[platform_channels] adds EventChannel Demo (#462)

platform_channels/lib/src/event_channel_demo.dart

@@ -0,0 +1,58 @@
+// 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/material.dart';
+import 'package:platform_channels/src/accelerometer_event_channel.dart';
+
+/// Demonstrates how to use [EventChannel] to listen continuous values
+/// of Accelerometer Sensor from platform.
+///
+/// The widget uses a [StreamBuilder] to rebuild it's descendant whenever it
+/// listens a new value from the [Accelerometer.readings] stream. It has three
+/// [Text] widgets to display the value of [AccelerometerReadings.x],
+/// [AccelerometerReadings.y], and [AccelerometerReadings.z] respectively.
+class EventChannelDemo extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final textStyle = Theme.of(context).textTheme.headline5;
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('EventChannel Demo'),
+      ),
+      body: Center(
+        child: StreamBuilder<AccelerometerReadings>(
+          stream: Accelerometer.readings,
+          builder: (context, snapshot) {
+            if (snapshot.hasError) {
+              return Text(snapshot.error.toString());
+            } else if (snapshot.hasData) {
+              return Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: [
+                  Text(
+                    'x axis: ' + snapshot.data.x.toStringAsFixed(3),
+                    style: textStyle,
+                  ),
+                  Text(
+                    'y axis: ' + snapshot.data.y.toStringAsFixed(3),
+                    style: textStyle,
+                  ),
+                  Text(
+                    'z axis: ' + snapshot.data.z.toStringAsFixed(3),
+                    style: textStyle,
+                  )
+                ],
+              );
+            }
+
+            return Text(
+              'No Data Available',
+              style: textStyle,
+            );
+          },
+        ),
+      ),
+    );
+  }
+}