[Android] Use UIResource for overscroll glow

ui/base/android/system_ui_resource_manager.h

+// Copyright 2014 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.
+
+#ifndef UI_BASE_ANDROID_SYSTEM_UI_RESOURCE_MANAGER_H_
+#define UI_BASE_ANDROID_SYSTEM_UI_RESOURCE_MANAGER_H_
+
+#include "cc/resources/ui_resource_client.h"
+#include "ui/base/ui_base_export.h"
+
+namespace ui {
+
+// Interface for loading and accessing shared system UI resources.
+class UI_BASE_EXPORT SystemUIResourceManager {
+ public:
+  enum ResourceType { OVERSCROLL_EDGE, OVERSCROLL_GLOW };
+
+  virtual ~SystemUIResourceManager() {}
+
+  // Trigger loading of the bitmap for the given |resource|, if necessary.
+  // Note that this operation may be asynchronous, and subsequent queries to
+  // |GetUIResourceId()| will yield a valid result only after loading finishes.
+  // This method is particularly useful for idly loading a resource before an
+  // explicit cc::UIResourceId is required.
+  virtual void EnsureResource(ResourceType resource) = 0;

[powei, 2014/07/31 17:24:28]

So it looks like overscroll is not using this right now.  I'm guessing you'll
use this for the other resources?  Is it to avoid janking on the first frame?

[jdduke (slow), 2014/07/31 17:40:56]

Yeah, it's unused, and I thought of just ripping it out.  The first time we
overscroll we'll just got a resource id of 0, so the effect itself will appear
to jank until we start getting non-zero id's, but we won't block the main
thread.  I tried it a few times on my Galaxy Nexus and didn't really notice any
jank (it seemed to behave better than the old approach of blocking resource
loading on the main thread).

Maybe I'll add calls to this when the OverscrollGlow is created, and probably
rename it to |PreloadResource|, what do you think?

+
+  // Return the resource id associated with |resource|. If loading hasn't yet
+  // begun for the given |resource|, it will be triggered immediately. If
+  // loading is asynchronous, 0 will be returned until loading has finished, and
+  // the caller is responsible for re-querying until a valid id is returned.
+  virtual cc::UIResourceId GetUIResourceId(ResourceType resource) = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_BASE_ANDROID_SYSTEM_UI_RESOURCE_MANAGER_H_