Add a mechanism for receiving platform messages (#6361)

packages/flutter/lib/src/services/binding.dart

@@ -3,12 +3,14 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:ui' as ui;
 
 import 'package:flutter/foundation.dart';
 
 import 'asset_bundle.dart';
 import 'image_cache.dart';
 import 'shell.dart';
+import 'platform_messages.dart';
 
 /// Ensures that the [MojoShell] singleton is created synchronously
 /// during binding initialization. This allows other binding classes
@@ -27,6 +29,8 @@ abstract class ServicesBinding extends BindingBase {
   @override
   void initInstances() {
     super.initInstances();
+    ui.window
+      ..onPlatformMessage = PlatformMessages.handlePlatformMessage;
     new MojoShell();
     LicenseRegistry.addLicense(_addLicenses);
   }

packages/flutter/lib/src/services/platform_messages.dart

@@ -7,8 +7,21 @@ import 'dart:convert';
 import 'dart:typed_data';
 import 'dart:ui' as ui;
 
-String _decodeUTF8(ByteData data) {
-  return data != null ? UTF8.decoder.convert(data.buffer.asUint8List()) : null;
+import 'package:flutter/foundation.dart';
+
+ByteData _encodeUTF8(String message) {
+  if (message == null)
+    return null;
+  Uint8List encoded = UTF8.encoder.convert(message);
+  return encoded.buffer.asByteData();
+}
+
+String _decodeUTF8(ByteData message) {
+  return message != null ? UTF8.decoder.convert(message.buffer.asUint8List()) : null;
+}
+
+String _encodeJSON(dynamic message) {
+  return message != null ? JSON.encode(message) : null;
 }
 
 dynamic _decodeJSON(String message) {
@@ -18,12 +31,52 @@ dynamic _decodeJSON(String message) {
 void _sendString(String name, String message, void callback(String reply)) {
   Uint8List encoded = UTF8.encoder.convert(message);
   ui.window.sendPlatformMessage(name, encoded.buffer.asByteData(), (ByteData reply) {
-    callback(_decodeUTF8(reply));
+    try {
+      callback(_decodeUTF8(reply));
+    } catch (exception, stack) {
+      FlutterError.reportError(new FlutterErrorDetails(
+        exception: exception,
+        stack: stack,
+        library: 'services library',
+        context: 'during a platform message response callback',
+      ));
+    }
   });
 }
 
-/// Sends messages to the hosting application.
+typedef Future<ByteData> _PlatformMessageHandler(ByteData message);
+
+/// Sends message to and receives messages from the underlying platform.
 class PlatformMessages {
+  static final Map<String, _PlatformMessageHandler> _handlers =
+      <String, _PlatformMessageHandler>{};
+
+  /// Calls the handler registered for the given name.
+  ///
+  /// Typically called by [ServicesBinding] to handle platform messages received
+  /// from [ui.window.onPlatformMessage].
+  ///
+  /// To register a handler for a given message name, see
+  /// [setStringMessageHandler] and [setJSONMessageHandler].
+  static Future<Null> handlePlatformMessage(
+        String name, ByteData data, ui.PlatformMessageResponseCallback callback) async {
+    ByteData response;
+    try {
+      _PlatformMessageHandler handler = _handlers[name];
+      if (handler != null)
+        response = await handler(data);
+    } catch (exception, stack) {
+      FlutterError.reportError(new FlutterErrorDetails(
+        exception: exception,
+        stack: stack,
+        library: 'services library',
+        context: 'during a platform message callback',
+      ));
+    } finally {
+      callback(response);
+    }
+  }
+
   /// Send a string message to the host application.
   static Future<String> sendString(String name, String message) {
     Completer<String> completer = new Completer<String>();
@@ -41,4 +94,33 @@ class PlatformMessages {
     });
     return completer.future;
   }
+
+  /// Set a callback for receiving binary messages from the platform.
+  ///
+  /// The given callback will replace the currently registered callback (if any).
+  static void setBinaryMessageHandler(String name, Future<ByteData> handler(ByteData message)) {
+    _handlers[name] = handler;
+  }
+
+  /// Set a callback for receiving string messages from the platform.
+  ///
+  /// The given callback will replace the currently registered callback (if any).
+  static void setStringMessageHandler(String name, Future<String> handler(String message)) {
+    setBinaryMessageHandler(name, (ByteData message) async {
+      return _encodeUTF8(await handler(_decodeUTF8(message)));
+    });
+  }
+
+  /// Set a callback for receiving JSON messages from the platform.
+  ///
+  /// Messages received are decoded as JSON before being passed to the given
+  /// callback. The result of the callback is encoded as JSON before being
+  /// returned as the response to the message.
+  ///
+  /// The given callback will replace the currently registered callback (if any).
+  static void setJSONMessageHandler(String name, Future<dynamic> handler(dynamic message)) {
+    setStringMessageHandler(name, (String message) async {
+      return _encodeJSON(await handler(_decodeJSON(message)));
+    });
+  }
 }