src/lib.rs

@@ -36,4 +36,77 @@ impl CanvasElement {
     }
 }
 
+use im::Vector;
+
+// A canvas contains all elements to be drawn
+pub type Canvas = Vector<CanvasElement>;
+
+#[derive(Clone, druid::Data)]
+pub struct VersionedCanvas {
+    // We internally guarantee that this vector
+    // is never empty
+    versions: Vector<Canvas>,
+    curr_version: usize,
+}
+
+impl VersionedCanvas {
+    pub fn new(canvas: Canvas) -> VersionedCanvas {
+        VersionedCanvas {
+            versions: im::vector![canvas],
+            curr_version: 0,
+        }
+    }
+
+    // Get current canvas version
+    pub fn get(&self) -> &Canvas {
+        let focus = self.versions.focus();
+        self.versions.get(self.curr_version).unwrap()
+    }
+
+    fn has_newer_versions(&self) -> bool {
+        self.curr_version + 1 < self.versions.len()
+    }
+
+    fn has_older_versions(&self) -> bool {
+        self.curr_version > 0
+    }
+
+    pub fn undo(&mut self) {
+        if self.has_older_versions() {
+            self.curr_version = self.curr_version - 1;
+        }
+    }
+
+    pub fn redo(&mut self) {
+        if self.has_newer_versions() {
+            self.curr_version = self.curr_version + 1;
+        }
+    }
+
+    pub fn update(&mut self, update_fn: impl FnOnce(&Canvas) -> Canvas) {
+        // This is a linear history,
+        // so we first check if there are newer versions, if so
+        // this means we are in the past, so a change in the past destroys the future
+        // and creates a new future.
+        if self.has_newer_versions() {
+            self.versions = self.versions.take(self.curr_version + 1);
+        }
+        let new_version = update_fn(self.get());
+        self.versions.push_back(new_version);
+        self.curr_version = self.curr_version + 1;
+    }
+
+    // Do inplace update, which will be irreversible
+    pub fn irreversible_update(&mut self, update_fn: impl FnOnce(&mut Canvas)) {
+        // This is a linear history,
+        // so we first check if there are newer versions, if so
+        // this means we are in the past, so a change in the past destroys the future
+        // and creates a new future.
+        if self.has_newer_versions() {
+            self.versions = self.versions.take(self.curr_version + 1);
+        }
+
+        update_fn(self.versions.back_mut().unwrap());
+    }
+}
 

src/main.rs

@@ -14,23 +14,36 @@
 //  You should have received a copy of the GNU Affero General Public License
 //  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-use druid::im::{vector, Vector};
+use druid::im::{vector};
 use druid::kurbo::BezPath;
 use druid::widget::prelude::*;
 use druid::{AppLauncher, Color, Data, Event, LocalizedString, WindowDesc};
 
-use stiletto::CanvasElement;
+use stiletto::{CanvasElement, VersionedCanvas, Canvas};
 
 #[derive(Clone, Data)]
 struct CanvasData {
     current_element: Option<CanvasElement>,
-    elements: Vector<CanvasElement>,
+    //elements: Vector<CanvasElement>,
+    elements: VersionedCanvas,
 }
 
 impl CanvasData {
     fn is_drawing(&self) -> bool {
         self.current_element.is_some()
     }
+
+    fn perform_undo(&mut self) {
+        if !self.is_drawing() {
+            self.elements.undo();
+        }
+    }
+
+    fn perform_redo(&mut self) {
+        if !self.is_drawing() {
+            self.elements.redo();
+        }
+    }
 }
 
 struct CanvasWidget;
@@ -60,7 +73,13 @@ impl Widget<CanvasData> for CanvasWidget {
             Event::MouseUp(_) => {
                 if data.is_drawing() {
                     if let Some(current_element) = data.current_element.take() {
-                        data.elements.push_back(current_element);
+
+                        data.elements.update(move |canvas: &Canvas| -> Canvas {
+                            let mut new_canvas = canvas.clone();
+                            new_canvas.push_back(current_element);
+                            return new_canvas;
+                        });
+
                     }
                 }
             }
@@ -124,7 +143,7 @@ impl Widget<CanvasData> for CanvasWidget {
         // and we only want to clear this widget's area.
         ctx.fill(rect, &Color::WHITE);
 
-        for element in &data.elements {
+        for element in data.elements.get().iter() {
             element.draw(ctx);
         }
         if let Some(element) = &data.current_element {
@@ -138,8 +157,10 @@ fn build_ui() -> impl Widget<CanvasData> {
     let toolbar = Flex::row()
         .cross_axis_alignment(CrossAxisAlignment::Center)
         .with_spacer(30.0)
-        .with_child(Button::new("Undo"))
-        .with_child(Button::new("Redo"));
+        .with_child(
+            Button::new("Undo").on_click(|_ctx: &mut EventCtx, data: &mut CanvasData, _env: &Env| data.perform_undo())
+        )
+        .with_child(Button::new("Redo").on_click(|_ctx: &mut EventCtx, data: &mut CanvasData, _env: &Env| data.perform_redo()));
 
     Flex::column()
         .cross_axis_alignment(CrossAxisAlignment::Center)
@@ -156,7 +177,7 @@ pub fn main() {
         );
     let canvas_data = CanvasData {
         current_element: None,
-        elements: vector![],
+        elements: VersionedCanvas::new(vector![]),
     };
     AppLauncher::with_window(window)
         .use_simple_logger()