Implement a simple image cache for Sky in Dart (no eviction)

sky/sdk/lib/framework/net/image_cache.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:sky' as sky;
+import 'dart:collection';
+
+class ImageCache {

[abarth-chromium, 2015/06/02 00:52:47]

There's no need for this class.  You can just put the state in the global scope
of this library and use _ prefixes to hide them.

Client then use the library as follows:

import "package:sky/framework/net/image_cache.dart" as image_cache;

image_cache.load(...)

[jackson, 2015/06/02 01:18:25]

Acknowledged.

+  static final ImageCache _sharedCache = new ImageCache();
+  static ImageCache shared() { return _sharedCache; }
+
+  final HashMap pendingRequests = new HashMap<String, List<sky.ImageLoaderCallback>>();
+  final HashMap completedRequests = new HashMap<String, sky.Image>();

[abarth-chromium, 2015/06/02 00:52:47]

It's more important to put the types on the declaration than on the constructor:

final HashMap<String, sky.Image> completedRequests = new HashMap();

However, I think we should put the types everywhere given that this is framework
code that will be read 10x more often than it is written.

[jackson, 2015/06/02 01:18:25]

Acknowledged.

+
+  void load(String url, sky.ImageLoaderCallback callback) {
+    if (completedRequests.containsKey(url)) {
+      callback(completedRequests[url]);
+    } else if (pendingRequests.containsKey(url)) {

[abarth-chromium, 2015/06/02 00:52:47]

Using putIfAbsent [1] is the idiomatic (and faster) way to implement this
pattern.


[1]
https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:core.Map...

[jackson, 2015/06/02 01:18:25]

Acknowledged.

+      pendingRequests[url].add(callback);
+    } else {
+      pendingRequests[url] = new List();
+      pendingRequests[url].add(callback);
+      new sky.ImageLoader(url, (image) {
+        completedRequests[url] = image;
+        pendingRequests[url].forEach((c) => c(image));
+        pendingRequests[url].remove(url);
+      }).load();
+    }
+  }
+}