Connection::cacheState property, load/saveState() tweaks and fixes

The property controls whether or not the rooms state is cached: if it's off, loadState() and saveState() become no-ops. Other changes:
* loadState/saveState properly deal with rooms in Invite state (this is not quite relevant to the current branch but very much is in the light of a concurrent kitsune-invite-kick PR);
* Profile loadState/saveState (because dumping and especially parsing JSON takes time);
* Use QJsonDocument::Compact layout, it's about 3 times smaller and quicker to parse than Indented, and we really don't care about the cache being human-friendly;
* Have a default path for the state cache, based on the connection's user id.

1 files changed, 36 insertions, 4 deletions

diff --git a/connection.h b/connection.h
index ad161d7c..bf50d7d3 100644
--- a/connection.h
+++ b/connection.h
@@ -39,6 +39,11 @@ namespace QMatrixClient
 
     class Connection: public QObject {
             Q_OBJECT
+
+            /** Whether or not the rooms state should be cached locally
+             * \sa loadState(), saveState()
+             */
+            Q_PROPERTY(bool cacheState READ cacheState WRITE setCacheState NOTIFY cacheStateChanged)
         public:
             explicit Connection(const QUrl& server, QObject* parent = nullptr);
             Connection();
@@ -86,10 +91,35 @@ namespace QMatrixClient
 
             /**
              * Call this before first sync to load from previously saved file.
-             * Uses QUrl to be QML-friendly.
-            */
-            Q_INVOKABLE void loadState(const QUrl &fromFile);
-            Q_INVOKABLE void saveState(const QUrl &toFile);
+             *
+             * \param fromFile A local path to read the state from. Uses QUrl
+             * to be QML-friendly. Empty parameter means using a path
+             * defined by stateCachePath().
+             */
+            Q_INVOKABLE void loadState(const QUrl &fromFile = {});
+            /**
+             * This method saves the current state of rooms (but not messages
+             * in them) to a local cache file, so that it could be loaded by
+             * loadState() on a next run of the client.
+             *
+             * \param toFile A local path to save the state to. Uses QUrl to be
+             * QML-friendly. Empty parameter means using a path defined by
+             * stateCachePath().
+             */
+            Q_INVOKABLE void saveState(const QUrl &toFile = {}) const;
+
+            /**
+             * The default path to store the cached room state, defined as
+             * follows:
+             *     QStandardPaths::writeableLocation(QStandardPaths::CacheLocation) + _safeUserId + "_state.json"
+             * where `_safeUserId` is userId() with `:` (colon) replaced with
+             * `_` (underscore)
+             * /see loadState(), saveState()
+             */
+            Q_INVOKABLE QString stateCachePath() const;
+
+            bool cacheState() const;
+            void setCacheState(bool newValue);
 
             template <typename JobT, typename... JobArgTs>
             JobT* callApi(JobArgTs... jobArgs) const
@@ -120,6 +150,8 @@ namespace QMatrixClient
             void syncError(QString error);
             //void jobError(BaseJob* job);
 
+            void cacheStateChanged();
+
         protected:
             /**
              * @brief Access the underlying ConnectionData class