[NTP Snippets] Refactor background scheduling for remote suggestions

components/ntp_snippets/remote/persistent_scheduler.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_REMOTE_PERSISTENT_SCHEDULER_H_
+#define COMPONENTS_NTP_SNIPPETS_REMOTE_PERSISTENT_SCHEDULER_H_
+
+#include "base/macros.h"
+#include "base/time/time.h"
+
+namespace ntp_snippets {
+
+// Interface to schedule persistent periodic fetches for remote suggestions, OS-
+// dependent. These persistent fetches must get triggered according to their
+// schedule independent of whether Chrome is running at that moment.
+//
+// Once per period, the concrete implementation should call
+// RemoteSuggestionsScheduler::OnFetchDue() where the scheduler object is
+// obtained from ContentSuggestionsService.
+//
+// The implementation may also call
+// RemoteSuggestionsScheduler::RescheduleFetching() when its own current
+// schedule got corrupted for whatever reason and needs to be applied again
+// (in turn, this will result in calling Schedule() on the implementation).
+class PersistentScheduler {
+ public:
+  // Schedule periodic fetching of remote suggestions, with different periods
+  // depending on network state. Any of the periods can be zero to indicate that
+  // the corresponding task should not be scheduled.
+  virtual bool Schedule(base::TimeDelta period_wifi,
+                        base::TimeDelta period_fallback) = 0;
+
+  // Cancel any scheduled tasks. Equivalent to Schedule(0, 0).

[Marc Treib, 2016/12/19 12:59:23]

Hm, we could just get rid of Unschedule. WDYT?

[jkrcal, 2016/12/20 16:39:46]

We could. I prefer them being split, though. It makes the application clearer,
IMO.
It also makes writing expectations in unit-tests way clearer.

If you feel strongly, I am willing to remove it.

+  virtual bool Unschedule() = 0;
+
+ protected:
+  PersistentScheduler() = default;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(PersistentScheduler);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_REMOTE_PERSISTENT_SCHEDULER_H_