QNotifiedProperty: pass old value to callback if requested

Check at compile time whether the static callback takes an argument
(which has to be of the same time as the type of the property). If so,
retrieve the old value and pass it to the callback.

Change-Id: Ib1c4c9e05b826b6be492b03f66fa72ad015963ee
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -95,7 +95,7 @@ QPropertyBase::~QPropertyBase()
 QUntypedPropertyBinding QPropertyBase::setBinding(const QUntypedPropertyBinding &binding,
                                                   void *propertyDataPtr,
                                                   void *staticObserver,
-                                                  void (*staticObserverCallback)(void*))
+                                                  void (*staticObserverCallback)(void*, void*))
 {
     QPropertyBindingPrivatePtr oldBinding;
     QPropertyBindingPrivatePtr newBinding = binding.d;

src/corelib/kernel/qproperty.h

@@ -374,11 +374,21 @@ namespace Qt {
     }
 }
 
-template <typename T, typename Class, void(Class::*Callback)()>
+namespace detail {
-class QNotifiedProperty<T, Callback>
+    template <typename F>
+    struct ExtractClassFromFunctionPointer;
+
+    template<typename T, typename C>
+    struct ExtractClassFromFunctionPointer<T C::*> { using Class = C; };
+}
+
+template <typename T, auto Callback>
+class QNotifiedProperty
 {
 public:
     using value_type = T;
+    using Class = typename detail::ExtractClassFromFunctionPointer<decltype(Callback)>::Class;
+    static_assert(std::is_invocable_v<decltype(Callback), Class, T> || std::is_invocable_v<decltype(Callback), Class>);
 
     QNotifiedProperty() = default;
 
@@ -420,45 +430,83 @@ public:
 
     void setValue(Class *owner, T &&newValue)
     {
+        if constexpr(std::is_invocable_v<decltype(Callback), Class>) {
             if (d.setValueAndReturnTrueIfChanged(std::move(newValue)))
                 notify(owner);
+        } else {
+            T oldValue = value(); // TODO: kind of pointless if there was no change
+            if (d.setValueAndReturnTrueIfChanged(std::move(newValue)))
+                notify(owner, &oldValue);
+        }
         d.priv.removeBinding();
     }
 
     void setValue(Class *owner, const T &newValue)
     {
+        if constexpr(std::is_invocable_v<decltype(Callback), Class>) {
             if (d.setValueAndReturnTrueIfChanged(newValue))
                 notify(owner);
+        } else {
+            T oldValue = value();
+            if (d.setValueAndReturnTrueIfChanged(newValue))
+                notify(owner, &oldValue);
+        }
         d.priv.removeBinding();
     }
 
     QPropertyBinding<T> setBinding(Class *owner, const QPropertyBinding<T> &newBinding)
     {
+        if constexpr(std::is_invocable_v<decltype(Callback), Class>) {
             QPropertyBinding<T> oldBinding(d.priv.setBinding(newBinding, &d, owner, [](void *o) {
                 (reinterpret_cast<Class *>(o)->*Callback)();
             }));
             notify(owner);
             return oldBinding;
+        } else {
+            T oldValue = value();
+            QPropertyBinding<T> oldBinding(d.priv.setBinding(newBinding, &d, owner, [](void *o, void *oldValue) {
+                (reinterpret_cast<Class *>(o)->*Callback)(*reinterpret_cast<T *>(oldValue));
+            }));
+            notify(owner, &oldValue);
+            return oldBinding;
+        }
     }
 
     QPropertyBinding<T> setBinding(Class *owner, QPropertyBinding<T> &&newBinding)
     {
         QPropertyBinding<T> b(std::move(newBinding));
-        QPropertyBinding<T> oldBinding(d.priv.setBinding(b, &d, owner, [](void *o) {
+        if constexpr(std::is_invocable_v<decltype(Callback), Class>) {
+            QPropertyBinding<T> oldBinding(d.priv.setBinding(b, &d, owner, [](void *o, void *) {
                 (reinterpret_cast<Class *>(o)->*Callback)();
             }));
             notify(owner);
             return oldBinding;
+        } else {
+            T oldValue = value();
+            QPropertyBinding<T> oldBinding(d.priv.setBinding(b, &d, owner, [](void *o, void *oldValue) {
+                (reinterpret_cast<Class *>(o)->*Callback)(*reinterpret_cast<T *>(oldValue));
+            }));
+            notify(owner, &oldValue);
+            return oldBinding;
+        }
     }
 
     bool setBinding(Class *owner, const QUntypedPropertyBinding &newBinding)
     {
         if (newBinding.valueMetaType().id() != qMetaTypeId<T>())
             return false;
-        d.priv.setBinding(newBinding, &d, owner, [](void *o) {
+        if constexpr(std::is_invocable_v<decltype(Callback), Class>) {
+            d.priv.setBinding(newBinding, &d, owner, [](void *o, void *) {
                 (reinterpret_cast<Class *>(o)->*Callback)();
             });
             notify(owner);
+        } else {
+            T oldValue = value();
+            d.priv.setBinding(newBinding, &d, owner, [](void *o, void *oldValue) {
+                (reinterpret_cast<Class *>(o)->*Callback)(*reinterpret_cast<T *>(oldValue));
+            });
+            notify(owner, &oldValue);
+        }
         return true;
     }
 
@@ -493,10 +541,13 @@ public:
     QPropertyChangeHandler<Functor> subscribe(Functor f);
 
 private:
-    void notify(Class *owner)
+    void notify(Class *owner, T *oldValue=nullptr)
     {
         d.priv.notifyObservers(&d);
+        if constexpr(std::is_invocable_v<decltype(Callback), Class>)
             (owner->*Callback)();
+        else
+            (owner->*Callback)(*oldValue);
     }
 
     Q_DISABLE_COPY_MOVE(QNotifiedProperty)
@@ -624,7 +675,7 @@ QPropertyChangeHandler<Functor> QProperty<T>::subscribe(Functor f)
     return onValueChanged(f);
 }
 
-template <typename T, typename Class, void(Class::*Callback)()>
+template <typename T, auto Callback>
 template<typename Functor>
 QPropertyChangeHandler<Functor> QNotifiedProperty<T, Callback>::onValueChanged(Functor f)
 {
@@ -634,7 +685,7 @@ QPropertyChangeHandler<Functor> QNotifiedProperty<T, Callback>::onValueChanged(F
     return QPropertyChangeHandler<Functor>(*this, f);
 }
 
-template <typename T, typename Class, void(Class::*Callback)()>
+template <typename T, auto Callback>
 template<typename Functor>
 QPropertyChangeHandler<Functor> QNotifiedProperty<T, Callback>::subscribe(Functor f)
 {

src/corelib/kernel/qpropertybinding.cpp

@@ -69,8 +69,15 @@ void QPropertyBindingPrivate::markDirtyAndNotifyObservers()
     dirty = true;
     if (firstObserver)
         firstObserver.notify(this, propertyDataPtr);
-    if (hasStaticObserver)
+    if (hasStaticObserver) {
-        staticObserverCallback(staticObserver);
+        if (metaType == QMetaType::fromType<bool>()) {
+            auto propertyPtr = reinterpret_cast<QPropertyBase *>(propertyDataPtr);
+            bool oldValue = propertyPtr->extraBit();
+            staticObserverCallback(staticObserver, &oldValue);
+        } else {
+            staticObserverCallback(staticObserver, propertyDataPtr);
+        }
+    }
 }
 
 bool QPropertyBindingPrivate::evaluateIfDirtyAndReturnTrueIfValueChanged()

src/corelib/kernel/qpropertybinding_p.h

@@ -80,7 +80,7 @@ private:
         ObserverArray inlineDependencyObservers;
         struct {
             void *staticObserver;
-            void (*staticObserverCallback)(void*);
+            void (*staticObserverCallback)(void*, void*);
         };
     };
     QScopedPointer<std::vector<QPropertyObserver>> heapObservers;
@@ -107,7 +107,7 @@ public:
 
     void setDirty(bool d) { dirty = d; }
     void setProperty(void *propertyPtr) { propertyDataPtr = propertyPtr; }
-    void setStaticObserver(void *observer, void (*callback)(void*))
+    void setStaticObserver(void *observer, void (*callback)(void*, void*))
     {
         if (observer) {
             if (!hasStaticObserver) {

src/corelib/kernel/qpropertyprivate.h

@@ -84,7 +84,7 @@ public:
 
     QUntypedPropertyBinding setBinding(const QUntypedPropertyBinding &newBinding,
                                        void *propertyDataPtr, void *staticObserver = nullptr,
-                                       void (*staticObserverCallback)(void*) = nullptr);
+                                       void (*staticObserverCallback)(void *, void *) = nullptr);
     QPropertyBindingPrivate *binding();
 
     void evaluateIfDirty();

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

@@ -72,6 +72,7 @@ private slots:
     void multipleObservers();
     void propertyAlias();
     void notifiedProperty();
+    void notifiedPropertyWithOldValueCallback();
 };
 
 void tst_QProperty::functorBinding()
@@ -779,6 +780,15 @@ struct ClassWithNotifiedProperty
     QNotifiedProperty<int, &ClassWithNotifiedProperty::callback> property;
 };
 
+struct ClassWithNotifiedProperty2
+{
+    QVector<int> recordedValues;
+
+    void callback(int oldValue) { recordedValues << oldValue; }
+
+    QNotifiedProperty<int, &ClassWithNotifiedProperty2::callback> property;
+};
+
 void tst_QProperty::notifiedProperty()
 {
     ClassWithNotifiedProperty instance;
@@ -854,6 +864,18 @@ void tst_QProperty::notifiedProperty()
     QCOMPARE(subscribedCount, 10);
 }
 
+void tst_QProperty::notifiedPropertyWithOldValueCallback()
+{
+    ClassWithNotifiedProperty2 instance;
+    instance.property.setValue(&instance, 1);
+    instance.property.setBinding(&instance, [](){return 2;});
+    instance.property.setBinding(&instance, [](){return 3;});
+    instance.property.setValue(&instance, 4);
+    QVector<int> expected {0, 1, 2, 3};
+    QCOMPARE(instance.recordedValues, expected);
+    QCOMPARE(instance.property.value(), 4);
+}
+
 QTEST_MAIN(tst_QProperty);
 
 #include "tst_qproperty.moc"