Implement focus_previous operation

native/src/widget/operation.rs

@@ -109,6 +109,46 @@ where
     }
 }
 
+pub fn focus_previous<T>() -> impl Operation<T> {
+    struct FocusPrevious {
+        count: FocusCount,
+        current: usize,
+    }
+
+    impl<T> Operation<T> for FocusPrevious {
+        fn focusable(
+            &mut self,
+            state: &mut dyn state::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_focusable(|count| FocusPrevious { count, current: 0 })
+}
+
 pub fn focus_next<T>() -> impl Operation<T> {
     struct FocusNext {
         count: FocusCount,
@@ -121,19 +161,10 @@ pub fn focus_next<T>() -> impl Operation<T> {
             state: &mut dyn state::Focusable,
             _id: Option<&Id>,
         ) {
-            if self.count.total == 0 {
-                return;
-            }
-
             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(),
-                Some(focused)
-                    if focused == self.count.total - 1 && self.current == 0 =>
-                {
-                    state.focus()
-                }
                 _ => {}
             }