QQuickTableView: preload one extra row and column at start-up

When a TableView initially loads as many rows and columns it can fit
inside the viewport, it will always be one less than the number it
will show while flicking. The reason is that, as soon as you flick
half a column out on the left, half a column will move in on the right.
And this will increase the number of visible columns by 1 (but without
reusing any items from the pool, since the first column is not out).

Since this is always the case, it makes sense to preload one extra
row and column at start-up, so that they're ready when the flicking
starts.

Note that this doesn't load more items in the background than
what we need (like the cache buffer would). The viewport will fit
_all_ the loaded items into the viewport once you start flicking.
But the extra items loaded at start-up will instead be moved direcly
to the pool for reuse, and the application will be informed about it
(using the onPooled signal).

Change-Id: Icea85c1d44f74ab54f1b96325489e8d6d1c0889e
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

1 files changed, 33 insertions, 9 deletions

diff --git a/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp b/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp
index 69bed8be43..09c2850247 100644
--- a/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp
+++ b/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp
@@ -85,6 +85,7 @@ private slots:
     void setAndGetModel();
     void emptyModel_data();
     void emptyModel();
+    void checkPreload();
     void checkZeroSizedDelegate();
     void checkImplicitSizeDelegate();
     void checkColumnWidthWithoutProvider();
@@ -191,6 +192,22 @@ void tst_QQuickTableView::emptyModel()
     QCOMPARE(tableViewPrivate->loadedItems.count(), 0);
 }
 
+void tst_QQuickTableView::checkPreload()
+{
+    // Check that the reuse pool is filled up with one extra row and
+    // column (pluss corner) after rebuilding the table.
+    LOAD_TABLEVIEW("plaintableview.qml");
+
+    auto model = TestModelAsVariant(100, 100);
+    tableView->setModel(model);
+
+    WAIT_UNTIL_POLISHED;
+
+    QSize visibleTableSize = tableViewPrivate->loadedTable.size();
+    int expectedPoolSize = visibleTableSize.height() + visibleTableSize.width() + 1;
+    QCOMPARE(tableViewPrivate->tableModel->poolSize(), expectedPoolSize);
+}
+
 void tst_QQuickTableView::checkZeroSizedDelegate()
 {
     // Check that if we assign a delegate with empty width and height, we
@@ -1127,9 +1144,12 @@ void tst_QQuickTableView::checkRowColumnCount()
 
     WAIT_UNTIL_POLISHED;
 
-    const int tableViewCount = tableViewPrivate->loadedItems.count();
+    // We expect that the number of created items after start-up should match
+    //the size of the visible table, pluss one extra preloaded row and column.
+    const QSize visibleTableSize = tableViewPrivate->loadedTable.size();
     const int qmlCountAfterInit = view->rootObject()->property(maxDelegateCountProp).toInt();
-    QCOMPARE(tableViewCount, qmlCountAfterInit);
+    const int expectedCount = (visibleTableSize.width() + 1) * (visibleTableSize.height() + 1);
+    QCOMPARE(qmlCountAfterInit, expectedCount);
 
     // This test will keep track of the maximum number of delegate items TableView
     // had to show at any point while flicking (in countingtableview.qml). Because
@@ -1304,6 +1324,7 @@ void tst_QQuickTableView::checkIfDelegatesAreReused()
     LOAD_TABLEVIEW("countingtableview.qml");
 
     const qreal delegateWidth = 100;
+    const qreal delegateHeight = 50;
     const int pageFlickCount = 4;
 
     auto model = TestModelAsVariant(100, 100);
@@ -1312,13 +1333,20 @@ void tst_QQuickTableView::checkIfDelegatesAreReused()
 
     WAIT_UNTIL_POLISHED;
 
-    // Flick half an item to the left, to force one extra column to load before we start.
+    // Flick half an item to the side, to force one extra row and column to load before we start.
     // This will make things less complicated below, when checking how many times the items
     // have been reused (all items will then report the same number).
     tableView->setContentX(delegateWidth / 2);
-    tableView->polish();
+    tableView->setContentY(delegateHeight / 2);
+    QCOMPARE(tableViewPrivate->tableModel->poolSize(), 0);
 
-    WAIT_UNTIL_POLISHED;
+    // Some items have already been pooled and reused after we moved the content view, because
+    // we preload one extra row and column at start-up. So reset the count-properties back to 0
+    // before we continue.
+    for (auto fxItem : tableViewPrivate->loadedItems) {
+        fxItem->item->setProperty("pooledCount", 0);
+        fxItem->item->setProperty("reusedCount", 0);
+    }
 
     const int visibleColumnCount = tableViewPrivate->loadedTable.width();
     const int visibleRowCount = tableViewPrivate->loadedTable.height();
@@ -1327,10 +1355,6 @@ void tst_QQuickTableView::checkIfDelegatesAreReused()
     for (int column = 1; column <= (visibleColumnCount * pageFlickCount); ++column) {
         // Flick columns to the left (and add one pixel to ensure the left column is completely out)
         tableView->setContentX((delegateWidth * column) + 1);
-        tableView->polish();
-
-        WAIT_UNTIL_POLISHED;
-
         // Check that the number of delegate items created so far is what we expect.
         const int delegatesCreatedCount = view->rootObject()->property(kDelegatesCreatedCountProp).toInt();
         int expectedCount = delegateCountAfterInit + (reuseItems ? 0 : visibleRowCount * column);