Fix unnecessary evaluation of dependent bindings

Given two simple bindings in this order

    property int firstVar: secondVar
    property int secondVar: ...

then the binding expression for "secondVar" ends up being evaluated
twice at run-time. The first time happens when enabling the binding
expression for "firstVar", which results in the engine detecting that
there is a dependency onto another binding that has not been enabled
yet. This is when QQmlData::flushPendingBinding(Impl) enables the
expression for secondVar and does an initial evaluation. Afterwards the
QQmlObjectCreator continues enabling the next binding in ::finalize(),
which will end up evaluating secondVar a second time, unnecessarily.

We can detect this case inside setEnabled and only call update() if we
transition from disabled to enabled state. This should also cover the
case of bindings created and assigned dynamically through QtQuick
PropertyChanges / States, as those call setEnabled(false) before
removing the binding (to replace it with something else) and
setEnabled(true) when reverting the state (in
QQmlPropertyPrivate::setBinding).

Change-Id: I447432891eabff2c4393f5abfee1092992746fa0
Task-number: QTBUG-66945
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

3 files changed, 51 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmllanguage/data/noDoubleEvaluationForFlushedBindings.2.qml b/tests/auto/qml/qqmllanguage/data/noDoubleEvaluationForFlushedBindings.2.qml
new file mode 100644
index 0000000000..ff2f0f5a2c
--- /dev/null
+++ b/tests/auto/qml/qqmllanguage/data/noDoubleEvaluationForFlushedBindings.2.qml
@@ -0,0 +1,9 @@
+import QtQml 2.0
+
+QtObject {
+    property int secondVar: {
+        stats.increaseEvaluationCounter()
+        return 1
+    }
+    property int firstVar: secondVar + 1
+}
diff --git a/tests/auto/qml/qqmllanguage/data/noDoubleEvaluationForFlushedBindings.qml b/tests/auto/qml/qqmllanguage/data/noDoubleEvaluationForFlushedBindings.qml
new file mode 100644
index 0000000000..0eb5e03642
--- /dev/null
+++ b/tests/auto/qml/qqmllanguage/data/noDoubleEvaluationForFlushedBindings.qml
@@ -0,0 +1,9 @@
+import QtQml 2.0
+
+QtObject {
+    property int firstVar: secondVar + 1
+    property int secondVar: {
+        stats.increaseEvaluationCounter()
+        return 1
+    }
+}
diff --git a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
index 5bb2b69565..403e8c5730 100644
--- a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
+++ b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
@@ -125,6 +125,8 @@ private slots:
     void dynamicObjectProperties();
     void dynamicSignalsAndSlots();
     void simpleBindings();
+    void noDoubleEvaluationForFlushedBindings_data();
+    void noDoubleEvaluationForFlushedBindings();
     void autoComponentCreation();
     void autoComponentCreationInGroupProperty();
     void propertyValueSource();
@@ -1478,6 +1480,37 @@ void tst_qqmllanguage::simpleBindings()
     QCOMPARE(object->property("objectProperty"), QVariant::fromValue(object));
 }
 
+class EvaluationCounter : public QObject
+{
+    Q_OBJECT
+public:
+    int counter = 0;
+    Q_INVOKABLE void increaseEvaluationCounter() { ++counter; }
+};
+
+void tst_qqmllanguage::noDoubleEvaluationForFlushedBindings_data()
+{
+    QTest::addColumn<QString>("fileName");
+    QTest::newRow("order1") << QString("noDoubleEvaluationForFlushedBindings.qml");
+    QTest::newRow("order2") << QString("noDoubleEvaluationForFlushedBindings.2.qml");
+}
+
+void tst_qqmllanguage::noDoubleEvaluationForFlushedBindings()
+{
+    QFETCH(QString, fileName);
+    QQmlEngine engine;
+
+    EvaluationCounter stats;
+    engine.rootContext()->setContextProperty("stats", &stats);
+
+    QQmlComponent component(&engine, testFileUrl(fileName));
+    VERIFY_ERRORS(0);
+    QScopedPointer<QObject> obj(component.create());
+    QVERIFY(!obj.isNull());
+
+    QCOMPARE(stats.counter, 1);
+}
+
 void tst_qqmllanguage::autoComponentCreation()
 {
     {