[Offline Pages] Set up the initial prefetching service.

components/offline_pages/core/prefetch/prefetch_service.h

+// Copyright 2017 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 COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_H_
+
+#include "components/keyed_service/core/keyed_service.h"
+#include "components/offline_pages/core/offline_page_item.h"
+
+class GURL;
+
+namespace offline_pages {
+
+// Main class for the Offline Pages Prefetching feature, which will ask a server
+// to package content from interesting URLs and send it to Chrome for
+// consumption while offline.
+class PrefetchService : public KeyedService {
+ public:
+  struct PrefetchURL {
+    // Used to differentiate URLs from different sources.  |name_space| should
+    // be unique per source. |id| can be anything useful to identify the page,
+    // but will not be used for deduplication.
+    ClientId client_id;
+
+    // This URL will be prefetched by the service.
+    GURL url;
+  };
+
+  ~PrefetchService() override = default;
+
+  // Called when a consumer has new URLs for the system to prefetch.

[carlosk, 2017/04/18 00:36:09]

nit: s/has new/has potentially new/ ?
I'm not really sure how to phrase it but I think it should be made clear here
that PrefetchService must be able to accept repeated URL suggestions.

[dewittj, 2017/04/18 18:20:25]

Done.

+  virtual void OnNewURLsToPrefetch(

[jianli, 2017/04/17 21:52:58]

Can we rename this to something like PrefetchURLs since OnNewURLsToPrefetch
sounds a bit permissive to understand?

[dewittj, 2017/04/17 22:22:18]

Done.

+      const std::vector<PrefetchURL>& prefetch_urls) = 0;
+
+  // Called when all existing suggestions are no longer considered valid for a
+  // given namespace.  The prefetch service should remove any URLs that
+  // have not yet started downloading within that namespace.

[carlosk, 2017/04/18 00:36:09]

IRT "have not yet started downloading": why not delete ongoing downloads too? Or
even why not delete all prefetched pages? Has the scope of content to be deleted
upon this event been discussed?

[dewittj, 2017/04/18 18:20:25]

I recall we want to treat pages that have started downloading as somewhat more
permanent, since we have spent potentially significant user data/power on it
already.  There is some discussion on the design doc. I prefer to land this then
modify the comment if we decide on something different.

[carlosk, 2017/04/18 20:17:22]

Acknowledged.

+  virtual void RemoveAllUnprocessedPrefetchURLs(
+      const std::string& name_space) = 0;
+
+  // Called to invalidate a single PrefetchURL entry identified by |client_id|.
+  // If multiple have the same |client_id|, they will all be removed.
+  virtual void RemovePrefetchURLsByClientId(const ClientId& client_id) = 0;
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_SERVICE_IMPL_H_