book: Use bounded channels instead of unbounded (#1522)

They should be enough in most situations.

Author: Hofer-Julian

book/listings/main_event_loop/3/main.rs

@@ -29,7 +29,8 @@ fn build_ui(app: &Application) {
         .build();
 
     // ANCHOR: callback
-    let (sender, receiver) = async_channel::unbounded();
+    // Create channel that can hold at most 1 message at a time
+    let (sender, receiver) = async_channel::bounded(1);
     // Connect to "clicked" signal of `button`
     button.connect_clicked(move |_| {
         let sender = sender.clone();

book/listings/main_event_loop/4/main.rs

@@ -26,7 +26,8 @@ fn build_ui(app: &Application) {
         .build();
 
     // ANCHOR: callback
-    let (sender, receiver) = async_channel::unbounded();
+    // Create channel that can hold at most 1 message at a time
+    let (sender, receiver) = async_channel::bounded(1);
     // Connect to "clicked" signal of `button`
     button.connect_clicked(move |_| {
         let main_context = MainContext::default();