NetworkImage occasionally does not grab the image

I haven't been able to reproduce this bug consistently, but my theory is that
the ImageDecoder was being garbage collected before it called its completion
callback. This patch prevents that by keeping a reference to the image decoder
while the callback is in flight.

Fixes #801

packages/flutter/lib/services.dart

@@ -16,6 +16,7 @@ export 'src/services/asset_bundle.dart';
 export 'src/services/embedder.dart';
 export 'src/services/fetch.dart';
 export 'src/services/image_cache.dart';
+export 'src/services/image_decoder.dart';
 export 'src/services/image_resource.dart';
 export 'src/services/keyboard.dart';
 export 'src/services/shell.dart';

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

@@ -11,6 +11,7 @@ import 'package:mojo/core.dart' as core;
 import 'package:mojo_services/mojo/asset_bundle/asset_bundle.mojom.dart';
 import 'package:sky/src/services/fetch.dart';
 import 'package:sky/src/services/image_cache.dart';
+import 'package:sky/src/services/image_decoder.dart';
 import 'package:sky/src/services/image_resource.dart';
 import 'package:sky/src/services/shell.dart';
 
@@ -66,13 +67,13 @@ class MojoAssetBundle extends AssetBundle {
     _imageCache = null;
   }
 
+  Future<sky.Image> _fetchImage(String key) async {
+    return await decodeImageFromDataPipe(await load(key));
+  }
+
   ImageResource loadImage(String key) {
     return _imageCache.putIfAbsent(key, () {
-      Completer<sky.Image> completer = new Completer<sky.Image>();
-      load(key).then((assetData) {
-        new sky.ImageDecoder.consume(assetData.handle.h, completer.complete);
-      });
-      return new ImageResource(completer.future);
+      return new ImageResource(_fetchImage(key));
     });
   }
 

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

@@ -7,8 +7,18 @@ import 'dart:collection';
 import 'dart:sky' as sky;
 
 import 'package:mojo/mojo/url_response.mojom.dart';
-import 'package:sky/src/services/image_resource.dart';
 import 'package:sky/src/services/fetch.dart';
+import 'package:sky/src/services/image_decoder.dart';
+import 'package:sky/src/services/image_resource.dart';
+
+Future<sky.Image> _fetchImage(String url) async {
+  UrlResponse response = await fetchUrl(url);
+  if (response.statusCode >= 400) {
+    print("Failed (${response.statusCode}) to load image ${url}");
+    return null;
+  }
+  return await decodeImageFromDataPipe(response.body);
+}
 
 class _ImageCache {
   _ImageCache._();
@@ -17,16 +27,7 @@ class _ImageCache {
 
   ImageResource load(String url) {
     return _cache.putIfAbsent(url, () {
-      Completer<sky.Image> completer = new Completer<sky.Image>();
-      fetchUrl(url).then((UrlResponse response) {
-        if (response.statusCode >= 400) {
-          print("Failed (${response.statusCode}) to load image ${url}");
-          completer.complete(null);
-        } else {
-          new sky.ImageDecoder.consume(response.body.handle.h, completer.complete);
-        }
-      });
-      return new ImageResource(completer.future);
+      return new ImageResource(_fetchImage(url));
     });
   }
 }

packages/flutter/lib/src/services/image_decoder.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 'dart:async';
+import 'dart:sky' show Image, ImageDecoder, ImageDecoderCallback;
+import 'dart:typed_data';
+
+import 'package:mojo/core.dart' show MojoDataPipeConsumer;
+
+final Set<ImageDecoder> _activeDecoders = new Set<ImageDecoder>();
+
+typedef ImageDecoder _DecoderFactory(ImageDecoderCallback callback);
+
+Future<Image> _decode(_DecoderFactory createDecoder) {
+  Completer<Image> completer = new Completer<Image>();
+  ImageDecoder decoder;
+  decoder = createDecoder((Image image) {
+    _activeDecoders.remove(decoder);
+    completer.complete(image);
+  });
+  _activeDecoders.add(decoder);
+  return completer.future;
+}
+
+Future<Image> decodeImageFromDataPipe(MojoDataPipeConsumer consumerHandle) {
+  return _decode((ImageDecoderCallback callback) => new ImageDecoder.consume(consumerHandle.handle.h, callback));
+}
+
+Future<Image> decodeImageFromList(Uint8List list) {
+  return _decode((ImageDecoderCallback callback) => new ImageDecoder.fromList(list, callback));
+}