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>

2 files changed, 11 insertions, 0 deletions

diff --git a/src/corelib/kernel/qproperty.cpp b/src/corelib/kernel/qproperty.cpp
index a5354532d2..7c8477e31a 100644
--- a/src/corelib/kernel/qproperty.cpp
+++ b/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;
diff --git a/src/corelib/kernel/qproperty_p.h b/src/corelib/kernel/qproperty_p.h
index 58a000e9bf..6a074efebb 100644
--- a/src/corelib/kernel/qproperty_p.h
+++ b/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)