Introduced ImageProvider for asynchronously loading images.

Updated image cache to use any ImageProvider instance.
Renamed RawImage to AsyncImage and updated the interface
to use any ImageProvider instance.

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

@@ -11,25 +11,45 @@ import 'fetch.dart';
 import 'image_decoder.dart';
 import 'image_resource.dart';
 
-Future<ui.Image> _fetchImage(String url) async {
-  UrlResponse response = await fetchUrl(url);
-  if (response.statusCode >= 400) {
-    print("Failed (${response.statusCode}) to load image $url");
-    return null;
+/// Implements a way to retrieve an image, for example by fetching it from the network.
+/// Also used as a key in the image cache.
+abstract class ImageProvider {
+  Future<ui.Image> loadImage();
+}
+
+class _UrlFetcher implements ImageProvider {
+  final String _url;
+
+  _UrlFetcher(this._url);
+
+  Future<ui.Image> loadImage() async {
+    UrlResponse response = await fetchUrl(_url);
+    if (response.statusCode >= 400) {
+      print("Failed (${response.statusCode}) to load image $_url");
+      return null;
+    }
+    return await decodeImageFromDataPipe(response.body);
   }
-  return await decodeImageFromDataPipe(response.body);
+
+  bool operator ==(other) => other is _UrlFetcher && _url == other._url;
+  int get hashCode => _url.hashCode;
 }
 
 class _ImageCache {
   _ImageCache._();
 
-  final Map<String, ImageResource> _cache = new Map<String, ImageResource>();
+  final Map<ImageProvider, ImageResource> _cache =
+      new Map<ImageProvider, ImageResource>();
 
-  ImageResource load(String url) {
-    return _cache.putIfAbsent(url, () {
-      return new ImageResource(_fetchImage(url));
+  ImageResource loadProvider(ImageProvider provider) {
+    return _cache.putIfAbsent(provider, () {
+      return new ImageResource(provider.loadImage());
     });
   }
+
+  ImageResource load(String url) {
+    return loadProvider(new _UrlFetcher(url));
+  }
 }
 
 final _ImageCache imageCache = new _ImageCache._();

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

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:typed_data';
 import 'dart:ui' as ui;
 
 import 'package:flutter/rendering.dart';
@@ -10,7 +9,6 @@ import 'package:flutter/services.dart';
 
 import 'framework.dart';
 
-export 'dart:typed_data' show Uint8List;
 export 'package:flutter/rendering.dart' show
     BackgroundImage,
     BlockDirection,
@@ -1170,10 +1168,10 @@ class DefaultAssetBundle extends InheritedWidget {
   bool updateShouldNotify(DefaultAssetBundle old) => bundle != old.bundle;
 }
 
-class RawImage extends StatelessComponent {
-  RawImage({
+class AsyncImage extends StatelessComponent {
+  AsyncImage({
     Key key,
-    this.bytes,
+    this.provider,
     this.width,
     this.height,
     this.colorFilter,
@@ -1182,7 +1180,7 @@ class RawImage extends StatelessComponent {
     this.centerSlice
   }) : super(key: key);
 
-  final Uint8List bytes;
+  final ImageProvider provider;
   final double width;
   final double height;
   final ColorFilter colorFilter;
@@ -1191,9 +1189,8 @@ class RawImage extends StatelessComponent {
   final Rect centerSlice;
 
   Widget build(BuildContext context) {
-    ImageResource image = new ImageResource(decodeImageFromList(bytes));
     return new ImageListener(
-      image: image,
+      image: imageCache.loadProvider(provider),
       width: width,
       height: height,
       colorFilter: colorFilter,