[traffic] Replace full invalidation with subscription recalculation

Signed-off-by: mvglasow <michael -at- vonglasow.com>

map/traffic_manager.cpp

@@ -166,8 +166,7 @@ void TrafficManager::SetEnabled(bool enabled)
     if (notifyUpdate)
       OnTrafficDataUpdate();
     else
-      // TODO After sorting out invalidation, figure out if we need that here.
-      Invalidate();
+      RecalculateSubscription();
     m_canSetMode = false;
   }
   else
@@ -254,36 +253,11 @@ void TrafficManager::OnRecoverSurface()
   Resume();
 }
 
-/*
- * TODO Revisit invalidation logic.
- * We currently invalidate when enabling and resuming, and when a new MWM file is downloaded
- * (behavior inherited from MapsWithMe).
- * Traffic data in MapsWithMe was a set of pre-decoded messages per MWM; the whole set would get
- * re-fetched periodically. Invalidation meant discarding and re-fetching all traffic data.
- * This logic is different for TraFF:
- * - Messages expire individually or get replaced by updates, thus there is hardly ever a reason
- *   to discard messages.
- * - Messages are decoded into segments in the app. Discarding decoded segments may be needed on
- *   a per-message basis for the following reasons:
- *   - the message is replaced by a new one and the location or traffic situation has changed
- *     (this is dealt with as part of the message update process)
- *   - one of the underlying MWMs has been updated to a new version
- *   - a new MWM has been added, and a message location that previously could not be decoded
- *     completely now can
- * The sensible equivalent in TraFF would be to discard and re-generate decoded locations, and
- * possibly poll for updates. Discarding and re-generating decoded locations could be done
- * selectively:
- * - compare map versions of decoded segments to current map version
- * - figure out when a new map has been added, and which segments are affected by it
- */
-void TrafficManager::Invalidate()
+void TrafficManager::RecalculateSubscription()
 {
   if (!IsEnabled())
     return;
 
-  m_lastDrapeMwmsByRect.clear();
-  m_lastRoutingMwmsByRect.clear();
-
   if (m_currentModelView.second)
     UpdateViewport(m_currentModelView.first);
   if (m_currentPosition.second)
@@ -1272,7 +1246,7 @@ void TrafficManager::Resume()
     return;
 
   m_isPaused = false;
-  Invalidate();
+  RecalculateSubscription();
 }
 
 void TrafficManager::SetSimplifiedColorScheme(bool simplified)

map/traffic_manager.hpp

@@ -150,9 +150,6 @@ public:
    * that will not get picked up. We need to extend `TrafficManager` to react to MWMs being added
    * (and removed) – note that this affects the `DataSource`, not the set of active MWMs.
    *
-   * @todo Enabling the traffic manager will invalidate its data, disabling it will notify the
-   * observer that traffic data has been cleared. This is old logic, to be reviewed/removed.
-   *
    * @todo State/pause/resume logic is not fully implemented ATM and needs to be revisited.
    *
    * @param enabled True to enable, false to disable
@@ -176,25 +173,30 @@ public:
   void UpdateMyPosition(MyPosition const & myPosition);
 
   /**
-   * @brief Invalidates traffic information.
+   * @brief Recalculates the TraFF subscription area.
    *
-   * Invalidating causes traffic data to be re-requested.
+   * The subscription area needs to be recalculated when the traffic manager goes from disabled to
+   * enabled, or when it is resumed after being paused, as the subscription area is not updated
+   * while the traffic manager is disabled or paused.
    *
-   * This happens when a new MWM file is downloaded, the traffic manager is enabled after being
-   * disabled or resumed after being paused.
+   * If the subscription are has changed, this triggers a change of the active TraFF subscription.
    *
-   * @todo this goes for the old MWM arch. For TraFF we need to refresh the MWM set for the decoder
-   * and possibly decode locations again (MWMs might have changed, or new ones added).
+   * No traffic data is discarded, but sources will be polled for an update, which may turn out
+   * larger than usual if the traffic manager was in disabled/paused state for an extended period of
+   * time or the subscription area has changed.
+   *
+   * @todo Routing is currently not considered (active MWMs are based on viewport and current
+   * position).
    */
-  void Invalidate();
+  void RecalculateSubscription();
 
   /**
-   * @brief Invalidates traffic information.
+   * @brief Invalidates traffic information for the specified MWM.
    *
-   * Invalidation happens when a new MWM file is downloaded (it may or may not replace an older
-   * version), and pertains to that MWM. Locations which refer to any version of this MWM, or whose
-   * enclosing rectangle overlaps with that of the MWM, are discarded and recreated to ensure the
-   * new MWM is considered.
+   * Invalidation of traffic data is always per MWM and affects locations which refer to any version
+   * of this MWM, or whose enclosing rectangle overlaps with that of the MWM. The decoded segments
+   * for these locations are discarded and decoded again, ensuring they are based on the new MWM.
+   * The TraFF messages themselves remain unchanged.
    *
    * @param mwmId The newly addded MWM.
    */
@@ -641,7 +643,6 @@ private:
    * Methods which use both, but in a different way:
    *
    * * ClearCache(), removes the requested MWM from the set but clears the vector completely.
-   * * Invalidate(), clears the vector but not the set.
    * * UpdateActiveMwms(), uses the vector to detect changes. If so, it updates both vector and set.
    *
    * Clear() clears both the set and the vector. (Clearing the set is currently disabled as it breaks ForEachActiveMwm.)