Remove unused callback from GUI exercise (#1293)

Here's a possible update to the GUI/TUI exercise, taking out the
callback and explaining (slightly).

See what you think.

---------

Co-authored-by: Martin Geisler <martin@geisler.net>

src/exercises/day-3/simple-gui.md

@@ -1,13 +1,14 @@
-# A Simple GUI Library
+# Drawing A Simple GUI
 
 Let us design a classical GUI library using our new knowledge of traits and
-trait objects.
+trait objects. We'll only implement the drawing of it (as text) for simplicity.
 
 We will have a number of widgets in our library:
 
 * `Window`: has a `title` and contains other widgets.
-* `Button`: has a `label` and a callback function which is invoked when the
-  button is pressed.
+* `Button`: has a `label`. In reality, it would also take a callback
+  function to allow the program to do something when the button is clicked
+  but we won't include that since we're only drawing the GUI.
 * `Label`: has a `label`.
 
 The widgets will implement a `Widget` trait, see below.

src/exercises/day-3/simple-gui.rs

@@ -43,14 +43,12 @@ impl Label {
 
 pub struct Button {
     label: Label,
-    callback: Box<dyn FnMut()>,
 }
 
 impl Button {
-    fn new(label: &str, callback: Box<dyn FnMut()>) -> Button {
+    fn new(label: &str) -> Button {
         Button {
             label: Label::new(label),
-            callback,
         }
     }
 }
@@ -158,8 +156,7 @@ fn main() {
     let mut window = Window::new("Rust GUI Demo 1.23");
     window.add_widget(Box::new(Label::new("This is a small text GUI demo.")));
     window.add_widget(Box::new(Button::new(
-        "Click me!",
-        Box::new(|| println!("You clicked the button!")),
+        "Click me!"
     )));
     window.draw();
 }

src/exercises/day-3/solutions-morning.md

@@ -1,6 +1,6 @@
 # Day 3 Morning Exercise
 
-## A Simple GUI Library
+## Drawing A Simple GUI
 
 ([back to exercise](simple-gui.md))