QJSEngine: Optimize conversion from QObject* to QString

This is commonly done for logging. With this in place we can have the
code generator use coerceType() for such constructs.

[ChangeLog][QML][Important Behavior Changes] You can implement custom
toString() methods for your QML objects in JavaScript or in C++. Those
methods don't actually have to return a string. Previously, whatever
return value the method generated was forwarded as-is. Now it is coerced
to a string.

Change-Id: I4a9721a6948be0c24a36b31d453a74bd747db729
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>

2 files changed, 19 insertions, 0 deletions

diff --git a/src/qml/jsapi/qjsengine.cpp b/src/qml/jsapi/qjsengine.cpp
index c15b5c1ccd..8b0b650c1f 100644
--- a/src/qml/jsapi/qjsengine.cpp
+++ b/src/qml/jsapi/qjsengine.cpp
@@ -920,6 +920,12 @@ bool QJSEngine::convertMetaType(QMetaType fromType, const void *from, QMetaType
     return QV4::ExecutionEngine::metaTypeFromJS(handle()->fromData(fromType, from), toType, to);
 }
 
+QString QJSEngine::convertQObjectToString(QObject *object)
+{
+    return QV4::QObjectWrapper::objectToString(
+                handle(), object ? object->metaObject() : nullptr, object);
+}
+
 /*! \fn template <typename T> QJSValue QJSEngine::toScriptValue(const T &value)
 
     Creates a QJSValue with the given \a value.
diff --git a/src/qml/jsapi/qjsengine.h b/src/qml/jsapi/qjsengine.h
index db78e1b9a9..9af6fd6e7c 100644
--- a/src/qml/jsapi/qjsengine.h
+++ b/src/qml/jsapi/qjsengine.h
@@ -101,6 +101,13 @@ public:
         if constexpr (std::is_same_v<T, QJSManagedValue>)
             return toManagedValue(value);
 
+        if constexpr (std::is_same_v<T, QString>) {
+            if (targetType.flags() & QMetaType::PointerToQObject) {
+                return convertQObjectToString(
+                            *reinterpret_cast<QObject *const *>(value.constData()));
+            }
+        }
+
         if (sourceType == QMetaType::fromType<QJSValue>())
             return fromScriptValue<T>(*reinterpret_cast<const QJSValue *>(value.constData()));
 
@@ -148,6 +155,11 @@ public:
         if constexpr (std::is_same_v<To, QVariant>)
             return QVariant::fromValue(from);
 
+        if constexpr (std::is_same_v<To, QString>
+                && std::is_base_of_v<QObject, std::remove_const_t<std::remove_pointer_t<To>>>) {
+            return convertQObjectToString(from);
+        }
+
         if constexpr (std::is_same_v<To, std::remove_const_t<std::remove_pointer_t<To>> const *>) {
             using nonConstTo = std::remove_const_t<std::remove_pointer_t<To>> *;
             if constexpr (std::is_same_v<From, nonConstTo>)
@@ -220,6 +232,7 @@ private:
 
     bool convertVariant(const QVariant &value, QMetaType metaType, void *ptr);
     bool convertMetaType(QMetaType fromType, const void *from, QMetaType toType, void *to);
+    QString convertQObjectToString(QObject *object);
 
     template<typename T>
     friend inline T qjsvalue_cast(const QJSValue &);