timecache: add timecache help for timed caches

Some things we definitely don't want to generate every frame, such as
relative-time formatted strings, as that would create a heap allocation
each frame.

Introduce TimeCached<T> which is responsible for updating some state
after some expiry.

Signed-off-by: William Casarin <jb55@jb55.com>

src/lib.rs

@@ -9,6 +9,7 @@ mod images;
 mod result;
 mod filter;
 mod ui;
+mod timecache;
 mod frame_history;
 mod timeline;
 

src/timecache.rs

@@ -0,0 +1,27 @@
+use std::time::{Duration, Instant};
+
+pub struct TimeCached<T> {
+    last_update: Instant,
+    expires_in: Duration,
+    value: Option<T>,
+    refresh: Box<dyn Fn() -> T + 'static>,
+}
+
+impl<T> TimeCached<T> {
+    pub fn new(expires_in: Duration, refresh: Box<dyn Fn() -> T + 'static>) -> Self {
+        TimeCached {
+            last_update: Instant::now(),
+            expires_in,
+            value: None,
+            refresh,
+        }
+    }
+
+    pub fn get(&mut self) -> &T {
+        if self.value.is_none() || self.last_update.elapsed() > self.expires_in {
+            self.last_update = Instant::now();
+            self.value = Some((self.refresh)());
+        }
+        self.value.as_ref().unwrap() // This unwrap is safe because we just set the value if it was None.
+    }
+}