Added brightness service that was missed in the last commit.

services/Brightness.js

@@ -0,0 +1,84 @@
+class BrightnessService extends Service {
+    // every subclass of GObject.Object has to register itself
+    static {
+        // takes three arguments
+        // the class itself
+        // an object defining the signals
+        // an object defining its properties
+        Service.register(
+            this,
+            {
+                // 'name-of-signal': [type as a string from GObject.TYPE_<type>],
+                'screen-changed': ['float'],
+            },
+            {
+                // 'kebab-cased-name': [type as a string from GObject.TYPE_<type>, 'r' | 'w' | 'rw']
+                // 'r' means readable
+                // 'w' means writable
+                // guess what 'rw' means
+                'screen-value': ['float', 'rw'],
+            },
+        );
+    }
+
+    // this Service assumes only one device with backlight
+    #interface = Utils.exec("sh -c 'ls -w1 /sys/class/backlight | head -1'");
+
+    // # prefix means private in JS
+    #screenValue = 0;
+    #max = Number(Utils.exec('brightnessctl max'));
+
+    // the getter has to be in snake_case
+    get screen_value() {
+        return this.#screenValue;
+    }
+
+    // the setter has to be in snake_case too
+    set screen_value(percent) {
+        if (percent < 0)
+            percent = 0;
+
+        if (percent > 1)
+            percent = 1;
+
+        Utils.execAsync(`brightnessctl set ${percent * 100}% -q`);
+        // the file monitor will handle the rest
+    }
+
+    constructor() {
+        super();
+
+        // setup monitor
+        const brightness = `/sys/class/backlight/${this.#interface}/brightness`;
+        Utils.monitorFile(brightness, () => this.#onChange());
+
+        // initialize
+        this.#onChange();
+    }
+
+    #onChange() {
+        this.#screenValue = Number(Utils.exec('brightnessctl get')) / this.#max;
+
+        // signals have to be explicitly emitted
+        this.emit('changed'); // emits "changed"
+        this.notify('screen-value'); // emits "notify::screen-value"
+
+        // or use Service.changed(propName: string) which does the above two
+        // this.changed('screen-value');
+
+        // emit screen-changed with the percent as a parameter
+        this.emit('screen-changed', this.#screenValue);
+    }
+
+    // overwriting the connect method, let's you
+    // change the default event that widgets connect to
+    connect(event = 'screen-changed', callback) {
+        return super.connect(event, callback);
+    }
+}
+
+// the singleton instance
+const service = new BrightnessService;
+
+// export to use in other modules
+export default service;