[Sync] Initial implementation of foreign sessions suggestions provider.

components/ntp_snippets/sessions/tab_delegate_sync_adapter.h

+// Copyright 2016 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_NTP_SNIPPETS_SESSIONS_TAB_DELEGATE_SYNC_ADAPTER_H_
+#define COMPONENTS_NTP_SNIPPETS_SESSIONS_TAB_DELEGATE_SYNC_ADAPTER_H_
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "components/ntp_snippets/sessions/foreign_sessions_suggestions_provider.h"
+#include "components/sync/driver/sync_service_observer.h"
+
+namespace sync_driver {
+class OpenTabsUIDelegate;
+class SyncService;
+}  // namespace sync_driver
+
+namespace ntp_snippets {
+
+// Adapter that sits on top of SyncService and provides simplified notifications
+// when foreign tab data may have changed.
+class TabDelegateSyncAdapter
+    : public sync_driver::SyncServiceObserver,
+      public ForeignSessionsSuggestionsProvider::OpenTabsUIDelegateProvider {
+ public:
+  explicit TabDelegateSyncAdapter(sync_driver::SyncService* sync_service);
+  ~TabDelegateSyncAdapter() override;
+
+  // OpenTabsUIDelegateProvider implementation.
+  sync_driver::OpenTabsUIDelegate* GetOpenTabsUIDelegate() override;
+  void SubscribeForForeignTabChange(
+      const OnChangeWithDelegate& change_callback) override;
+
+  // sync_driver::SyncServiceObserver implementation.

[Marc Treib, 2016/09/16 12:26:49]

These can (and IMO should) be private.

[skym, 2016/09/16 18:18:50]

Can you explain why these should be private? Like what's the advantage? If
someone knows this class is a TabDelegateSyncAdapter, then they also know that
we're a sync_driver::SyncServiceObserver, which means they know we have these
public (and virtual) methods. And since (afaik) we're only allowed to use public
inheritance within the code base, I don't see how making these private does
anything.

[tschumann, 2016/09/17 15:52:56]

I guess you could argue that we use the adapter to deal with (and hide away) the
details of the sync service.  So any client of the adapter really only wants the
ForeignSessionsProvider semantics. Marking the SyncServiceObserver functions
private makes the interface clearer and easier to use.
I'm not feeling strongly, but would be in favor of having them private, too.

[Marc Treib, 2016/09/19 09:24:08]

Pretty much what Tim says: To me, it's an implementation detail that this class
implements SyncServiceObserver; these methods aren't really part of the public
interface. By marking them private, you make it clear to the casual reader that
they're not meant to call these methods.

[skym, 2016/09/19 18:54:47]

Done.

[skym, 2016/09/19 18:54:47]

Done.

+  void OnStateChanged() override;
+  void OnSyncConfigurationCompleted() override;
+  void OnForeignSessionUpdated() override;
+
+ private:
+  void InvokeCallback();
+
+  sync_driver::SyncService* sync_service_;
+  OnChangeWithDelegate change_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(TabDelegateSyncAdapter);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_SESSIONS_TAB_DELEGATE_SYNC_ADAPTER_H_