Support querying indirect extensions for a given object

Introduce a private version of qmlExtendedObject() that returns an
index-based extension (where 0 represents an extension on the leaf type
and N represents a (N - 1)th base type's extension)

Teach QQmlProxyMetaObject to distinguish different extension proxies.
Its custom metaCall can now query up to 128 extensions (should be enough
for the user needs)

Change-Id: I5520a1e84501f1f9fe6a8e77d8269009a12c255c
Reviewed-by: Maximilian Goldstein <max.goldstein@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

4 files changed, 32 insertions, 7 deletions

diff --git a/src/qml/qml/qqml.cpp b/src/qml/qml/qqml.cpp
index 728250dcf2..3c9f930eb9 100644
--- a/src/qml/qml/qqml.cpp
+++ b/src/qml/qml/qqml.cpp
@@ -151,6 +151,11 @@ QObject *qmlAttachedPropertiesObject(QObject *object, QQmlAttachedPropertiesFunc
 
 QObject *qmlExtendedObject(QObject *object)
 {
+    return QQmlPrivate::qmlExtendedObject(object, 0);
+}
+
+QObject *QQmlPrivate::qmlExtendedObject(QObject *object, int index)
+{
     if (!object)
         return nullptr;
 
@@ -161,8 +166,8 @@ QObject *qmlExtendedObject(QObject *object)
 
     const int id = d->metaObject->metaCall(
                 object, QMetaObject::CustomCall,
-                QQmlProxyMetaObject::ExtensionObjectId, &result);
-    if (id != QQmlProxyMetaObject::ExtensionObjectId)
+                QQmlProxyMetaObject::extensionObjectId(index), &result);
+    if (id != QQmlProxyMetaObject::extensionObjectId(index))
         return nullptr;
 
     return static_cast<QObject *>(result);
diff --git a/src/qml/qml/qqmlprivate.h b/src/qml/qml/qqmlprivate.h
index 817f25c112..9754f9dcf3 100644
--- a/src/qml/qml/qqmlprivate.h
+++ b/src/qml/qml/qqmlprivate.h
@@ -1087,6 +1087,8 @@ namespace QQmlPrivate
         };
     }
 
+    Q_QML_EXPORT QObject *qmlExtendedObject(QObject *, int);
+
 } // namespace QQmlPrivate
 
 QT_END_NAMESPACE
diff --git a/src/qml/qml/qqmlproxymetaobject.cpp b/src/qml/qml/qqmlproxymetaobject.cpp
index e05c63794c..4844ca3720 100644
--- a/src/qml/qml/qqmlproxymetaobject.cpp
+++ b/src/qml/qml/qqmlproxymetaobject.cpp
@@ -144,11 +144,15 @@ int QQmlProxyMetaObject::metaCall(QObject *o, QMetaObject::Call c, int id, void
 
         break;
     }
-    case QMetaObject::CustomCall:
-        if (id != ExtensionObjectId)
+    case QMetaObject::CustomCall: {
+        if ((id & ~MaxExtensionCount) != ExtensionObjectId)
             break;
-        a[0] = getProxy(0);
+        int index = id & MaxExtensionCount;
+        if (qsizetype(index) >= metaObjects->size())
+            break;
+        a[0] = getProxy(index);
         return id;
+    }
     default:
         break;
     }
diff --git a/src/qml/qml/qqmlproxymetaobject_p.h b/src/qml/qml/qqmlproxymetaobject_p.h
index a58818f22a..965d86c70f 100644
--- a/src/qml/qml/qqmlproxymetaobject_p.h
+++ b/src/qml/qml/qqmlproxymetaobject_p.h
@@ -65,8 +65,6 @@ QT_BEGIN_NAMESPACE
 class QQmlProxyMetaObject : public QDynamicMetaObjectData
 {
 public:
-    enum { ExtensionObjectId = std::numeric_limits<int>::min() };
-
     struct ProxyData {
         typedef QObject *(*CreateFunc)(QObject *);
         QMetaObject *metaObject;
@@ -78,6 +76,13 @@ public:
     QQmlProxyMetaObject(QObject *, QList<ProxyData> *);
     ~QQmlProxyMetaObject();
 
+    static constexpr int extensionObjectId(int id) noexcept
+    {
+        Q_ASSERT(id >= 0);
+        Q_ASSERT(id <= MaxExtensionCount); // MaxExtensionCount is a valid index
+        return ExtensionObjectId | id;
+    }
+
 protected:
     int metaCall(QObject *o, QMetaObject::Call _c, int _id, void **_a) override;
     QMetaObject *toDynamicMetaObject(QObject *) override;
@@ -91,6 +96,15 @@ private:
     QDynamicMetaObjectData *parent;
     QMetaObject *metaObject;
     QObject *object;
+
+    // ExtensionObjectId acts as a flag for whether we should interpret a
+    // QMetaObject::CustomCall as a call to fetch the extension object (see
+    // QQmlProxyMetaObject::metaCall()). MaxExtensionCount is a limit on how
+    // many extensions we can query via such mechanism
+    enum : int {
+        MaxExtensionCount = 127, // magic number so that low bits are all '1'
+        ExtensionObjectId = ~MaxExtensionCount,
+    };
 };
 
 QT_END_NAMESPACE