Store a QV4::ReturnedValue in QJSValue

Being careful, we can now save primitive values inline. We use the heap
pointer of QV4::Value as either QString* or QV4::Value* for complex
types. We cannot store persistent managed QV4::Value without the double
indirection as those need to be allocated in a special place.

The generic QVariant case is not supported anymore. The only place where
it was actually needed were the stream operators for QJSValue. Those
were fundamentally broken:

* A managed QJSValue saved and loaded from a stream was converted to a
  QVariant-type QJSValue
* QVariant-type QJSValues were not callable, could not be objects or
  arrays, or any of the special types.
* Cyclic references were forcibly broken when saving to a data stream.

In general the support for saving and loading of managed types to/from
a data stream was so abysmally bad that we don't lose much by dropping
it.

[ChangeLog][QML][Important Behavior Changes] When saving a QJSValue to a
QDataStream only primitive values or strings will be retained. Support
for objects and arrays was incomplete and unreliable already before. It
cannot work correctly as we don't necessarily have a JavaScript heap
when loading a QJSValue from a stream. Therefore, we don't have a proper
place to keep any managed values. Using QVariant to keep them instead is
a bad idea because QVariant cannot represent everything a QJSValue can
contain.

Fixes: QTBUG-75174
Change-Id: I75697670639bca8d4b1668763d7020c4cf871bda
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 5 insertions, 4 deletions

diff --git a/src/imports/statemachine/signaltransition.cpp b/src/imports/statemachine/signaltransition.cpp
index 69edaa4e48..51b2501a7b 100644
--- a/src/imports/statemachine/signaltransition.cpp
+++ b/src/imports/statemachine/signaltransition.cpp
@@ -106,15 +106,16 @@ void SignalTransition::setSignal(const QJSValue &signal)
     if (m_signal.strictlyEquals(signal))
         return;
 
-    m_signal = signal;
-
     QV4::ExecutionEngine *jsEngine = QQmlEngine::contextForObject(this)->engine()->handle();
     QV4::Scope scope(jsEngine);
 
     QObject *sender;
     QMetaMethod signalMethod;
 
-    QV4::ScopedValue value(scope, QJSValuePrivate::convertedToValue(jsEngine, m_signal));
+    m_signal = signal;
+    QJSValuePrivate::manageStringOnV4Heap(jsEngine, &m_signal);
+
+    QV4::ScopedValue value(scope, QJSValuePrivate::asReturnedValue(&m_signal));
 
     // Did we get the "slot" that can be used to invoke the signal?
     if (QV4::QObjectMethod *signalSlot = value->as<QV4::QObjectMethod>()) {
@@ -170,7 +171,7 @@ void SignalTransition::connectTriggered()
 
     QV4::ExecutionEngine *jsEngine = QQmlEngine::contextForObject(this)->engine()->handle();
     QV4::Scope scope(jsEngine);
-    QV4::Scoped<QV4::QObjectMethod> qobjectSignal(scope, QJSValuePrivate::convertedToValue(jsEngine, m_signal));
+    QV4::Scoped<QV4::QObjectMethod> qobjectSignal(scope, QJSValuePrivate::asReturnedValue(&m_signal));
     Q_ASSERT(qobjectSignal);
     QMetaMethod metaMethod = target->metaObject()->method(qobjectSignal->methodIndex());
     int signalIndex = QMetaObjectPrivate::signalIndex(metaMethod);