add new example: stopwatch (#503)

Co-authored-by: Mckol <mckol363@gmail.com>

Readme.md

@@ -94,6 +94,7 @@ Here are some cool applications using cursive:
 * [fui](https://github.com/xliiv/fui): Add CLI & form interface to your program.
 * [grin-tui](https://github.com/mimblewimble/grin): Minimal implementation of the MimbleWimble protocol.
 * [ripasso](https://github.com/cortex/ripasso): A simple password manager written in Rust.
+* [clock-cli](https://github.com/TianyiShi2001/clock-cli-rs): A clock with stopwatch and countdown timer functionalities
 * [sudoku-tui](https://github.com/TianyiShi2001/sudoku-tui): Play sudoku on the command line.
 
 ## Goals

examples/src/bin/stopwatch.rs

@@ -0,0 +1,91 @@
+//! A simple stopwatch implementation.
+//! Also check out `clock-cli` (https://github.com/TianyiShi2001/cl!ock-cli-rs),
+//! which aims to implement a fully-fledged clock with stopwatch, countdown
+//! timer, and possibly more functionalities.
+
+use cursive::traits::{Nameable, Resizable};
+use cursive::views::{Button, Canvas, Dialog, LinearLayout};
+use std::time::{Duration, Instant};
+
+fn main() {
+    let mut siv = cursive::default();
+
+    siv.add_layer(
+        Dialog::new()
+            .title("Stopwatch")
+            .content(
+                LinearLayout::horizontal()
+                    .child(
+                        Canvas::new(Watch {
+                            last_started: Instant::now(),
+                            last_elapsed: Duration::default(),
+                            running: true,
+                        })
+                        .with_draw(|s, printer| {
+                            printer.print(
+                                (0, 1),
+                                &format!("{:.2?}", s.elapsed()),
+                            );
+                        })
+                        .with_name("stopwatch")
+                        .fixed_size((8, 3)),
+                    )
+                    .child(
+                        LinearLayout::vertical()
+                            .child(Button::new("Start", run(Watch::start)))
+                            .child(Button::new("Pause", run(Watch::pause)))
+                            .child(Button::new("Stop", run(Watch::stop))),
+                    ),
+            )
+            .button("Quit", |s| s.quit())
+            .h_align(cursive::align::HAlign::Center),
+    );
+
+    siv.set_fps(20);
+
+    siv.run();
+}
+
+struct Watch {
+    last_started: Instant,
+    last_elapsed: Duration,
+    running: bool,
+}
+
+impl Watch {
+    fn start(&mut self) {
+        self.running = true;
+        self.last_started = Instant::now();
+    }
+
+    fn elapsed(&self) -> Duration {
+        self.last_elapsed
+            + if self.running {
+                Instant::now() - self.last_started
+            } else {
+                Duration::default()
+            }
+    }
+
+    fn pause(&mut self) {
+        self.last_elapsed = self.elapsed();
+        self.running = false;
+    }
+
+    fn stop(&mut self) {
+        self.running = false;
+        self.last_elapsed = Duration::default();
+    }
+}
+
+// Helper function to find the stopwatch view and run a closure on it.
+fn run<F>(f: F) -> impl Fn(&mut cursive::Cursive)
+where
+    F: Fn(&mut Watch),
+{
+    move |s| {
+        s.call_on_name("stopwatch", |c: &mut Canvas<Watch>| {
+            f(c.state_mut());
+        });
+    }
+}