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>

4 files changed, 58 insertions, 29 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 &);
diff --git a/src/qml/jsruntime/qv4qobjectwrapper.cpp b/src/qml/jsruntime/qv4qobjectwrapper.cpp
index e83c111251..2d33b5f36e 100644
--- a/src/qml/jsruntime/qv4qobjectwrapper.cpp
+++ b/src/qml/jsruntime/qv4qobjectwrapper.cpp
@@ -1023,6 +1023,38 @@ int QObjectWrapper::virtualMetacall(Object *object, QMetaObject::Call call, int
     return 0;
 }
 
+QString QObjectWrapper::objectToString(
+        ExecutionEngine *engine, const QMetaObject *metaObject, QObject *object)
+{
+    if (!metaObject)
+        return QLatin1String("null");
+
+    if (!object)
+        return QString::fromUtf8(metaObject->className()) + QLatin1String("(0x0)");
+
+    const int id = metaObject->indexOfMethod("toString()");
+    if (id >= 0) {
+        const QMetaMethod method = metaObject->method(id);
+        const QMetaType returnType = method.returnMetaType();
+        QVariant result(returnType);
+        method.invoke(object, QGenericReturnArgument(returnType.name(), result.data()));
+        if (result.metaType() == QMetaType::fromType<QString>())
+            return result.toString();
+        QV4::Scope scope(engine);
+        QV4::ScopedValue value(scope, engine->fromVariant(result));
+        return value->toQString();
+    }
+
+    QString result;
+    result += QString::fromUtf8(metaObject->className()) +
+            QLatin1String("(0x") + QString::number(quintptr(object), 16);
+    QString objectName = object->objectName();
+    if (!objectName.isEmpty())
+        result += QLatin1String(", \"") + objectName + QLatin1Char('\"');
+    result += QLatin1Char(')');
+    return result;
+}
+
 struct QObjectSlotDispatcher : public QtPrivate::QSlotObjectBase
 {
     PersistentValue function;
@@ -2385,12 +2417,9 @@ static QObject *qObject(const Value *v)
     return nullptr;
 }
 
-ReturnedValue QObjectMethod::method_toString(ExecutionEngine *engine, const QV4::Value *thisObject) const
+ReturnedValue QObjectMethod::method_toString(
+        ExecutionEngine *engine, const QV4::Value *thisObject) const
 {
-    const auto encode = [engine](const QString &result) {
-        return engine->newString(result)->asReturnedValue();
-    };
-
     QObject *o = qObject(thisObject);
 
     d()->assertIntegrity(o);
@@ -2398,31 +2427,9 @@ ReturnedValue QObjectMethod::method_toString(ExecutionEngine *engine, const QV4:
     const QMetaObject *metaObject = d()->metaObject();
     if (!metaObject && o)
         metaObject = o->metaObject();
-    if (metaObject) {
-        if (QObject *qobject = o ? o : d()->object()) {
-            const int id = metaObject->indexOfMethod("toString()");
-            if (id >= 0) {
-                const QMetaMethod method = metaObject->method(id);
-                const QMetaType returnType = method.returnMetaType();
-                QVariant result(returnType);
-                method.invoke(qobject, QGenericReturnArgument(returnType.name(), result.data()));
-                return engine->fromVariant(result);
-            }
-
-            QString result;
-            result += QString::fromUtf8(metaObject->className()) +
-                    QLatin1String("(0x") + QString::number(quintptr(qobject), 16);
-            QString objectName = qobject->objectName();
-            if (!objectName.isEmpty())
-                result += QLatin1String(", \"") + objectName + QLatin1Char('\"');
-            result += QLatin1Char(')');
-            return encode(result);
-        } else {
-            return encode(QString::fromUtf8(metaObject->className()) + QLatin1String("(0x0)"));
-        }
-    }
 
-    return encode(QLatin1String("null"));
+    return engine->newString(QObjectWrapper::objectToString(
+                                 engine, metaObject, o ? o : d()->object()))->asReturnedValue();
 }
 
 ReturnedValue QObjectMethod::method_destroy(ExecutionEngine *engine, const QV4::Value *thisObject, const Value *args, int argc) const
diff --git a/src/qml/jsruntime/qv4qobjectwrapper_p.h b/src/qml/jsruntime/qv4qobjectwrapper_p.h
index 043afe79e7..d17b12ea4a 100644
--- a/src/qml/jsruntime/qv4qobjectwrapper_p.h
+++ b/src/qml/jsruntime/qv4qobjectwrapper_p.h
@@ -194,6 +194,9 @@ struct Q_QML_EXPORT QObjectWrapper : public Object
 
     static int virtualMetacall(Object *object, QMetaObject::Call call, int index, void **a);
 
+    static QString objectToString(
+            ExecutionEngine *engine, const QMetaObject *metaObject, QObject *object);
+
 protected:
     static bool virtualIsEqualTo(Managed *that, Managed *o);
     static ReturnedValue create(ExecutionEngine *engine, QObject *object);