Tumbler: calculate proper displacement when count <= visibleItemCount

Task-number: QTBUG-53120
Change-Id: I78035b2cf75b2bc18326b7f735be3f9c930d472b
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

2 files changed, 35 insertions, 22 deletions

diff --git a/src/quicktemplates2/qquicktumbler.cpp b/src/quicktemplates2/qquicktumbler.cpp
index 546e699c..852803f0 100644
--- a/src/quicktemplates2/qquicktumbler.cpp
+++ b/src/quicktemplates2/qquicktumbler.cpp
@@ -403,7 +403,7 @@ public:
     QQuickTumblerAttachedPrivate(QQuickItem *delegateItem) :
         tumbler(nullptr),
         index(-1),
-        displacement(1)
+        displacement(0)
     {
         if (!delegateItem->parentItem()) {
             qWarning() << "Tumbler: attached properties must be accessed from within a delegate item that has a parent";
@@ -489,8 +489,10 @@ void QQuickTumblerAttachedPrivate::_q_calculateDisplacement()
     if (contentType == PathViewContentItem) {
         offset = tumbler->contentItem()->property("offset").toReal();
 
-        displacement = count - index - offset;
-        int halfVisibleItems = tumbler->visibleItemCount() / 2 + 1;
+        displacement = count > 1 ? count - index - offset : 0;
+        // Don't add 1 if count <= visibleItemCount
+        const int visibleItems = tumbler->visibleItemCount();
+        int halfVisibleItems = visibleItems / 2 + (visibleItems < count ? 1 : 0);
         if (displacement > halfVisibleItems)
             displacement -= count;
         else if (displacement < -halfVisibleItems)
@@ -522,6 +524,8 @@ QQuickTumblerAttached::QQuickTumblerAttached(QQuickItem *delegateItem) :
         const char *contentItemSignal = contentType == PathViewContentItem
             ? SIGNAL(offsetChanged()) : SIGNAL(contentYChanged());
         connect(d->tumbler->contentItem(), contentItemSignal, this, SLOT(_q_calculateDisplacement()));
+
+        d->_q_calculateDisplacement();
     }
 }
 
diff --git a/tests/auto/controls/data/tst_tumbler.qml b/tests/auto/controls/data/tst_tumbler.qml
index ec716f32..2de52924 100644
--- a/tests/auto/controls/data/tst_tumbler.qml
+++ b/tests/auto/controls/data/tst_tumbler.qml
@@ -237,25 +237,34 @@ TestCase {
     function test_displacement_data() {
         var data = [
             // At 0 offset, the first item is current.
-            { index: 0, offset: 0, expectedDisplacement: 0 },
-            { index: 1, offset: 0, expectedDisplacement: -1 },
-            { index: 5, offset: 0, expectedDisplacement: 1 },
+            { count: 6, index: 0, offset: 0, expectedDisplacement: 0 },
+            { count: 6, index: 1, offset: 0, expectedDisplacement: -1 },
+            { count: 6, index: 5, offset: 0, expectedDisplacement: 1 },
             // When we start to move the first item down, the second item above it starts to become current.
-            { index: 0, offset: 0.25, expectedDisplacement: -0.25 },
-            { index: 1, offset: 0.25, expectedDisplacement: -1.25 },
-            { index: 5, offset: 0.25, expectedDisplacement: 0.75 },
-            { index: 0, offset: 0.5, expectedDisplacement: -0.5 },
-            { index: 1, offset: 0.5, expectedDisplacement: -1.5 },
-            { index: 5, offset: 0.5, expectedDisplacement: 0.5 },
+            { count: 6, index: 0, offset: 0.25, expectedDisplacement: -0.25 },
+            { count: 6, index: 1, offset: 0.25, expectedDisplacement: -1.25 },
+            { count: 6, index: 5, offset: 0.25, expectedDisplacement: 0.75 },
+            { count: 6, index: 0, offset: 0.5, expectedDisplacement: -0.5 },
+            { count: 6, index: 1, offset: 0.5, expectedDisplacement: -1.5 },
+            { count: 6, index: 5, offset: 0.5, expectedDisplacement: 0.5 },
             // By this stage, the delegate at index 1 is destroyed, so we can't test its displacement.
-            { index: 0, offset: 0.75, expectedDisplacement: -0.75 },
-            { index: 5, offset: 0.75, expectedDisplacement: 0.25 },
-            { index: 0, offset: 4.75, expectedDisplacement: 1.25 },
-            { index: 1, offset: 4.75, expectedDisplacement: 0.25 },
-            { index: 0, offset: 4.5, expectedDisplacement: 1.5 },
-            { index: 1, offset: 4.5, expectedDisplacement: 0.5 },
-            { index: 0, offset: 4.25, expectedDisplacement: 1.75 },
-            { index: 1, offset: 4.25, expectedDisplacement: 0.75 }
+            { count: 6, index: 0, offset: 0.75, expectedDisplacement: -0.75 },
+            { count: 6, index: 5, offset: 0.75, expectedDisplacement: 0.25 },
+            { count: 6, index: 0, offset: 4.75, expectedDisplacement: 1.25 },
+            { count: 6, index: 1, offset: 4.75, expectedDisplacement: 0.25 },
+            { count: 6, index: 0, offset: 4.5, expectedDisplacement: 1.5 },
+            { count: 6, index: 1, offset: 4.5, expectedDisplacement: 0.5 },
+            { count: 6, index: 0, offset: 4.25, expectedDisplacement: 1.75 },
+            { count: 6, index: 1, offset: 4.25, expectedDisplacement: 0.75 },
+            // count == visibleItemCount
+            { count: 3, index: 0, offset: 0, expectedDisplacement: 0 },
+            { count: 3, index: 1, offset: 0, expectedDisplacement: -1 },
+            { count: 3, index: 2, offset: 0, expectedDisplacement: 1 },
+            // count < visibleItemCount
+            { count: 2, index: 0, offset: 0, expectedDisplacement: 0 },
+            { count: 2, index: 1, offset: 0, expectedDisplacement: 1 },
+            // count == 1
+            { count: 1, index: 0, offset: 0, expectedDisplacement: 0 }
         ];
         for (var i = 0; i < data.length; ++i) {
             var row = data[i];
@@ -283,8 +292,8 @@ TestCase {
         // TODO: test setting these in the opposite order (delegate after model
         // doesn't seem to cause a change in delegates in PathView)
         tumbler.delegate = displacementDelegate;
-        tumbler.model = 6;
-        compare(tumbler.count, 6);
+        tumbler.model = data.count;
+        compare(tumbler.count, data.count);
 
         var delegate = findChild(tumbler.contentItem, "delegate" + data.index);
         verify(delegate);