Fix QProperty property interaction with aliases

With this change, an alias of a bindable property is also bindable, and
shares its bindable interface with the target.

Moreover, the logic in QQmlTypeCompiler is adjusted so that a change
handler of an alias uses the bindable interface if possible, instead of
connecting to the alias' change signal. That would never be emitted if
the target is a QProperty without a notify signal.

Alias properties still have a change signal, but those never get
emitted.

Change-Id: I857dfdbe51048a2b604ad632982e7f4adac6b907
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 44 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmlecmascript/data/aliasOfQProperty.qml b/tests/auto/qml/qqmlecmascript/data/aliasOfQProperty.qml
new file mode 100644
index 0000000000..6c37a33f39
--- /dev/null
+++ b/tests/auto/qml/qqmlecmascript/data/aliasOfQProperty.qml
@@ -0,0 +1,7 @@
+import Qt.test 1.0
+ClassWithQProperty {
+    id: root
+    property alias myAlias: root.value
+    property int changeCounter: 0
+    onMyAliasChanged: ++changeCounter
+}
diff --git a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
index ef2eb5ad03..e3396a430f 100644
--- a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
+++ b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
@@ -382,6 +382,7 @@ private slots:
     void semicolonAfterProperty();
     void hugeStack();
     void bindingOnQProperty();
+    void aliasOfQProperty();
     void bindingOnQPropertyContextProperty();
     void bindingContainingQProperty();
     void urlConstruction();
@@ -9177,6 +9178,42 @@ void tst_qqmlecmascript::bindingOnQProperty()
     QVERIFY(qprop.hasBinding());
 }
 
+void tst_qqmlecmascript::aliasOfQProperty() {
+    QQmlEngine engine;
+    QQmlComponent component(&engine, testFileUrl("aliasOfQProperty.qml"));
+    QVERIFY2(component.isReady(), qPrintable(component.errorString()));
+    QScopedPointer<QObject> root(component.create());
+    QProperty<float> &qprop = static_cast<ClassWithQProperty*>(root.data())->value;
+    QProperty<float> otherProperty;
+
+
+    qprop.setBinding([&](){return otherProperty.value();});
+    // changing the target properties value triggers the alias property's change handler exactly
+    // once, and doesn't require reading the value (binding is no longer lazy)
+    otherProperty.setValue(42.0);
+    QCOMPARE(root->property("changeCounter").toInt(), 1);
+    // reading the value afterwards doesn't trigger the observer again
+    QCOMPARE(root->property("myAlias").toFloat(), 42.0);
+    QCOMPARE(root->property("changeCounter").toInt(), 1);
+
+    // writing to the alias breaks the binding
+    root->setProperty("myAlias", 12.0);
+    QCOMPARE(qprop.value(), 12.0);
+    QVERIFY(!qprop.hasBinding());
+
+    // it is possible to obtain the bindable interface of the target from the bindable property
+    QUntypedBindable bindable;
+    void *argv[] = { &bindable };
+    int myaliasPropertyIndex = root->metaObject()->indexOfProperty("myAlias");
+    root->metaObject()->metacall(root.get(), QMetaObject::BindableProperty, myaliasPropertyIndex, argv);
+    QVERIFY(bindable.isValid());
+
+    bool ok = bindable.setBinding(Qt::makePropertyBinding([&]() -> float {return 13.0; }  ));
+    QVERIFY(ok);
+    QVERIFY(qprop.hasBinding());
+    QCOMPARE(qprop.value(), 13.0);
+}
+
 void tst_qqmlecmascript::bindingOnQPropertyContextProperty()
 {
     QSKIP("Test needs to be adjusted");