Actually populate the view host proxy

Now that the engine lets us take the view host handle, we can actually
access the view host from Dart.

examples/widgets/flutter.yaml

 name: widgets
 assets:
   - assets/starcircle.png
-  - assets/ali_connors.png
 material-design-icons:
   - name: action/account_circle
   - name: action/alarm

examples/widgets/mozart.dart

+// Copyright 2015 The Chromium Authors. 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';
+
+const List<String> _kKnownApps = const <String>[
+  'mojo:noodles_view',
+  'mojo:shapes_view',
+];
+
+const Size _kSmallWindowSize = const Size(400.0, 400.0);
+const Size _kBigWindowSize = const Size(600.0, 600.0);
+
+class Window extends StatefulComponent {
+  Window({ Key key, this.child }) : super(key: key);
+
+  final ChildViewConnection child;
+
+  _WindowState createState() => new _WindowState();
+}
+
+class _WindowState extends State<Window> {
+  Offset _offset = Offset.zero;
+  bool _isSmall = true;
+
+  void _handlePanUpdate(Offset delta) {
+    setState(() {
+      _offset += delta;
+    });
+  }
+
+  void _handleTap() {
+    setState(() {
+      _isSmall = !_isSmall;
+    });
+  }
+
+  Widget build(BuildContext context) {
+    Size size = _isSmall ? _kSmallWindowSize : _kBigWindowSize;
+    return new Positioned(
+      left: _offset.dx,
+      top: _offset.dy,
+      width: size.width,
+      height: size.height,
+      child: new GestureDetector(
+        onPanUpdate: _handlePanUpdate,
+        onTap: _handleTap,
+        child: new ChildView(child: config.child)
+      )
+    );
+  }
+}
+
+class WindowManager extends StatefulComponent {
+  _WindowManagerState createState() => new _WindowManagerState();
+}
+
+class _WindowManagerState extends State<WindowManager> {
+  List<ChildViewConnection> _windows = <ChildViewConnection>[];
+
+  void _handleTap() {
+    setState(() {
+      _windows.add(new ChildViewConnection(url: _kKnownApps[_windows.length % _kKnownApps.length]));
+    });
+  }
+
+  Widget build(BuildContext context) {
+    return new GestureDetector(
+      onTap: _handleTap,
+      child: new Container(
+        decoration: new BoxDecoration(
+          backgroundColor: Colors.blue[500]
+        ),
+        child: new Stack(
+          children: _windows.map((ChildViewConnection child) {
+            return new Window(
+              key: new ObjectKey(child),
+              child: child
+            );
+          }).toList()
+        )
+      )
+    );
+  }
+}
+
+void main() {
+  runApp(new MaterialApp(
+    title: 'Mozart',
+    routes: <String, RouteBuilder>{ '/': (_) => new WindowManager() }
+  ));
+}

packages/flutter/lib/src/rendering/child_view.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:ui_internals' as internals;
 
 import 'package:flutter/services.dart';
 import 'package:mojo_services/mojo/gfx/composition/scene_token.mojom.dart' as mojom;
@@ -10,20 +11,39 @@ import 'package:mojo_services/mojo/ui/layouts.mojom.dart' as mojom;
 import 'package:mojo_services/mojo/ui/view_provider.mojom.dart' as mojom;
 import 'package:mojo_services/mojo/ui/views.mojom.dart' as mojom;
 import 'package:mojo/application.dart';
+import 'package:mojo/core.dart' as core;
 import 'package:mojo/mojo/service_provider.mojom.dart' as mojom;
 
 import 'box.dart';
 import 'object.dart';
 
-// TODO(abarth): Populate the view host.
-final mojom.ViewHost _viewHost = null;
+mojom.ViewHostProxy _initViewHostProxy() {
+  int viewHost = internals.takeViewHostHandle();
+  assert(() {
+    if (viewHost == 0)
+      debugPrint('Child view are supported only when running in Mojo shell.');
+    return true;
+  });
+  return new mojom.ViewHostProxy.fromHandle(new core.MojoHandle(viewHost));
+}
+
+// TODO(abarth): The view host is a unique resource. We should structure how we
+// take the handle from the engine so that multiple libraries can interact with
+// the view host safely. Unfortunately, the view host has a global namespace of
+// view keys, which means any scheme for sharing the view host also needs to
+// provide a mechanism for coordinating about view keys.
+final mojom.ViewHostProxy _viewHostProxy = _initViewHostProxy();
+final mojom.ViewHost _viewHost = _viewHostProxy?.ptr;
 
 class ChildViewConnection {
   ChildViewConnection({ this.url }) {
     mojom.ServiceProviderProxy incomingServices = new mojom.ServiceProviderProxy.unbound();
     mojom.ServiceProviderStub outgoingServices = new mojom.ServiceProviderStub.unbound();
+    assert(_viewToken == null);
+    mojom.ViewProviderProxy viewProvider = new mojom.ViewProviderProxy.unbound();
+    shell.connectToService(url, viewProvider);
+    _unresolvedViewToken = _awaitResponse(viewProvider.ptr.createView(incomingServices, outgoingServices), viewProvider);
     _connection = new ApplicationConnection(outgoingServices, incomingServices);
-    _unresolvedViewToken = _connectToView(incomingServices, outgoingServices);
   }
 
   final String url;
@@ -34,14 +54,11 @@ class ChildViewConnection {
   Future<mojom.ViewToken> _unresolvedViewToken;
   mojom.ViewToken _viewToken;
 
-  Future<mojom.ViewToken> _connectToView(
-    mojom.ServiceProviderProxy incomingServices,
-    mojom.ServiceProviderStub outgoingServices
+  Future<mojom.ViewToken> _awaitResponse(
+    Future<mojom.ViewProviderCreateViewResponseParams> response,
+    mojom.ViewProviderProxy viewProvider
   ) async {
-    assert(_viewToken == null);
-    mojom.ViewProviderProxy viewProvider = new mojom.ViewProviderProxy.unbound();
-    shell.connectToService(url, proxy);
-    mojom.ViewToken viewToken = (await viewProvider.ptr.createView(incomingServices, outgoingServices)).viewToken;
+    mojom.ViewToken viewToken = (await response).viewToken;
     viewProvider.close();
     assert(_viewToken == null);
     _viewToken = viewToken;
@@ -125,7 +142,10 @@ class RenderChildView extends RenderBox {
   RenderChildView({
     ChildViewConnection child,
     double scale
-  }) : _child = child, _scale = scale;
+  }) : _child = child, _scale = scale {
+    if (_child != null)
+      _awaitViewToken();
+  }
 
   ChildViewConnection get child => _child;
   ChildViewConnection _child;
@@ -150,10 +170,14 @@ class RenderChildView extends RenderBox {
       // repaint now (to remove any old child view), and we need to watch for
       // the view token resolving before attempting layout.
       markNeedsPaint();
-      _child._unresolvedViewToken.then(_handleViewTokenResolved);
+      _awaitViewToken();
     }
   }
 
+  void _awaitViewToken() {
+    _child._unresolvedViewToken.then(_handleViewTokenResolved);
+  }
+
   double get scale => _scale;
   double _scale;
   void set scale (double value) {
@@ -182,7 +206,7 @@ class RenderChildView extends RenderBox {
   }
 
   void performLayout() {
-    if (_child != null)
+    if (_child != null && _child._viewToken != null)
       _child._layout(size: size, scale: scale).then(_handleLayoutInfoChanged);
   }
 
@@ -200,6 +224,8 @@ class RenderChildView extends RenderBox {
       markNeedsLayout();
   }
 
+  bool hitTestSelf(Point position) => true;
+
   void paint(PaintingContext context, Offset offset) {
     assert(needsCompositing);
     if (_layoutInfo != null)

packages/flutter/lib/src/widgets/child_view.dart

@@ -15,6 +15,7 @@ class ChildView extends StatelessComponent {
   final ChildViewConnection child;
 
   Widget build(BuildContext context) {
+    assert(MediaQuery.of(context) != null);
     return new _ChildViewWidget(
       child: child,
       scale: MediaQuery.of(context).devicePixelRatio