QML: emit messages when breaking bindings

Add a new logging category that can be used to debug issues with
unexpected binding breakages caused by assignment to previously bound
properties. If enabled, outputs a message when a binding is broken with
detailed location and property/value information.

[ChangeLog][QtQml] The QML engine can now emit informational messages
(in the "qt.qml.binding.removal" logging category) whenever a binding is
lost due to an imperative assignment. This can be used to debug issues
due to broken bindings.

Change-Id: Ie31e5a198450b6f998c1266acfc97e8f33a92e3d
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmlbinding/tst_qqmlbinding.cpp b/tests/auto/qml/qqmlbinding/tst_qqmlbinding.cpp
index 6f1d82eca5..4b485d2ce8 100644
--- a/tests/auto/qml/qqmlbinding/tst_qqmlbinding.cpp
+++ b/tests/auto/qml/qqmlbinding/tst_qqmlbinding.cpp
@@ -50,6 +50,7 @@ private slots:
     void disabledOnUnknownProperty();
     void disabledOnReadonlyProperty();
     void delayed();
+    void bindingOverwriting();
 
 private:
     QQmlEngine engine;
@@ -303,6 +304,21 @@ void tst_qqmlbinding::delayed()
     delete item;
 }
 
+void tst_qqmlbinding::bindingOverwriting()
+{
+    QQmlTestMessageHandler messageHandler;
+    QLoggingCategory::setFilterRules(QStringLiteral("qt.qml.binding.removal.info=true"));
+
+    QQmlEngine engine;
+    QQmlComponent c(&engine, testFileUrl("bindingOverwriting.qml"));
+    QQuickItem *item = qobject_cast<QQuickItem*>(c.create());
+    QVERIFY(item);
+    delete item;
+
+    QLoggingCategory::setFilterRules(QString());
+    QCOMPARE(messageHandler.messages().count(), 2);
+}
+
 QTEST_MAIN(tst_qqmlbinding)
 
 #include "tst_qqmlbinding.moc"