OpacityAnimator: apply opacity animation to the first frame in SG

When using OpacityAnimator, an opacity node will be created and inserted into
nodes tree to perform opacity animation. However, since the default value of
opacity node is 1, the opacity animation will start from 1 even if we set
it from 0 to 1.

Fixes this issue by updating the value of opacity just after creating a new
opacity node.

Fixes: QTBUG-79199
Change-Id: I2e462f0c56892fda040836ffde6685145769e60c
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

3 files changed, 58 insertions, 0 deletions

diff --git a/src/quick/util/qquickanimatorjob.cpp b/src/quick/util/qquickanimatorjob.cpp
index a9caedec4f..84480ddd18 100644
--- a/src/quick/util/qquickanimatorjob.cpp
+++ b/src/quick/util/qquickanimatorjob.cpp
@@ -582,6 +582,7 @@ void QQuickOpacityAnimatorJob::postSync()
         }
 
         d->extra.value().opacityNode = m_opacityNode;
+        updateCurrentTime(0);
     }
     Q_ASSERT(m_opacityNode);
 }
diff --git a/tests/auto/quick/qquickanimations/data/opacityAnimationFromZero.qml b/tests/auto/quick/qquickanimations/data/opacityAnimationFromZero.qml
new file mode 100644
index 0000000000..bfb8211706
--- /dev/null
+++ b/tests/auto/quick/qquickanimations/data/opacityAnimationFromZero.qml
@@ -0,0 +1,20 @@
+import QtQuick 2.14
+import QtQuick.Window 2.14
+
+Window {
+    width: 640
+    height: 480
+    Rectangle {
+        id: rect
+        width: 200
+        height: 200
+        color: "black"
+        OpacityAnimator {
+            target: rect
+            from: 0
+            to: 1
+            duration: 1000
+            running: true
+        }
+    }
+}
diff --git a/tests/auto/quick/qquickanimations/tst_qquickanimations.cpp b/tests/auto/quick/qquickanimations/tst_qquickanimations.cpp
index 48f779a490..9bd0cd777f 100644
--- a/tests/auto/quick/qquickanimations/tst_qquickanimations.cpp
+++ b/tests/auto/quick/qquickanimations/tst_qquickanimations.cpp
@@ -112,6 +112,7 @@ private slots:
     void replacingTransitions();
     void animationJobSelfDestruction();
     void fastFlickingBug();
+    void opacityAnimationFromZero();
 };
 
 #define QTIMED_COMPARE(lhs, rhs) do { \
@@ -1774,6 +1775,42 @@ void tst_qquickanimations::fastFlickingBug()
     }
 }
 
+void tst_qquickanimations::opacityAnimationFromZero()
+{
+    if ((QGuiApplication::platformName() == QLatin1String("offscreen"))
+        || (QGuiApplication::platformName() == QLatin1String("minimal")))
+        QSKIP("Skipping due to grabWindow not functional on offscreen/minimimal platforms");
+
+    // not easy to verify this in threaded render loop
+    // since it's difficult to capture the first frame when scene graph
+    // is renderred in another thread
+    qputenv("QSG_RENDER_LOOP", "basic");
+    auto cleanup = qScopeGuard([]() { qputenv("QSG_RENDER_LOOP", ""); });
+
+    QQmlEngine engine;
+    QQmlComponent c(&engine, testFileUrl("opacityAnimationFromZero.qml"));
+    QScopedPointer<QQuickWindow> win(qobject_cast<QQuickWindow*>(c.create()));
+    if (!c.errors().isEmpty())
+        qDebug() << c.errorString();
+    QVERIFY(win);
+    win->setTitle(QTest::currentTestFunction());
+    win->show();
+    QVERIFY(QTest::qWaitForWindowExposed(win.data()));
+
+    QImage img;
+    bool firstFrameSwapped = false;
+    QObject::connect(win.get(), &QQuickWindow::frameSwapped, win.get(), [&win, &img, &firstFrameSwapped]() {
+        if (firstFrameSwapped)
+            return;
+        else
+            firstFrameSwapped = true;
+        img = win->grabWindow();
+        if (img.width() < win->width())
+            QSKIP("Skipping due to grabWindow not functional");
+    });
+    QTRY_VERIFY(!img.isNull() && img.pixel(100, 100) > qRgb(10, 10, 10));
+}
+
 QTEST_MAIN(tst_qquickanimations)
 
 #include "tst_qquickanimations.moc"