Add selection toolbar and working delete action

src/main.rs

@@ -120,6 +120,18 @@ fn build_ui() -> impl Widget<StilettoState> {
             |_ctx: &mut EventCtx, data: &mut StilettoState, _env: &Env| data.canvas.perform_redo(),
         ));
 
+    let selection_buttons = Flex::row()
+        .cross_axis_alignment(CrossAxisAlignment::Center)
+        .with_child(Button::new("Copy").on_click(
+            |_ctx: &mut EventCtx, data: &mut StilettoState, _env: &Env| {},
+        ))
+        .with_child(Button::new("Cut").on_click(
+            |_ctx: &mut EventCtx, data: &mut StilettoState, _env: &Env| {},
+        ))
+        .with_child(Button::new("Delete").on_click(
+            |_ctx: &mut EventCtx, data: &mut StilettoState, _env: &Env| data.canvas.delete_selected(),
+        ));
+
     let stlt = FileSpec::new("Stiletto notebook", &["stlt"]);
     let save_dialog_options = FileDialogOptions::new()
         .allowed_types(vec![stlt])
@@ -178,6 +190,8 @@ fn build_ui() -> impl Widget<StilettoState> {
         .with_spacer(30.0)
         .with_flex_child(Align::left(history_buttons), 1.0)
         .with_spacer(10.0)
+        .with_flex_child(Align::left(selection_buttons), 1.0)
+        .with_spacer(10.0)
         .with_flex_child(Align::left(tool_buttons), 2.0)
         .with_spacer(20.0)
         .with_flex_child(Align::right(save_buttons), 1.0)

src/widget/canvas.rs

@@ -17,7 +17,7 @@
 use im::Vector;
 
 use super::tool_ctx::{CanvasToolCtx, CanvasToolState};
-use crate::canvas::Canvas;
+use crate::canvas::{Canvas, CanvasElement};
 use crate::tool::CanvasToolParams;
 use crate::{DocumentSnapshot, VersionedCanvas};
 
@@ -79,6 +79,33 @@ impl CanvasState {
         }
     }
 
+    pub fn delete_selected(&mut self) {
+        let selected_elements = self.versioned_canvas.get().selected_elements();
+        if !selected_elements.is_empty() {
+            if !selected_elements.is_empty() {
+                let elements = self.versioned_canvas.get().elements();
+                // TODO(enrico): is this memory efficient?
+                //               what is the difference between this and
+                //               im::Vector::retain?
+                let new_elements = elements.iter().enumerate()
+                    .filter_map(|(i, elem)| {
+                        if !selected_elements.contains(&i) {
+                            Some(elem)
+                        } else {
+                            None
+                        }
+                    })
+                    .cloned()
+                    .collect::<Vector<CanvasElement>>();
+
+                self.versioned_canvas.update(move |canvas: &mut Canvas| {
+                    *canvas = Canvas::new_with_elements(new_elements);
+                });
+            }
+        }
+    }
+
+
     /// Changes tool while mantaining the current CanvasToolCtx
     pub fn change_tool(&mut self, tool_params: CanvasToolParams) {
         self.tool_ctx.set_params(tool_params);