[CrOS Tether] Create HostScanCache, which caches scan results and inserts them into the network sta…

chromeos/components/tether/host_scan_cache.h

Index: chromeos/components/tether/host_scan_cache.h
diff --git a/chromeos/components/tether/host_scan_cache.h b/chromeos/components/tether/host_scan_cache.h
new file mode 100644
index 0000000000000000000000000000000000000000..23f3e46ba2281c9d030186c1a803921305370a80
--- /dev/null
+++ b/chromeos/components/tether/host_scan_cache.h
@@ -0,0 +1,132 @@
+// 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 CHROMEOS_COMPONENTS_TETHER_HOST_SCAN_CACHE_H_
+#define CHROMEOS_COMPONENTS_TETHER_HOST_SCAN_CACHE_H_
+
+#include <memory>
+#include <unordered_map>
+
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/timer/timer.h"
+#include "chromeos/components/tether/tether_host_response_recorder.h"
+
+namespace chromeos {
+
+class NetworkStateHandler;
+
+namespace tether {
+
+class ActiveHost;
+class DeviceIdTetherNetworkGuidMap;
+class TetherHostResponseRecorder;
+
+// Caches scan results and inserts them into the network stack.
+class HostScanCache : public TetherHostResponseRecorder::Observer {
+ public:
+  // The number of minutes that a cache entry is considered to be valid before
+  // it becomes stale. Once a cache entry is inserted, it will be automatically
+  // removed after this amount of time passes unless it corresponds to the
+  // active host. This value was chosen for two reasons:
+  //     (1) Tether properties such as battery percentage and signal strength
+  //         are ephemeral in nature, so keeping these values cached for more
+  //         than a short period makes it likely that the values will be wrong.
+  //     (2) Tether networks rely on proximity to a tether host device, so it is
+  //         possible that host devices have physically moved away from each
+  //         other. We assume that the devices do not stay in proximity to one
+  //         another until a new scan result is received which proves that they
+  //         are still within the distance needed to communicate.
+  static constexpr int kNumMinutesBeforeCacheEntryExpires = 5;
+
+  HostScanCache(
+      NetworkStateHandler* network_state_handler,
+      ActiveHost* active_host,
+      TetherHostResponseRecorder* tether_host_response_recorder,
+      DeviceIdTetherNetworkGuidMap* device_id_tether_network_guid_map);
+  virtual ~HostScanCache();
+
+  // Updates the cache to include this scan result. If no scan result for
+  // |tether_network_guid| exists in the cache, a scan result will be added;
+  // if a scan result is already present, it is updated with the new data
+  // provided as parameters to this function. Once scan results have been
+  // updated, a timer starts counting down |kNumMinutesBeforeCacheEntryExpires|
+  // minutes. Once the timer fires, the scan result is automatically removed
+  // from the cache unless it corresponds to the active host.
+  // Note: |signal_strength| should be in the range [0, 100]. This is different
+  // from the |connection_strength| field received in ConnectTetheringResponse
+  // and KeepAliveTickleResponse messages (the range is [0, 4] in those cases).
+  virtual void SetHostScanResult(const std::string& tether_network_guid,
+                                 const std::string& device_name,
+                                 const std::string& carrier,
+                                 int battery_percentage,
+                                 int signal_strength);
+
+  // Removes the scan result with GUID |tether_network_guid| from the cache. If
+  // no cache result with that GUID was present in the cache, this function is
+  // a no-op. Returns whether a scan result was actually removed.
+  virtual bool RemoveHostScanResult(const std::string& tether_network_guid);
+
+  // Removes all scan results from the cache unless they correspond to the
+  // active host; the active host must always remain in the cache while
+  // connecting/connected to ensure the UI is up to date.
+  virtual void ClearCacheExceptForActiveHost();
+
+  // TetherHostResponseRecorder::Observer:
+  void OnPreviouslyConnectedHostIdsChanged() override;
+
+ private:
+  friend class HostScanCacheTest;
+
+  class TimerFactory {
+   public:
+    // Creates a timer which will be used to determine if the cache contains
+    // stale data for the Tether network with GUID |tether_network_guid|.

[stevenjb, 2017/05/01 18:26:11]

Why does the timer vary based on |tether_network_guid|? I'm confused.

[Kyle Horimoto, 2017/05/01 21:14:43]

Removed this.

+    virtual std::unique_ptr<base::Timer> CreateOneShotTimer(
+        const std::string& tether_network_guid) = 0;
+  };
+
+  class TimerFactoryImpl : public TimerFactory {
+   public:
+    TimerFactoryImpl();
+    ~TimerFactoryImpl();
+
+    // TimerFactory:
+    std::unique_ptr<base::Timer> CreateOneShotTimer(
+        const std::string& tether_network_guid) override;
+  };

[stevenjb, 2017/05/01 18:26:11]

Does this need to be a class member?

[Kyle Horimoto, 2017/05/01 21:14:43]

Yes; otherwise, we would not be able to inject MockTimers in the test.

[stevenjb, 2017/05/01 22:13:01]

TimerFactory needs to be public, but TimerFactoryImpl shouldn't need to be, nor
should it need to be part of the class. i.e. in the C++ file:

namespace {

TimerFactoryImpl : public HostScanCache::TimerFactory {
  ...
};

}  // namespace

[Kyle Horimoto, 2017/05/02 00:02:18]

Done.

+
+  HostScanCache(
+      std::unique_ptr<TimerFactory> timer_factory,
+      NetworkStateHandler* network_state_handler,
+      ActiveHost* active_host,
+      TetherHostResponseRecorder* tether_host_response_recorder,
+      DeviceIdTetherNetworkGuidMap* device_id_tether_network_guid_map);
+
+  bool HasConnectedToHost(const std::string& tether_network_guid);
+  void StartTimer(const std::string& tether_network_guid);
+  void OnTimerFired(const std::string& tether_network_guid);
+
+  std::unique_ptr<TimerFactory> timer_factory_;
+  NetworkStateHandler* network_state_handler_;
+  ActiveHost* active_host_;
+  TetherHostResponseRecorder* tether_host_response_recorder_;
+  DeviceIdTetherNetworkGuidMap* device_id_tether_network_guid_map_;
+
+  // Maps from the Tether network GUID to a Timer object. While a scan result is
+  // active in the cache, the corresponding Timer object starts running; if the
+  // timer fires, the result is removed (unless it corresponds to the active
+  // host).
+  std::unordered_map<std::string, std::unique_ptr<base::Timer>>
+      tether_guid_to_timer_map_;
+  base::WeakPtrFactory<HostScanCache> weak_ptr_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(HostScanCache);
+};
+
+}  // namespace tether
+
+}  // namespace chromeos
+
+#endif  // CHROMEOS_COMPONENTS_TETHER_HOST_SCAN_CACHE_H_