[platform_channels] adds EventChannel implementation for iOS (#595)

platform_channels/ios/Runner/AccelerometerStreamHandler.swift

@@ -0,0 +1,38 @@
+// 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 CoreMotion
+
+class AccelerometerStreamHandler: NSObject, FlutterStreamHandler {
+    
+    var motionManger: CMMotionManager;
+    
+    override init() {
+        motionManger = CMMotionManager()
+    }
+    
+    func onListen(withArguments arguments: Any?, eventSink events: @escaping FlutterEventSink) -> FlutterError? {
+        
+        if !motionManger.isAccelerometerAvailable {
+            events(FlutterError(code: "SENSOR_UNAVAILABLE", message: "Accelerometer is not available", details: nil))
+        }
+        
+        motionManger.accelerometerUpdateInterval = 0.1
+        
+        motionManger.startAccelerometerUpdates(to: OperationQueue.main) {(data, error) in
+            guard let accelerationData = data?.acceleration else {
+                events(FlutterError(code: "DATA_UNAVAILABLE", message: "Cannot get accelerometer data", details: nil ))
+                return
+            }
+            
+            events([accelerationData.x, accelerationData.y, accelerationData.z])
+        }
+        
+        return nil
+    }
+    
+    func onCancel(withArguments arguments: Any?) -> FlutterError? {
+        return nil
+    }
+}