refs #581 Store a TTL value in the data cache revision file, skip loading a cached value if its timestamp is too old.

src/blackmisc/datacache.cpp

@@ -99,6 +99,11 @@ namespace BlackMisc
         }
     }
 
+    void CDataCache::setTimeToLive(const QString &key, int ttl)
+    {
+        QTimer::singleShot(0, &m_serializer, [this, key, ttl] { m_revision.setTimeToLive(key, ttl); });
+    }
+
     QString lockFileError(const QLockFile &lock)
     {
         switch (lock.error())
@@ -250,10 +255,13 @@ namespace BlackMisc
                 }
                 m_uuid = uuid;
 
+                auto timesToLive = fromJson(json.value("ttl").toObject());
                 auto newTimestamps = fromJson(json.value("timestamps").toObject());
                 for (auto it = newTimestamps.cbegin(); it != newTimestamps.cend(); ++it)
                 {
-                    if (timestamps.value(it.key(), 0) < it.value())
+                    auto current = timestamps.value(it.key(), 0);
+                    auto ttl = timesToLive.value(it.key(), -1);
+                    if (current < it.value() && (ttl < 0 || QDateTime::currentMSecsSinceEpoch() < it.value() + ttl))
                     {
                         m_timestamps.insert(it.key(), it.value());
                     }
@@ -302,6 +310,7 @@ namespace BlackMisc
         QJsonObject json;
         json.insert("uuid", m_uuid.toString());
         json.insert("timestamps", toJson(timestamps));
+        json.insert("ttl", toJson(m_timesToLive));
         revisionFile.write(QJsonDocument(json).toJson());
     }
 
@@ -368,6 +377,13 @@ namespace BlackMisc
         return std::move(m_promises); // move into the return value, so m_promises becomes empty
     }
 
+    void CDataCacheRevision::setTimeToLive(const QString &key, int ttl)
+    {
+        Q_ASSERT(! m_updateInProgress);
+
+        m_timesToLive.insert(key, ttl);
+    }
+
     QJsonObject CDataCacheRevision::toJson(const QMap<QString, qint64> &timestamps)
     {
         QJsonObject result;

src/blackmisc/datacache.h

@@ -72,6 +72,9 @@ namespace BlackMisc
         //! Returns (by move) the container of promises to load values.
         std::vector<std::tuple<QObject *, QString, std::promise<CVariant>>> loadedValuePromises();
 
+        //! Set TTL value that will be written to the revision file.
+        void setTimeToLive(const QString &key, int ttl);
+
     private:
         mutable QMutex m_mutex { QMutex::Recursive };
         bool m_updateInProgress = false;
@@ -81,6 +84,7 @@ namespace BlackMisc
         QLockFile m_lockFile { m_basename + "/.lock" };
         QUuid m_uuid;
         QMap<QString, qint64> m_timestamps;
+        QMap<QString, qint64> m_timesToLive;
         std::vector<std::tuple<QObject *, QString, std::promise<CVariant>>> m_promises;
 
         static QJsonObject toJson(const QMap<QString, qint64> &timestamps);
@@ -149,6 +153,9 @@ namespace BlackMisc
         //! Method used for implementing CData::syncLoad.
         std::future<CVariant> syncLoad(QObject *pageOwner, const QString &key);
 
+        //! Method used for implementing TTL.
+        void setTimeToLive(const QString &key, int ttl);
+
     private:
         CDataCache();
 
@@ -183,7 +190,9 @@ namespace BlackMisc
         CData(T *owner, NotifySlot<T> slot = nullptr) :
             CData::CCached(CDataCache::instance(), Trait::key(), Trait::isValid, Trait::defaultValue(), owner, slot),
             m_owner(owner)
-        {}
+        {
+            if (Trait::timeToLive() >= 0) { CDataCache::instance()->setTimeToLive(Trait::key(), Trait::timeToLive()); }
+        }
 
         //! Reset the data to its default value.
         void setDefault() { this->set(Trait::defaultValue()); }
@@ -191,6 +200,9 @@ namespace BlackMisc
         //! Return the file that is used for persistence for this value.
         QString getFilename() const { return CDataCache::filenameForKey(this->getKey()); }
 
+        //! True if the current timestamp is older than the TTL (time to live).
+        bool isStale() const { return this->getTimestamp() + Trait::timeToLive() > QDateTime::currentMSecsSinceEpoch(); }
+
         //! Return a future providing the value. If the value is still loading, the future will wait for it.
         //! If the value is not present, the variant is null. Bypasses async get and inhibits notification slot.
         std::future<CVariant> syncLoad() { return CDataCache::instance()->syncLoad(m_owner, this->getKey()); }
@@ -222,6 +234,10 @@ namespace BlackMisc
         //! Default implementation returns a default-constructed value.
         static const T &defaultValue() { static const T def {}; return def; }
 
+        //! Number of milliseconds after which cached value becomes stale.
+        //! Default is -1 which means value never becomes stale.
+        static int timeToLive() { return -1; }
+
         //! Deleted default constructor.
         CDataTrait() = delete;