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>

1 files changed, 75 insertions, 24 deletions

diff --git a/src/corelib/kernel/qproperty.h b/src/corelib/kernel/qproperty.h
index f0156fac02..b1b4dbeae1 100644
--- a/src/corelib/kernel/qproperty.h
+++ b/src/corelib/kernel/qproperty.h
@@ -374,11 +374,21 @@ namespace Qt {
     }
 }
 
-template <typename T, typename Class, void(Class::*Callback)()>
-class QNotifiedProperty<T, Callback>
+namespace detail {
+    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 (d.setValueAndReturnTrueIfChanged(std::move(newValue)))
-            notify(owner);
+        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 (d.setValueAndReturnTrueIfChanged(newValue))
-            notify(owner);
+        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)
     {
-        QPropertyBinding<T> oldBinding(d.priv.setBinding(newBinding, &d, owner, [](void *o) {
-            (reinterpret_cast<Class *>(o)->*Callback)();
-        }));
-        notify(owner);
-        return oldBinding;
+        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) {
-            (reinterpret_cast<Class *>(o)->*Callback)();
-        }));
-        notify(owner);
-        return oldBinding;
+        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) {
-            (reinterpret_cast<Class *>(o)->*Callback)();
-        });
-        notify(owner);
+        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);
-        (owner->*Callback)();
+        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)
 {