emit QAbstractSlider::valueChanged() only on value change

The signal has been emitted also, when the slider position has changed
without a value change (e.g. on QWidget::show() or on resizing).

This patch stops emitting the signal without a value change.
It adds a verification to tst_QAbstractSlider::setValue(), that
valueChanged is not emitted on a no-op value change.

As a drive-by, the patch removes an unnecessary assignment and
changes signal spy constructions to PTMF syntax.

Pick-to: 6.5 6.4 6.2
Task-number: QTBUG-25365
Change-Id: I9932e45d4e680aa53422ca75aa42a16306213a38
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 9 insertions, 2 deletions

diff --git a/src/widgets/widgets/qabstractslider.cpp b/src/widgets/widgets/qabstractslider.cpp
index 6e44b2d432..4e65fbac88 100644
--- a/src/widgets/widgets/qabstractslider.cpp
+++ b/src/widgets/widgets/qabstractslider.cpp
@@ -498,18 +498,25 @@ void QAbstractSlider::setValue(int value)
     value = d->bound(value);
     if (d->value == value && d->position == value)
         return;
+
+    // delay signal emission until sliderChanged() has been called
+    const bool emitValueChanged = (value != d->value);
     d->value = value;
+
     if (d->position != value) {
         d->position = value;
         if (d->pressed)
-            emit sliderMoved((d->position = value));
+            emit sliderMoved(d->position);
     }
 #if QT_CONFIG(accessibility)
     QAccessibleValueChangeEvent event(this, d->value);
     QAccessible::updateAccessibility(&event);
 #endif
     sliderChange(SliderValueChange);
-    emit valueChanged(value);
+
+    if (emitValueChanged)
+        emit valueChanged(value);
+
 }
 
 /*!