S, Item> {
+    pub(super) fn get(&self, item: &Item) -> Option<&S> {
+        for list in &self.lists {
+            for option in &list.options {
+                if &option.1 == item {
+                    return Some(&option.0);
+                }
+            }
+        }
+
+        None
+    }
+
+    pub(super) fn next(&self) -> Option<&(S, Item)> {
+        let Some(item) = self.selected.as_ref() else {
+            return None;
+        };
+
+        let mut next = false;
+        for list in &self.lists {
+            for option in &list.options {
+                if next {
+                    return Some(option);
+                }
+
+                if &option.1 == item {
+                    next = true;
+                }
+            }
+        }
+
+        None
+    }
+
+    pub fn clear(&mut self) {
+        self.lists.clear();
+    }
+
+    pub fn insert(&mut self, list: List<S, Item>) {
+        self.lists.push(list);
+    }
+}
+
+/// A list for a multi-list dropdown widget.
+pub struct List<S, Item> {
+    pub description: Option<S>,
+    pub options: Vec<(S, Item)>,
+}