[Sync] Creating sync_sessions component, moving revisit logic.

components/sync_driver/revisit/sessions_page_revisit_observer.cc

-// 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.
-
-#include "components/sync_driver/revisit/sessions_page_revisit_observer.h"
-
-#include "base/metrics/histogram_macros.h"
-#include "base/thread_task_runner_handle.h"
-#include "components/sessions/core/session_types.h"
-#include "components/sync_driver/glue/synced_session.h"
-#include "components/sync_driver/revisit/current_tab_matcher.h"
-#include "components/sync_driver/revisit/offset_tab_matcher.h"
-#include "components/sync_driver/revisit/page_equality.h"
-
-namespace sync_driver {
-
-SessionsPageRevisitObserver::SessionsPageRevisitObserver(
-    scoped_ptr<ForeignSessionsProvider> provider)
-    : provider_(provider.Pass()) {}
-
-SessionsPageRevisitObserver::~SessionsPageRevisitObserver() {}
-
-void SessionsPageRevisitObserver::OnPageVisit(const GURL& url,
-                                              const TransitionType transition) {
-  // We need to be invoked and eventually execute on the thread which owns the
-  // session objects the provider will give us. However, this work is not
-  // especially time sensitive, and so we post a task to perform this to get out
-  // of the way of any currently executing logic.
-
-  // Bind this task to this->AsWeakPtr() so that if we're destructed this task
-  // just vanish instead of breaking things.
-  base::ThreadTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE, base::Bind(&SessionsPageRevisitObserver::CheckForRevisit,
-                            this->AsWeakPtr(), url, transition));
-}
-
-void SessionsPageRevisitObserver::CheckForRevisit(
-    const GURL& url,
-    const TransitionType transition) {
-  base::TimeTicks start(base::TimeTicks::Now());
-
-  // We want to match tabs/navigation entries in two slightly different ways. We
-  // value the current url/navigation entry of a tab more highly, and want to
-  // actually seperate metrics from the backwards/forwards entries. And then
-  // to make things a little bit messier, we only have an accurate modified time
-  // for the tabs/current entries. So use index offset for forward/back entries.
-  PageEquality page_equality(url);
-  CurrentTabMatcher current_matcher(page_equality);
-  OffsetTabMatcher offset_matcher(page_equality);
-
-  std::vector<const sync_driver::SyncedSession*> foreign_sessions;
-  if (provider_->GetAllForeignSessions(&foreign_sessions)) {
-    for (const sync_driver::SyncedSession* session : foreign_sessions) {
-      for (const std::pair<const SessionID::id_type, sessions::SessionWindow*>&
-               key_value : session->windows) {
-        for (const sessions::SessionTab* tab : key_value.second->tabs) {
-          // These matchers look identical and could easily implement an
-          // interface and we could iterate through a vector of matchers here.
-          // However this would cause quite a bit of overhead at the inner most
-          // loop of something that takes linear time in relation to the number
-          // of open foreign tabs. A small fraction of users have thousands of
-          // open tabs.
-          current_matcher.Check(tab);
-          offset_matcher.Check(tab);
-        }
-      }
-    }
-  }
-
-  // emit even if there are no foreign sessions so that that counts all match.
-  current_matcher.Emit(transition);
-  offset_matcher.Emit(transition);
-
-  base::TimeDelta duration(base::TimeTicks::Now() - start);
-  UMA_HISTOGRAM_TIMES("Sync.PageRevisitSessionDuration", duration);
-}
-
-}  // namespace sync_driver