QPropertyBinding: Add sticky mode

A sticky QPropertyBinding is a binding that does not get removed when a
write occurs. This is used in the QML engine to implement support for
the QQmlPropertyData::DontRemoveBinding flag.

Task-number: QTBUG-91689
Change-Id: Ib575b49abe634215318ccc7ba46212cc21eb4dad
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -508,6 +508,9 @@ void QPropertyBindingData::removeBinding_helper()
 
     auto *existingBinding = d.binding();
     Q_ASSERT(existingBinding);
+    if (existingBinding->isSticky()) {
+        return;
+    }
 
     auto observer = existingBinding->takeObservers();
     d_ref() = 0;

src/corelib/kernel/qproperty_p.h

@@ -184,6 +184,11 @@ private:
     bool hasBindingWrapper:1;
     // used to detect binding loops for eagerly evaluated properties
     bool isQQmlPropertyBinding:1;
+    /* a sticky binding does not get removed in removeBinding
+       this is used to support QQmlPropertyData::DontRemoveBinding
+       in qtdeclarative
+    */
+    bool m_sticky:1;
 
     const QtPrivate::BindingFunctionVTable *vtable;
 
@@ -234,11 +239,14 @@ public:
     size_t dependencyObserverCount = 0;
 
     bool isUpdating() {return updating;}
+    void setSticky(bool keep = true) {m_sticky = keep;}
+    bool isSticky() {return m_sticky;}
 
     QPropertyBindingPrivate(QMetaType metaType, const QtPrivate::BindingFunctionVTable *vtable,
                             const QPropertyBindingSourceLocation &location, bool isQQmlPropertyBinding=false)
         : hasBindingWrapper(false)
         , isQQmlPropertyBinding(isQQmlPropertyBinding)
+        , m_sticky(false)
         , vtable(vtable)
         , location(location)
         , metaType(metaType)

tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp

@@ -56,6 +56,7 @@ private slots:
     void avoidDependencyAllocationAfterFirstEval();
     void boolProperty();
     void takeBinding();
+    void stickyBinding();
     void replaceBinding();
     void changeHandler();
     void propertyChangeHandlerApi();
@@ -307,6 +308,30 @@ void tst_QProperty::takeBinding()
     QVERIFY(!existingBinding.isNull());
 }
 
+void tst_QProperty::stickyBinding()
+{
+    QProperty<int> prop;
+    QProperty<int> prop2 {2};
+    prop.setBinding([&](){ return prop2.value(); });
+    QCOMPARE(prop.value(), 2);
+    auto privBinding = QPropertyBindingPrivate::get(prop.binding());
+    // If we make a binding sticky,
+    privBinding->setSticky();
+    // then writing to the property does not remove it
+    prop = 1;
+    QVERIFY(prop.hasBinding());
+    // but the value still changes.
+    QCOMPARE(prop.value(), 1);
+    // The binding continues to work normally.
+    prop2 = 3;
+    QCOMPARE(prop.value(), 3);
+    // If we remove the stickiness
+    privBinding->setSticky(false);
+    // the binding goes away on the next write
+    prop = 42;
+    QVERIFY(!prop.hasBinding());
+}
+
 void tst_QProperty::replaceBinding()
 {
     QProperty<int> first(100);