adds macOS implementation for battery plugin (#589)

experimental/federated_plugin/federated_plugin_macos/macos/Classes/FederatedPluginMacosPlugin.swift

@@ -0,0 +1,37 @@
+// Copyright 2020 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import Cocoa
+import FlutterMacOS
+import IOKit.ps
+
+public class FederatedPluginMacosPlugin: NSObject, FlutterPlugin {
+  public static func register(with registrar: FlutterPluginRegistrar) {
+    let channel = FlutterMethodChannel(name: "battery", binaryMessenger: registrar.messenger)
+    let instance = FederatedPluginMacosPlugin()
+    registrar.addMethodCallDelegate(instance, channel: channel)
+  }
+
+  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
+    switch call.method {
+    case "getBatteryLevel":
+        getBatteryLevel(result)
+    default:
+      result(FlutterMethodNotImplemented)
+    }
+  }
+
+    private func getBatteryLevel(_ result: FlutterResult) {
+        let snapshot = IOPSCopyPowerSourcesInfo().takeRetainedValue()
+        let sources = IOPSCopyPowerSourcesList(snapshot).takeRetainedValue() as Array
+        let sourceInfo : NSDictionary = IOPSGetPowerSourceDescription(snapshot, sources[0]).takeUnretainedValue()
+        
+        guard let capacity = sourceInfo[kIOPSCurrentCapacityKey] as? Int else {
+            result(FlutterError(code: "STATUS_UNAVAILABLE", message: "Not able to determine battery level", details: nil))
+            return
+        }
+        
+        result(capacity)
+    }
+}