Make sure events get loaded if the timeline is empty

After fixing the race in the room switching procedure (see 4ce59f7)
another race showed up, this time between the model reset and updating
root.room property in QML. Because this property is distinct from
messageModel.room, it gets updated later, specifically after
onModelReset() is called. onModelReset() itself already uses
messageModel.room instead of root.room exactly for that reason but
ensurePreviousContent() called from there still operates on root.room.
That alone wouldn't be much of a hurdle (ensurePreviousContent() could
be rewired to messageModel.room just as well) but at the moment
onModelReset() is called chatView has not created any delegates for
the model yet - meaning that contentY and originY used in
ensurePreviousContent() are 0 even when the model has plenty of events
loaded, leading to a false-positive condition and an unneeded request
to the homeserver for more messages.

This commit moves requesting the initial batch of historical events
to the model. It does not request a lot, therefore the turnaround is
short; and that solves the timeline bootstrapping after a room switch.
Eventually this might even move to libQuotient, because events have
to be loaded to the room when it is displayed, regardless of
the client - but that's something to ponder separately.

And while we're at it, the property tracking the number of requested
historical events has been moved to QuaternionRoom, anticipating its
further move to libQuotient.

client/models/messageeventmodel.cpp

@@ -181,6 +181,9 @@ void MessageEventModel::changeRoom(QuaternionRoom* room)
         qCDebug(EVENTMODEL)
             << "Event model connected to room" << room->objectName() //
             << "as" << room->localUser()->id();
+        // If the timeline isn't loaded, ask for at least something right away
+        if (room->timelineSize() == 0)
+            room->getHistory(30);
     }
     endResetModel();
     emit readMarkerUpdated();

client/qml/Timeline.qml

@@ -256,10 +256,6 @@ Page {
             contentHeight > 0 && count > 0 ? count / contentHeight : 0.03
             // 0.03 is just an arbitrary reasonable number
 
-        property int lastRequestedEvents: 0
-        readonly property int currentRequestedEvents:
-            room && room.eventsHistoryJob ? lastRequestedEvents : 0
-
         property var textEditWithSelection
         property real readMarkerContentPos: originY
         readonly property real readMarkerViewportPos:
@@ -287,10 +283,8 @@ Page {
             // Check if we're about to bump into the ceiling in
             // 2 seconds and if yes, request the amount of messages
             // enough to scroll at this rate for 3 more seconds
-            if (velocity > 0 && contentY - velocity*2 < originY) {
-                lastRequestedEvents = velocity * eventDensity * 3
-                room.getPreviousContent(lastRequestedEvents)
-            }
+            if (velocity > 0 && contentY - velocity*2 < originY)
+                room.getHistory(velocity * eventDensity * 3)
         }
         onContentYChanged: ensurePreviousContent()
         onContentHeightChanged: ensurePreviousContent()
@@ -346,11 +340,9 @@ Page {
                 chatView.saveViewport(true)
             }
             function onModelReset() {
-                if (messageModel.room) {
-                    // Load events if there are not enough of them
-                    chatView.ensurePreviousContent()
-                    chatView.positionViewAtBeginning()
-                }
+                // NB: at this point, the actual delegates are not instantiated
+                //     yet, so defer all actions to when at least some are
+                scrollFinisher.scrollViewTo(0, ListView.Contain)
             }
         }
 
@@ -479,7 +471,7 @@ Page {
 
         // A proxy property for animation
         property int requestedHistoryEventsCount:
-            chatView.currentRequestedEvents
+            room ? room.requestedEventsCount : 0
         AnimationBehavior on requestedHistoryEventsCount {
             NormalNumberAnimation { }
         }
@@ -619,8 +611,8 @@ Page {
         color: palette.alternateBase
         property bool shown:
             (chatView.bottommostVisibleIndex >= 0
-                  && (scrollerArea.containsMouse || scrollAnimation.running))
-                 || chatView.currentRequestedEvents > 0
+                && (scrollerArea.containsMouse || scrollAnimation.running))
+            || (room && room.requestedEventsCount > 0)
 
         onShownChanged: {
             if (shown) {
@@ -648,10 +640,10 @@ Page {
                             chatView.bottommostVisibleIndex))
                        + "\n" + qsTr("%Ln events cached", "", chatView.count)
                    : "")
-                  + (chatView.currentRequestedEvents > 0
+                  + (room && room.requestedEventsCount > 0
                      ? (chatView.count > 0 ? "\n" : "")
                        + qsTr("%Ln events requested from the server",
-                              "", chatView.currentRequestedEvents)
+                              "", room.requestedEventsCount)
                      : "")
             horizontalAlignment: Label.AlignRight
         }
@@ -720,7 +712,7 @@ Page {
                 scrollFinisher.scrollViewTo(messageModel.readMarkerVisualIndex,
                                             ListView.Center)
             else
-                room.getPreviousContent(chatView.count / 2) // FIXME, #799
+                room.getHistory(chatView.count / 2) // FIXME, #799
         }
     }
 }

client/quaternionroom.cpp

@@ -22,6 +22,8 @@ QuaternionRoom::QuaternionRoom(Connection* connection, QString roomId,
 {
     connect(this, &Room::namesChanged,
             this, &QuaternionRoom::htmlSafeDisplayNameChanged);
+    connect(this, &Room::eventsHistoryJobChanged,
+            this, &QuaternionRoom::requestedEventsCountChanged);
 }
 
 const QString& QuaternionRoom::cachedUserFilter() const
@@ -80,6 +82,17 @@ QString QuaternionRoom::htmlSafeDisplayName() const
     return displayName().toHtmlEscaped();
 }
 
+int QuaternionRoom::requestedEventsCount() const
+{
+    return eventsHistoryJob() != nullptr ? m_requestedEventsCount : 0;
+}
+
+void QuaternionRoom::getHistory(int limit)
+{
+    m_requestedEventsCount = limit;
+    getPreviousContent(m_requestedEventsCount);
+}
+
 void QuaternionRoom::onAddNewTimelineEvents(timeline_iter_t from)
 {
     std::for_each(from, messageEvents().cend(),

client/quaternionroom.h

@@ -14,6 +14,7 @@ class QuaternionRoom: public Quotient::Room
 {
         Q_OBJECT
         Q_PROPERTY(QString htmlSafeDisplayName READ htmlSafeDisplayName NOTIFY htmlSafeDisplayNameChanged)
+        Q_PROPERTY(int requestedEventsCount READ requestedEventsCount NOTIFY requestedEventsCountChanged)
     public:
         QuaternionRoom(Quotient::Connection* connection,
                        QString roomId, Quotient::JoinState joinState);
@@ -29,16 +30,24 @@ class QuaternionRoom: public Quotient::Room
                                       bool force = false);
 
         QString htmlSafeDisplayName() const;
+        int requestedEventsCount() const;
+
+    public slots:
+        // TODO, 0.0.96: move logic to libQuotient 0.9 and get rid of it here
+        void getHistory(int limit);
 
     signals:
         // Gotta wrap the Room::namesChanged signal because it has parameters
         // and moc cannot use signals with parameters defined in the parent
         // class as NOTIFY targets
         void htmlSafeDisplayNameChanged();
+        // TODO, 0.0.96: same as for getHistory()
+        void requestedEventsCountChanged();
 
     private:
         QSet<const Quotient::RoomEvent*> highlights;
         QString m_cachedUserFilter;
+        int m_requestedEventsCount = 0;
 
         void onAddNewTimelineEvents(timeline_iter_t from) override;
         void onAddHistoricalTimelineEvents(rev_iter_t from) override;