add assets channel

lib/assetsChannel.dart

+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+class PlatformChannel extends StatefulWidget {
+  @override
+  _PlatformChannelState createState() => _PlatformChannelState();
+}
+
+class _PlatformChannelState extends State<PlatformChannel> {
+  static const MethodChannel methodChannel =
+      MethodChannel('GMAssetsMethodChannelName');
+  static const EventChannel eventChannel =
+      EventChannel('GMAssetsEventChannelName');
+
+  String _batteryLevel = 'Battery level: unknown.';
+  ByteData _imageData;
+
+  Future<void> _editPortrait() async {
+    String batteryLevel;
+    try {
+      final int result = await methodChannel.invokeMethod('editPortrait');
+      batteryLevel = 'Battery level: $result%.';
+    } on PlatformException {
+      batteryLevel = 'Failed to get battery level.';
+    }
+    setState(() {
+      _batteryLevel = batteryLevel;
+    });
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    eventChannel.receiveBroadcastStream().listen(_onEvent, onError: _onError);
+  }
+
+  void _onEvent(Object event) {
+    _imageData = event;
+    print(event.runtimeType);
+    setState(() {
+    });
+  }
+
+  void _onError(Object error) {
+    setState(() {
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Material(
+      child: Scaffold(
+        appBar: AppBar(
+          title: Text('editPortrait test'),
+        ),
+        body: Center(
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+            children: <Widget>[
+              Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: <Widget>[
+                  Text(_batteryLevel, key: const Key('editPortrait')),
+                  Padding(
+                    padding: const EdgeInsets.all(16.0),
+                    child: RaisedButton(
+                      child: const Text('editPortrait'),
+                      onPressed: _editPortrait,
+                    ),
+                  ),
+                ],
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}