Prepare to support nested operations

experiments/2024-12-09/src/ui.rs

@@ -2,14 +2,14 @@ use crate::geometry::Operation;
 
 pub struct OperationView {
     operation: Box<dyn Operation>,
-    selected: usize,
+    selected: Option<usize>,
 }
 
 impl OperationView {
     pub fn new(operation: impl Operation + 'static) -> Self {
         Self {
             operation: Box::new(operation),
-            selected: 0,
+            selected: None,
         }
     }
 
@@ -18,29 +18,35 @@ impl OperationView {
             .children()
             .into_iter()
             .enumerate()
-            .map(|(i, op)| (op, i == self.selected))
+            .map(|(i, op)| (op, Some(i) == self.selected))
             .collect()
     }
 
     pub fn select_last(&mut self) {
         let last_index = self.operations().len().saturating_sub(1);
-        self.selected = last_index;
+        self.selected = Some(last_index);
     }
 
     pub fn select_next(&mut self) {
-        if self.selected + 1 < self.operations().len() {
-            self.selected += 1;
+        if let Some(selected) = self.selected {
+            if selected + 1 < self.operations().len() {
+                self.selected = Some(selected + 1);
+            }
         }
     }
 
     pub fn select_previous(&mut self) {
-        self.selected = self.selected.saturating_sub(1);
+        if let Some(selected) = self.selected {
+            self.selected = Some(selected.saturating_sub(1));
+        }
     }
 
     pub fn selected(&self) -> Option<Box<dyn Operation>> {
+        let selected = self.selected?;
+
         self.operations()
             .into_iter()
-            .nth(self.selected)
+            .nth(selected)
             .map(|(op, _)| op)
     }
 }