Convert image_cache to using Futures rather than callbacks

Future<sky.Image> is easier to work with than a callback that provides an
image. It will also let us use other images sources that vend futures to
images.

R=jackson@google.com

Review URL: https://codereview.chromium.org/1216303005.

sdk/example/game/lib/image_map.dart

@@ -18,7 +18,7 @@ class ImageMap {
   }
 
   void _addURL(String url) {
-    image_cache.load(url, (Image image) {
+    image_cache.load(url).then((Image image) {
       // Store reference to image
       _images[url] = image;
 
@@ -34,4 +34,4 @@ class ImageMap {
   Image getImage(String url) => _images[url];
 
   Image operator [](String url) => _images[url];
-}
+}

sdk/example/raw/spinning_image.dart

@@ -59,7 +59,7 @@ bool handleEvent(Event event) {
   }
 
   if (event.type == "pointerup") {
-    image_cache.load(url2, handleImageLoad);
+    image_cache.load(url2).then(handleImageLoad);
     return true;
   }
 
@@ -67,8 +67,7 @@ bool handleEvent(Event event) {
 }
 
 void main() {
-  image_cache.load(url1, handleImageLoad);
-  image_cache.load(url1, handleImageLoad);
+  image_cache.load(url1).then(handleImageLoad);
   view.setEventCallback(handleEvent);
   view.setBeginFrameCallback(beginFrame);
 }

sdk/lib/mojo/asset_bundle.dart

@@ -12,7 +12,7 @@ import 'shell.dart' as shell;
 import 'net/fetch.dart';
 
 Future<sky.Image> _decodeImage(core.MojoDataPipeConsumer assetData) {
-  Completer<sky.Image> completer = new Completer();
+  Completer<sky.Image> completer = new Completer<sky.Image>();
   new sky.ImageDecoder(assetData.handle.h, completer.complete);
   return completer.future;
 }

sdk/lib/mojo/net/image_cache.dart

@@ -2,27 +2,28 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:sky';
+import 'dart:async';
+import 'dart:sky' as sky;
 import 'dart:collection';
 
 import 'package:mojom/mojo/url_response.mojom.dart';
 
 import 'fetch.dart';
 
-final HashMap<String, List<ImageDecoderCallback>> _pendingRequests =
-  new HashMap<String, List<ImageDecoderCallback>>();
+final HashMap<String, List<sky.ImageDecoderCallback>> _pendingRequests =
+  new HashMap<String, List<sky.ImageDecoderCallback>>();
 
-final HashMap<String, Image> _completedRequests =
-  new HashMap<String, Image>();
+final HashMap<String, sky.Image> _completedRequests =
+  new HashMap<String, sky.Image>();
 
-void _loadComplete(url, image) {
+void _loadComplete(String url, sky.Image image) {
   _completedRequests[url] = image;
   _pendingRequests[url].forEach((c) => c(image));
   _pendingRequests.remove(url);
 }
 
-void load(String url, ImageDecoderCallback callback) {
-  Image result = _completedRequests[url];
+void _load(String url, sky.ImageDecoderCallback callback) {
+  sky.Image result = _completedRequests[url];
   if (result != null) {
     callback(_completedRequests[url]);
     return;
@@ -31,7 +32,7 @@ void load(String url, ImageDecoderCallback callback) {
   bool newRequest = false;
   _pendingRequests.putIfAbsent(url, () {
     newRequest = true;
-    return new List<ImageDecoderCallback>();
+    return new List<sky.ImageDecoderCallback>();
   }).add(callback);
   if (newRequest) {
     fetchUrl(url).then((UrlResponse response) {
@@ -39,8 +40,15 @@ void load(String url, ImageDecoderCallback callback) {
         _loadComplete(url, null);
         return;
       }
-      new ImageDecoder(response.body.handle.h,
-          (image) => _loadComplete(url, image));
+      new sky.ImageDecoder(response.body.handle.h, (image) {
+        _loadComplete(url, image);
+      });
     });
   }
 }
+
+Future<sky.Image> load(String url) {
+  Completer<sky.Image> completer = new Completer<sky.Image>();
+  _load(url, completer.complete);
+  return completer.future;
+}

sdk/lib/painting/box_painter.dart

@@ -140,7 +140,7 @@ class BackgroundImage {
     this.fit: BackgroundFit.scaleDown,
     this.repeat: BackgroundRepeat.noRepeat
   }) {
-    image_cache.load(src, (image) {
+    image_cache.load(src).then((image) {
       if (image == null)
         return;
       _image = image;

sdk/lib/rendering/box.dart

@@ -904,7 +904,7 @@ class RenderImage extends RenderBox {
     if (value == _src)
       return;
     _src = value;
-    image_cache.load(_src, (result) {
+    image_cache.load(_src).then((result) {
       _image = result;
       if (requestedSize.width == null || requestedSize.height == null)
         markNeedsLayout();