QQuickTableView: handle RebuildOption::ViewportOnly

When rebuildOptions have ViewportOnly set, we now let the top-left
item be the same as before (and at the same position as before), and
start rebuilding from there. This will greatly increase performance
if e.g the table has been flicked far down to row 1000 when the
rebuild needs to happen (e.g because the model got a new row).

Change-Id: I30beb34a7beccedff8dc406f9a524119a2893eb3
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 32 insertions, 13 deletions

diff --git a/src/quick/items/qquicktableview.cpp b/src/quick/items/qquicktableview.cpp
index e1ec5aff75..a1a5520239 100644
--- a/src/quick/items/qquicktableview.cpp
+++ b/src/quick/items/qquicktableview.cpp
@@ -771,13 +771,11 @@ void QQuickTableViewPrivate::layoutHorizontalEdge(Qt::Edge tableEdge)
 
 void QQuickTableViewPrivate::layoutTopLeftItem()
 {
-    // ###todo: support starting with other top-left items than 0,0
     const QPoint cell = loadRequest.firstCell();
-    Q_TABLEVIEW_ASSERT(cell == QPoint(0, 0), loadRequest.toString());
     auto topLeftItem = loadedTableItem(cell);
     auto item = topLeftItem->item;
 
-    item->setPosition(QPoint(tableMargins.left(), tableMargins.top()));
+    item->setPosition(loadRequest.startPosition());
     item->setSize(QSizeF(resolveColumnWidth(cell.x()), resolveRowHeight(cell.y())));
     topLeftItem->setVisible(true);
     qCDebug(lcTableViewDelegateLifecycle) << "geometry:" << topLeftItem->geometry();
@@ -938,18 +936,35 @@ void QQuickTableViewPrivate::beginRebuildTable()
     if (loadRequest.isActive())
         cancelLoadRequest();
 
-    releaseLoadedItems(QQmlTableInstanceModel::NotReusable);
+    QPoint topLeft;
+    QPointF topLeftPos;
+    calculateTableSize();
+
+    if (rebuildOptions & RebuildOption::All) {
+        releaseLoadedItems(QQmlTableInstanceModel::NotReusable);
+        topLeft = QPoint(0, 0);
+        topLeftPos = QPointF(tableMargins.left(), tableMargins.top());
+        q->setContentX(0);
+        q->setContentY(0);
+    } else if (rebuildOptions & RebuildOption::ViewportOnly) {
+        // Rebuild the table without flicking the content view back to origin, and
+        // start building from the same top left item that is currently showing
+        // (unless it has been removed from the model).
+        releaseLoadedItems(reusableFlag);
+        topLeft = loadedTable.topLeft();
+        topLeftPos = loadedTableOuterRect.topLeft();
+        topLeft.setX(qMin(topLeft.x(), tableSize.width() - 1));
+        topLeft.setY(qMin(topLeft.y(), tableSize.height() - 1));
+    } else {
+        Q_TABLEVIEW_UNREACHABLE(rebuildOptions);
+    }
 
     loadedTable = QRect();
     loadedTableOuterRect = QRect();
     loadedTableInnerRect = QRect();
     contentSizeBenchMarkPoint = QPoint(-1, -1);
 
-    q->setContentX(0);
-    q->setContentY(0);
-
-    calculateTableSize();
-    loadInitialTopLeftItem();
+    loadInitialTopLeftItem(topLeft, topLeftPos);
     loadAndUnloadVisibleEdges();
 }
 
@@ -965,7 +980,7 @@ void QQuickTableViewPrivate::layoutAfterLoadingInitialTable()
     }
 }
 
-void QQuickTableViewPrivate::loadInitialTopLeftItem()
+void QQuickTableViewPrivate::loadInitialTopLeftItem(const QPoint &cell, const QPointF &pos)
 {
     Q_TABLEVIEW_ASSERT(loadedItems.isEmpty(), "");
 
@@ -980,7 +995,7 @@ void QQuickTableViewPrivate::loadInitialTopLeftItem()
 
     // Load top-left item. After loaded, loadItemsInsideRect() will take
     // care of filling out the rest of the table.
-    loadRequest.begin(QPoint(0, 0), QQmlIncubator::AsynchronousIfNested);
+    loadRequest.begin(cell, pos, QQmlIncubator::AsynchronousIfNested);
     processLoadRequest();
 }
 
diff --git a/src/quick/items/qquicktableview_p_p.h b/src/quick/items/qquicktableview_p_p.h
index 81127adae4..1e8c10c285 100644
--- a/src/quick/items/qquicktableview_p_p.h
+++ b/src/quick/items/qquicktableview_p_p.h
@@ -86,7 +86,7 @@ public:
         // done by QQuickTableView.
 
     public:
-        void begin(const QPoint &cell, QQmlIncubator::IncubationMode incubationMode)
+        void begin(const QPoint &cell, const QPointF &pos, QQmlIncubator::IncubationMode incubationMode)
         {
             Q_ASSERT(!active);
             active = true;
@@ -95,6 +95,7 @@ public:
             mode = incubationMode;
             cellCount = 1;
             currentIndex = 0;
+            startPos = pos;
             qCDebug(lcTableViewDelegateLifecycle()) << "begin top-left:" << toString();
         }
 
@@ -125,6 +126,8 @@ public:
         inline Qt::Edge edge() { return tableEdge; }
         inline QQmlIncubator::IncubationMode incubationMode() { return mode; }
 
+        inline QPointF startPosition() { return startPos; }
+
         QString toString()
         {
             QString str;
@@ -154,6 +157,7 @@ public:
         int cellCount = 0;
         bool active = false;
         QQmlIncubator::IncubationMode mode = QQmlIncubator::AsynchronousIfNested;
+        QPointF startPos;
 
         QPoint cellAt(int index)
         {
@@ -304,7 +308,7 @@ public:
     void unloadItem(const QPoint &cell);
     void unloadItems(const QLine &items);
 
-    void loadInitialTopLeftItem();
+    void loadInitialTopLeftItem(const QPoint &cell, const QPointF &pos);
     void loadEdge(Qt::Edge edge, QQmlIncubator::IncubationMode incubationMode);
     void unloadEdge(Qt::Edge edge);
     void loadAndUnloadVisibleEdges();