add update function

src/main.rs

@@ -60,6 +60,9 @@ fn event(app: &App, model: &mut Model, event: WindowEvent) {
     println!("{:?}", event);
 }
 
+// Update the state of your application here. By default, this gets called right before `view`.
+fn update(_app: &App, _model: &mut Model, _update: Update) {}
+
 // Draw the state of your `Model` into the given `Frame` here.
 fn view(app: &App, model: &Model, frame: Frame) {
     let canvas = app.draw();
@@ -72,5 +75,5 @@ fn view(app: &App, model: &Model, frame: Frame) {
 }
 
 fn main() {
-    nannou::app(model).run();
+    nannou::app(model).update(update).run();
 }