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.

Task-number: QTBUG-25365
Change-Id: I9932e45d4e680aa53422ca75aa42a16306213a38
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
(cherry picked from commit 8b84374892512faa43e5deeeef22ca558ac5543a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 16 insertions, 5 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);
+
 }
 
 /*!
diff --git a/tests/auto/widgets/widgets/qabstractslider/tst_qabstractslider.cpp b/tests/auto/widgets/widgets/qabstractslider/tst_qabstractslider.cpp
index 17caa091c6..170c29922a 100644
--- a/tests/auto/widgets/widgets/qabstractslider/tst_qabstractslider.cpp
+++ b/tests/auto/widgets/widgets/qabstractslider/tst_qabstractslider.cpp
@@ -2021,9 +2021,9 @@ void tst_QAbstractSlider::setValue()
     slider->setRange(minimum, maximum);
     slider->setSliderDown(down);
     slider->setValue(49); // to force a valueChanged() below
-    QSignalSpy spy1(slider, SIGNAL(sliderMoved(int)));
-    QSignalSpy spy2(slider, SIGNAL(valueChanged(int)));
-    QSignalSpy spy3(slider, SIGNAL(actionTriggered(int)));
+    QSignalSpy spy1(slider, &QAbstractSlider::sliderMoved);
+    QSignalSpy spy2(slider, &QAbstractSlider::valueChanged);
+    QSignalSpy spy3(slider, &QAbstractSlider::actionTriggered);
     slider->setValue(50);
     QCOMPARE(spy1.size(), down ? 1 : 0);
     QCOMPARE(spy2.size(), 1);
@@ -2032,6 +2032,10 @@ void tst_QAbstractSlider::setValue()
     QCOMPARE(slider->sliderPosition(), reportedSliderPosition);
     if (down)
         QVERIFY(sliderMovedTimeStamp < valueChangedTimeStamp);
+
+    slider->setValue(50);
+    QApplication::processEvents();
+    QCOMPARE(spy2.size(), 1);
 }
 
 void tst_QAbstractSlider::waitUntilTimeElapsed(const QElapsedTimer &t, int ms)