WIP: UPower and backlight handling in battery applet

applets/cosmic-applet-power-battery/src/backlight.rs

@@ -0,0 +1,41 @@
+use std::{
+    fs::{self, File},
+    io::{self, Read},
+    path::PathBuf,
+    str::FromStr,
+};
+
+struct Backlight(PathBuf);
+
+impl Backlight {
+    fn brightness(&self) -> Option<i64> {
+        self.prop("brightness")
+    }
+
+    fn max_brightness(&self) -> Option<i64> {
+        self.prop("max_brightness")
+    }
+
+    fn prop<T: FromStr>(&self, name: &str) -> Option<T> {
+        let mut file = File::open(&self.0.join(name)).ok()?;
+        let mut s = String::new();
+        file.read_to_string(&mut s).ok()?;
+        s.parse().ok()
+    }
+}
+
+// Choose backlight with most "precision". This is what `light` does.
+fn backlight() -> io::Result<Option<Backlight>> {
+    let mut best_backlight = None;
+    let mut best_max_brightness = 0;
+    for i in fs::read_dir("/sys/class/backlight")? {
+        let backlight = Backlight(i?.path());
+        if let Some(max_brightness) = backlight.max_brightness() {
+            if max_brightness > best_max_brightness {
+                best_backlight = Some(backlight);
+                best_max_brightness = max_brightness;
+            }
+        }
+    }
+    Ok(best_backlight)
+}