Update scroll indicator position on content size change

If scroll indicator is at the beginning and data prepended to
contentItem, scroll indicator should change position to previous content
beginning.

This is especially important with so-called "infinite scrolling", when
scrolling goes upwards and new content added at the top of the view.

Task-number: QTBUG-50795
Change-Id: I250d6535b1146a54c6a70062b659cc49ed43709f
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

4 files changed, 126 insertions, 0 deletions

diff --git a/src/controls/Private/ScrollViewHelper.qml b/src/controls/Private/ScrollViewHelper.qml
index b4a589bf..bc3d9eb8 100644
--- a/src/controls/Private/ScrollViewHelper.qml
+++ b/src/controls/Private/ScrollViewHelper.qml
@@ -131,6 +131,11 @@ Item {
         anchors.right: cornerFill.left
         anchors.leftMargin:  leftMargin
         anchors.bottomMargin: bottomMargin
+        onScrollAmountChanged: {
+            if (flickableItem && (flickableItem.atXBeginning || flickableItem.atXEnd)) {
+                value = flickableItem.contentX - flickableItem.originX
+            }
+        }
         onValueChanged: {
             if (!blockUpdates) {
                 flickableItem.contentX = value + flickableItem.originX
@@ -180,6 +185,11 @@ Item {
         anchors.top: parent.top
         anchors.topMargin: __scrollBarTopMargin + topMargin
         anchors.rightMargin: rightMargin
+        onScrollAmountChanged: {
+            if (flickableItem && (flickableItem.atYBeginning || flickableItem.atYEnd)) {
+                value = flickableItem.contentY - flickableItem.originY
+            }
+        }
         onValueChanged: {
             if (flickableItem && !blockUpdates && enabled) {
                 flickableItem.contentY = value + flickableItem.originY
diff --git a/tests/auto/controls/data/tst_scrollview.qml b/tests/auto/controls/data/tst_scrollview.qml
index e687c9bc..cb96c3db 100644
--- a/tests/auto/controls/data/tst_scrollview.qml
+++ b/tests/auto/controls/data/tst_scrollview.qml
@@ -41,6 +41,7 @@
 import QtQuick 2.2
 import QtTest 1.0
 import QtQuick.Controls 1.2
+import QtQuick.Controls.Styles 1.1
 import QtQuickControlsTests 1.0
 
 Item {
@@ -90,6 +91,68 @@ TestCase {
         scrollView.destroy()
     }
 
+    Component {
+        id: dragFetchAppendComponent
+
+        ScrollView {
+            width: 400; height: 400
+            frameVisible: false
+            style: ScrollViewStyle {
+                transientScrollBars: false
+                handle: Rectangle {
+                    implicitWidth: 16; implicitHeight: 16
+                    color: "red"
+                }
+                scrollBarBackground: Item {width: 16 ; height: 16}
+                incrementControl: Rectangle {
+                    width: 16; height: 16
+                    color: "blue"
+                }
+                decrementControl: Rectangle {
+                    width: 16; height: 16
+                    color: "blue"
+                }
+            }
+            ListView {
+                id: view
+
+                verticalLayoutDirection: ListView.BottomToTop
+                model: TestFetchAppendModel { }
+                delegate: Text {
+                    width: view.width
+                    height: 60
+                    verticalAlignment: Text.AlignVCenter
+                    horizontalAlignment: Text.AlignHCenter
+                    text: "Item %1".arg(model.index)
+                }
+            }
+        }
+    }
+
+    function test_dragFetchAppend() {   // QTBUG-50795
+        var scrollView = dragFetchAppendComponent.createObject(container)
+        verify(scrollView !== null, "view created is null")
+        waitForRendering(scrollView)
+        verify(scrollView.flickableItem.contentHeight === 60 * 20)
+
+        // After scrolling to the end, view should ask the model to fetch more
+        // data, content height should increase and scrollbar handle should move
+        // to the center.
+        mouseDrag(scrollView, scrollView.width - 2, scrollView.height - 8 - 16, 0, -scrollView.height + 8 + 16)
+        waitForRendering(scrollView)
+
+        // Move it again to fetch more data from the model.
+        mouseDrag(scrollView, scrollView.width - 2, scrollView.height / 2, 0, -scrollView.height / 2  + 8 + 16)
+        waitForRendering(scrollView)
+
+        mouseRelease(scrollView, scrollView.width - 2, 8 + 16)
+        waitForRendering(scrollView)
+
+        verify(Math.round(scrollView.flickableItem.contentHeight) > 60 * 20)
+        verify(Math.round(scrollView.flickableItem.contentY) < -(60 * 20))
+
+        scrollView.destroy()
+    }
 
     function test_scrollbars() {
         var component = scrollViewComponent
diff --git a/tests/auto/testplugin/testcppmodels.h b/tests/auto/testplugin/testcppmodels.h
index f5cc6298..6298bd13 100644
--- a/tests/auto/testplugin/testcppmodels.h
+++ b/tests/auto/testplugin/testcppmodels.h
@@ -96,6 +96,58 @@ private:
     QList<TestObject> m_testobject;
 };
 
+class TestFetchAppendModel : public QAbstractListModel
+{
+    Q_OBJECT
+public:
+    explicit TestFetchAppendModel(QObject *parent = 0)
+        : QAbstractListModel(parent) { }
+
+    virtual int rowCount(const QModelIndex &parent) const
+    {
+        if (parent.isValid()) {
+            return 0;
+        }
+
+        return m_data.size();
+    }
+    virtual QVariant data(const QModelIndex &index, int role) const
+    {
+        if (!index.isValid() || role != Qt::DisplayRole) {
+            return QVariant();
+        }
+
+        return QVariant::fromValue(index.row());
+    }
+
+    virtual bool canFetchMore(const QModelIndex &parent) const
+    {
+        Q_UNUSED(parent)
+
+        return true;
+    }
+    virtual void fetchMore(const QModelIndex & parent)
+    {
+        Q_UNUSED(parent)
+
+        addMoreData();
+    }
+
+private:
+    void addMoreData()
+    {
+        static const int insertCount = 20;
+
+        beginInsertRows(QModelIndex(), m_data.size(), m_data.size() + insertCount - 1);
+        for (int i = 0; i < insertCount; i++) {
+            m_data.append(int());
+        }
+        endInsertRows();
+    }
+
+    QList<int> m_data;
+};
+
 
 #endif // TESTCPPMODELS_H
 
diff --git a/tests/auto/testplugin/testplugin.cpp b/tests/auto/testplugin/testplugin.cpp
index 0f23f0b4..ee6a9711 100644
--- a/tests/auto/testplugin/testplugin.cpp
+++ b/tests/auto/testplugin/testplugin.cpp
@@ -48,6 +48,7 @@ void TestPlugin::registerTypes(const char *uri)
     qmlRegisterType<TestObject>(uri, 1, 0, "TestObject");
     qmlRegisterType<TestItemModel>(uri, 1, 0, "TestItemModel");
     qmlRegisterType<TestModel>(uri, 1, 0, "TreeModel");
+    qmlRegisterType<TestFetchAppendModel>(uri, 1, 0, "TestFetchAppendModel");
 }
 
 void TestPlugin::initializeEngine(QQmlEngine *engine, const char * /*uri*/)