[platform_channels] adds EventChannel Demo (#462)

platform_channels/lib/src/accelerometer_event_channel.dart

@@ -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 'package:flutter/services.dart';
+
+/// This class includes the implementation for [EventChannel] to listen to value
+/// changes from the Accelerometer sensor from native side. It has a [readings]
+/// getter to provide a stream of [AccelerometerReadings].
+class Accelerometer {
+  static final _eventChannel = const EventChannel('eventChannelDemo');
+
+  /// Method responsible for providing a stream of [AccelerometerReadings] to listen
+  /// to value changes from the Accelerometer sensor.
+  static Stream<AccelerometerReadings> get readings {
+    return _eventChannel.receiveBroadcastStream().map(
+          (dynamic event) => AccelerometerReadings(
+            event[0] as double,
+            event[1] as double,
+            event[2] as double,
+          ),
+        );
+  }
+}
+
+class AccelerometerReadings {
+  /// Acceleration force along the x-axis.
+  final double x;
+
+  /// Acceleration force along the y-axis.
+  final double y;
+
+  /// Acceleration force along the z-axis.
+  final double z;
+
+  AccelerometerReadings(this.x, this.y, this.z);
+}