From 917199197f9719bbb3f6f98c63985cb64dfd147c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?H=C3=A9ctor=20Ram=C3=B3n=20Jim=C3=A9nez?=
 <hector0193@gmail.com>
Date: Sun, 3 May 2020 02:43:20 +0200
Subject: [PATCH] Allow erasing cells in `game_of_life`

---
 examples/game_of_life/src/main.rs | 35 ++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/examples/game_of_life/src/main.rs b/examples/game_of_life/src/main.rs
index c2f80dfc..080d55c0 100644
--- a/examples/game_of_life/src/main.rs
+++ b/examples/game_of_life/src/main.rs
@@ -156,6 +156,7 @@ mod grid {
     #[derive(Debug, Clone)]
     pub enum Message {
         Populate(Cell),
+        Unpopulate(Cell),
         Ticked {
             result: Result<Life, TickError>,
             tick_duration: Duration,
@@ -217,6 +218,10 @@ mod grid {
                     self.state.populate(cell);
                     self.life_cache.clear();
                 }
+                Message::Unpopulate(cell) => {
+                    self.state.unpopulate(&cell);
+                    self.life_cache.clear();
+                }
                 Message::Ticked {
                     result: Ok(life),
                     version,
@@ -293,20 +298,25 @@ mod grid {
 
             let cursor_position = cursor.position_in(&bounds)?;
             let cell = Cell::at(self.project(cursor_position, bounds.size()));
+            let is_populated = self.state.contains(&cell);
 
-            let populate = if self.state.contains(&cell) {
-                None
+            let (populate, unpopulate) = if is_populated {
+                (None, Some(Message::Unpopulate(cell)))
             } else {
-                Some(Message::Populate(cell))
+                (Some(Message::Populate(cell)), None)
             };
 
             match event {
                 Event::Mouse(mouse_event) => match mouse_event {
                     mouse::Event::ButtonPressed(button) => match button {
                         mouse::Button::Left => {
-                            self.interaction = Interaction::Drawing;
+                            self.interaction = if is_populated {
+                                Interaction::Erasing
+                            } else {
+                                Interaction::Drawing
+                            };
 
-                            populate
+                            populate.or(unpopulate)
                         }
                         mouse::Button::Right => {
                             self.interaction = Interaction::Panning {
@@ -321,6 +331,7 @@ mod grid {
                     mouse::Event::CursorMoved { .. } => {
                         match self.interaction {
                             Interaction::Drawing => populate,
+                            Interaction::Erasing => unpopulate,
                             Interaction::Panning { translation, start } => {
                                 self.translation = translation
                                     + (cursor_position - start)
@@ -504,6 +515,7 @@ mod grid {
         ) -> mouse::Interaction {
             match self.interaction {
                 Interaction::Drawing => mouse::Interaction::Crosshair,
+                Interaction::Erasing => mouse::Interaction::Crosshair,
                 Interaction::Panning { .. } => mouse::Interaction::Grabbing,
                 Interaction::None if cursor.is_over(&bounds) => {
                     mouse::Interaction::Crosshair
@@ -541,6 +553,14 @@ mod grid {
             }
         }
 
+        fn unpopulate(&mut self, cell: &Cell) {
+            if self.is_ticking {
+                let _ = self.births.remove(cell);
+            } else {
+                self.life.unpopulate(cell);
+            }
+        }
+
         fn update(&mut self, mut life: Life) {
             self.births.drain().for_each(|cell| life.populate(cell));
 
@@ -592,6 +612,10 @@ mod grid {
             self.cells.insert(cell);
         }
 
+        fn unpopulate(&mut self, cell: &Cell) {
+            let _ = self.cells.remove(cell);
+        }
+
         fn tick(&mut self) {
             let mut adjacent_life = FxHashMap::default();
 
@@ -706,6 +730,7 @@ mod grid {
     enum Interaction {
         None,
         Drawing,
+        Erasing,
         Panning { translation: Vector, start: Point },
     }
 }