[battery] Add BatteryStatusManager

Summary:
This adds a simple service to monitor whether the computer is plugged in
or not.

Test Plan: Run locally, verify that we correctly detect battery charging state

Reviewers: evan, spang, juan

Reviewed By: evan, spang, juan

Differential Revision: https://phab.nylas.com/D3937

src/global/nylas-exports.es6

@@ -219,8 +219,10 @@ lazyLoad(`QuotedHTMLTransformer`, 'services/quoted-html-transformer');
 lazyLoad(`InlineStyleTransformer`, 'services/inline-style-transformer');
 lazyLoad(`SearchableComponentMaker`, 'searchable-components/searchable-component-maker');
 lazyLoad(`QuotedPlainTextTransformer`, 'services/quoted-plain-text-transformer');
+lazyLoad(`BatteryStatusManager`, 'services/battery-status-manager');
 lazyLoadWithGetter(`BackoffScheduler`, () => require('../services/backoff-schedulers').BackoffScheduler);
 lazyLoadWithGetter(`ExponentialBackoffScheduler`, () => require('../services/backoff-schedulers').ExponentialBackoffScheduler);
+
 // Errors
 lazyLoadWithGetter(`APIError`, () => require('../flux/errors').APIError);
 

src/services/battery-status-manager.es6

@@ -0,0 +1,42 @@
+
+class BatteryStatusManager {
+  constructor() {
+    this._callbacks = [];
+    this._battery = null;
+  }
+
+  async activate() {
+    if (this._battery) {
+      return;
+    }
+    this._battery = await navigator.getBattery();
+    this._battery.addEventListener('chargingchange', this._onChargingChange);
+  }
+
+  deactivate() {
+    if (!this._battery) {
+      return;
+    }
+    this._battery.removeEventListener('chargingchange', this._onChargingChange);
+    this._battery = null;
+  }
+
+  _onChargingChange = () => {
+    this._callbacks.forEach(cb => cb());
+  }
+
+  onChange(callback) {
+    this._callbacks.push(callback);
+  }
+
+  isBatteryCharging() {
+    if (!this._battery) {
+      return false;
+    }
+    return this._battery.charging;
+  }
+}
+
+const manager = new BatteryStatusManager();
+manager.activate();
+export default manager;