Introduce required properties to QML

[ChangeLog][QtQml]
"required" is now a (contextual) keyword in QML, and users can
mark properties with it to specify that those properties must be set
when the component gets instantiated.
This can be done either declaratively via standard property
bindings from QML, or imperatively by using the functions to set initial
properties (QQmlCompoent::setInitalProperties and related functions in
C++, Qt.createObject, Loader.setSource,... in QML/JS).
Logic has been added to QQmlComponent::create and the various QQmlIncubator
classes to verify that the required properties were set. If properties
marked as required are not set, a warning will be printed at runtime,
and the component will not be created.

Change-Id: I8e38227fc8f173b053b689c1597dc7fd40e835e7
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

12 files changed, 293 insertions, 20 deletions

diff --git a/src/qml/common/qv4compileddata_p.h b/src/qml/common/qv4compileddata_p.h
index c3ddce5884..11de506a53 100644
--- a/src/qml/common/qv4compileddata_p.h
+++ b/src/qml/common/qv4compileddata_p.h
@@ -75,7 +75,7 @@ QT_BEGIN_NAMESPACE
 // Also change the comment behind the number to describe the latest change. This has the added
 // benefit that if another patch changes the version too, it will result in a merge conflict, and
 // not get removed silently.
-#define QV4_DATA_STRUCTURE_VERSION 0x24 // Collect function parameter types
+#define QV4_DATA_STRUCTURE_VERSION 0x26// support required properties
 
 class QIODevice;
 class QQmlTypeNameCache;
@@ -656,7 +656,8 @@ struct Property
 {
     quint32_le nameIndex;
     union {
-        quint32_le_bitfield<0, 29> builtinTypeOrTypeNameIndex;
+        quint32_le_bitfield<0, 28> builtinTypeOrTypeNameIndex;
+        quint32_le_bitfield<28, 1> isRequired;
         quint32_le_bitfield<29, 1> isBuiltinType;
         quint32_le_bitfield<30, 1> isList;
         quint32_le_bitfield<31, 1> isReadOnly;
diff --git a/src/qml/compiler/qqmlirbuilder.cpp b/src/qml/compiler/qqmlirbuilder.cpp
index afe68b72cb..fbb6a07b1b 100644
--- a/src/qml/compiler/qqmlirbuilder.cpp
+++ b/src/qml/compiler/qqmlirbuilder.cpp
@@ -847,6 +847,7 @@ bool IRBuilder::visit(QQmlJS::AST::UiPublicMember *node)
 
             Property *property = New<Property>();
             property->isReadOnly = node->isReadonlyMember;
+            property->isRequired = node->isRequired;
 
             QV4::CompiledData::BuiltinType builtinPropertyType = Parameter::stringToBuiltinType(memberType);
             bool typeFound = builtinPropertyType != QV4::CompiledData::BuiltinType::InvalidBuiltin;
diff --git a/src/qml/parser/qqmljs.g b/src/qml/parser/qqmljs.g
index b54232442c..cc560c5912 100644
--- a/src/qml/parser/qqmljs.g
+++ b/src/qml/parser/qqmljs.g
@@ -89,6 +89,7 @@
 %token T_STATIC "static"
 %token T_EXPORT "export"
 %token T_FROM "from"
+%token T_REQUIRED "required"
 
 --- template strings
 %token T_NO_SUBSTITUTION_TEMPLATE"(no subst template)"
@@ -121,7 +122,7 @@
 %token T_FOR_LOOKAHEAD_OK "(for lookahead ok)"
 
 --%left T_PLUS T_MINUS
-%nonassoc T_IDENTIFIER T_COLON T_SIGNAL T_PROPERTY T_READONLY T_ON T_SET T_GET T_OF T_STATIC T_FROM T_AS
+%nonassoc T_IDENTIFIER T_COLON T_SIGNAL T_PROPERTY T_READONLY T_ON T_SET T_GET T_OF T_STATIC T_FROM T_AS T_REQUIRED
 %nonassoc REDUCE_HERE
 %right T_THEN T_ELSE
 
@@ -1216,7 +1217,6 @@ UiObjectMemberPropertyNoInitialiser: T_PROPERTY UiPropertyType QmlIdentifier Sem
 ./
 
 
-
 UiObjectMember: UiObjectMemberPropertyNoInitialiser;
 
 UiObjectMember: T_DEFAULT UiObjectMemberPropertyNoInitialiser;
@@ -1245,6 +1245,17 @@ OptionalSemicolon: | Semicolon;
   and then we would miss a semicolon (see tests/auto/quick/qquickvisualdatamodel/data/objectlist.qml)*/
  ./
 
+UiObjectMember: T_REQUIRED UiObjectMemberPropertyNoInitialiser;
+/.
+    case $rule_number: {
+        AST::UiPublicMember *node = sym(2).UiPublicMember;
+        node->requiredToken = loc(1);
+        node->isRequired = true;
+        sym(1).Node = node;
+    } break;
+./
+
+
 UiObjectMemberWithScriptStatement: T_PROPERTY UiPropertyType QmlIdentifier T_COLON UiScriptStatement OptionalSemicolon;
 /.
     case $rule_number: {
@@ -1459,6 +1470,7 @@ QmlIdentifier: T_GET;
 QmlIdentifier: T_SET;
 QmlIdentifier: T_FROM;
 QmlIdentifier: T_OF;
+QmlIdentifier: T_REQUIRED;
 
 JsIdentifier: T_IDENTIFIER;
 JsIdentifier: T_PROPERTY;
@@ -1471,6 +1483,7 @@ JsIdentifier: T_FROM;
 JsIdentifier: T_STATIC;
 JsIdentifier: T_OF;
 JsIdentifier: T_AS;
+JsIdentifier: T_REQUIRED;
 
 IdentifierReference: JsIdentifier;
 BindingIdentifier: IdentifierReference;
diff --git a/src/qml/parser/qqmljsast_p.h b/src/qml/parser/qqmljsast_p.h
index 6c28903066..4247785905 100644
--- a/src/qml/parser/qqmljsast_p.h
+++ b/src/qml/parser/qqmljsast_p.h
@@ -3308,6 +3308,8 @@ public:
         return defaultToken;
       else if (readonlyToken.isValid())
           return readonlyToken;
+      else if (requiredToken.isValid())
+          return requiredToken;
 
       return propertyToken;
     }
@@ -3331,10 +3333,13 @@ public:
     UiObjectMember *binding; // initialized with a QML object or array.
     bool isDefaultMember;
     bool isReadonlyMember;
+    bool isRequired = false;
     UiParameterList *parameters;
+    // TODO: merge source locations
     SourceLocation defaultToken;
     SourceLocation readonlyToken;
     SourceLocation propertyToken;
+    SourceLocation requiredToken;
     SourceLocation typeModifierToken;
     SourceLocation typeToken;
     SourceLocation identifierToken;
diff --git a/src/qml/parser/qqmljskeywords_p.h b/src/qml/parser/qqmljskeywords_p.h
index 96b3709162..3eb054341f 100644
--- a/src/qml/parser/qqmljskeywords_p.h
+++ b/src/qml/parser/qqmljskeywords_p.h
@@ -743,6 +743,18 @@ static inline int classify8(const QChar *s, int parseModeFlags) {
             }
           }
         }
+      } else if (s[2].unicode() == 'q') {
+        if (s[3].unicode() == 'u') {
+          if (s[4].unicode() == 'i') {
+            if (s[5].unicode() == 'r') {
+              if (s[6].unicode() == 'e') {
+                if (s[7].unicode() == 'd') {
+                  return Lexer::T_REQUIRED;
+                }
+              }
+            }
+          }
+        }
       }
     }
   }
diff --git a/src/qml/qml/qqmlcomponent.cpp b/src/qml/qml/qqmlcomponent.cpp
index ed8c41a582..b72c745490 100644
--- a/src/qml/qml/qqmlcomponent.cpp
+++ b/src/qml/qml/qqmlcomponent.cpp
@@ -340,6 +340,11 @@ void QQmlComponentPrivate::fromTypeData(const QQmlRefPointer<QQmlTypeData> &data
     }
 }
 
+RequiredProperties &QQmlComponentPrivate::requiredProperties()
+{
+    return state.creator->requiredProperties();
+}
+
 void QQmlComponentPrivate::clear()
 {
     if (typeData) {
@@ -364,8 +369,8 @@ QObject *QQmlComponentPrivate::doBeginCreate(QQmlComponent *q, QQmlContext *cont
 
 bool QQmlComponentPrivate::setInitialProperty(QObject *component, const QString& name, const QVariant &value)
 {
-    QQmlProperty prop(component, name);
-    auto privProp = QQmlPropertyPrivate::get(prop);
+    QQmlProperty prop = QQmlComponentPrivate::removePropertyFromRequired(component, name, requiredProperties());
+    QQmlPropertyPrivate *privProp = QQmlPropertyPrivate::get(prop);
     if (!prop.isValid() || !privProp->writeValueProperty(value, nullptr)) {
         QQmlError error{};
         error.setUrl(url);
@@ -809,6 +814,10 @@ QObject *QQmlComponent::create(QQmlContext *context)
     QObject *rv = d->doBeginCreate(this, context);
     if (rv)
         completeCreate();
+    if (rv && !d->requiredProperties().empty()) {
+        delete  rv;
+        return nullptr;
+    }
     return rv;
 }
 
@@ -828,6 +837,10 @@ QObject *QQmlComponent::createWithInitialProperties(const QVariantMap& initialPr
         setInitialProperties(rv, initialProperties);
         completeCreate();
     }
+    if (!d->requiredProperties().empty()) {
+        d->requiredProperties().clear();
+        return nullptr;
+    }
     return rv;
 }
 
@@ -980,6 +993,57 @@ void QQmlComponentPrivate::complete(QQmlEnginePrivate *enginePriv, ConstructionS
 }
 
 /*!
+ * \internal
+ * Finds the matching toplevel property with name \a name of the component \a createdComponent.
+ * If it was a required property or an alias to a required property contained in \a
+ * requiredProperties, it is removed from it.
+ *
+ * If wasInRequiredProperties is non-null, the referenced boolean is set to true iff the property
+ * was found in requiredProperties.
+ *
+ * Returns the QQmlProperty with name \a name (which might be invalid if there is no such property),
+ * for further processing (for instance, actually setting the property value).
+ *
+ * Note: This method is used in QQmlComponent and QQmlIncubator to manage required properties. Most
+ * classes which create components should not need it and should only need to call
+ * setInitialProperties.
+ */
+QQmlProperty QQmlComponentPrivate::removePropertyFromRequired(QObject *createdComponent, const QString &name, RequiredProperties &requiredProperties, bool* wasInRequiredProperties)
+{
+    QQmlProperty prop(createdComponent, name);
+    auto privProp = QQmlPropertyPrivate::get(prop);
+    if (prop.isValid()) {
+        // resolve outstanding required properties
+        auto targetProp = &privProp->core;
+        if (targetProp->isAlias()) {
+            auto target = createdComponent;
+            QQmlPropertyIndex originalIndex(targetProp->coreIndex());
+            QQmlPropertyIndex propIndex;
+            QQmlPropertyPrivate::findAliasTarget(target, originalIndex, &target, &propIndex);
+            QQmlData *data = QQmlData::get(target);
+            Q_ASSERT(data && data->propertyCache);
+            targetProp = data->propertyCache->property(propIndex.coreIndex());
+        } else {
+            // we need to get the pointer from the property cache instead of directly using
+            // targetProp else the lookup will fail
+            QQmlData *data = QQmlData::get(createdComponent);
+            Q_ASSERT(data && data->propertyCache);
+            targetProp = data->propertyCache->property(targetProp->coreIndex());
+        }
+        auto it = requiredProperties.find(targetProp);
+        if (it != requiredProperties.end()) {
+            if (wasInRequiredProperties)
+                *wasInRequiredProperties = true;
+            requiredProperties.erase(it);
+        } else {
+            if (wasInRequiredProperties)
+                *wasInRequiredProperties = false;
+        }
+    }
+    return prop;
+}
+
+/*!
     This method provides advanced control over component instance creation.
     In general, programmers should use QQmlComponent::create() to create a
     component.
@@ -998,6 +1062,11 @@ void QQmlComponent::completeCreate()
 
 void QQmlComponentPrivate::completeCreate()
 {
+    const RequiredProperties& unsetRequiredProperties = requiredProperties();
+    for (const auto& unsetRequiredProperty: unsetRequiredProperties) {
+        QQmlError error = unsetRequiredPropertyToQQmlError(unsetRequiredProperty);
+        state.errors.push_back(error);
+    }
     if (state.completePending) {
         ++creationDepth.localData();
         QQmlEnginePrivate *ep = QQmlEnginePrivate::get(engine);
@@ -1185,7 +1254,7 @@ struct QmlIncubatorObject : public QV4::Object
     static ReturnedValue method_forceCompletion(const FunctionObject *, const Value *thisObject, const Value *argv, int argc);
 
     void statusChanged(QQmlIncubator::Status);
-    void setInitialState(QObject *);
+    void setInitialState(QObject *, RequiredProperties &requiredProperties);
 };
 
 }
@@ -1210,7 +1279,8 @@ public:
     void setInitialState(QObject *o) override {
         QV4::Scope scope(incubatorObject.engine());
         QV4::Scoped<QV4::QmlIncubatorObject> i(scope, incubatorObject.as<QV4::QmlIncubatorObject>());
-        i->setInitialState(o);
+        auto d = QQmlIncubatorPrivate::get(this);
+        i->setInitialState(o, d->requiredProperties());
     }
 
     QV4::PersistentValue incubatorObject; // keep a strong internal reference while incubating
@@ -1282,7 +1352,7 @@ static void QQmlComponent_setQmlParent(QObject *me, QObject *parent)
 */
 
 
-void QQmlComponentPrivate::setInitialProperties(QV4::ExecutionEngine *engine, QV4::QmlContext *qmlContext, const QV4::Value &o, const QV4::Value &v)
+void QQmlComponentPrivate::setInitialProperties(QV4::ExecutionEngine *engine, QV4::QmlContext *qmlContext, const QV4::Value &o, const QV4::Value &v, RequiredProperties &requiredProperties, QObject *createdComponent)
 {
     QV4::Scope scope(engine);
     QV4::ScopedObject object(scope);
@@ -1301,6 +1371,7 @@ void QQmlComponentPrivate::setInitialProperties(QV4::ExecutionEngine *engine, QV
             break;
         object = o;
         const QStringList properties = name->toQString().split(QLatin1Char('.'));
+        bool isTopLevelProperty = properties.size() == 1;
         for (int i = 0; i < properties.length() - 1; ++i) {
             name = engine->newString(properties.at(i));
             object = object->get(name);
@@ -1317,12 +1388,40 @@ void QQmlComponentPrivate::setInitialProperties(QV4::ExecutionEngine *engine, QV
         if (engine->hasException) {
             engine->hasException = false;
             continue;
+        } else if (isTopLevelProperty) {
+            auto prop = removePropertyFromRequired(createdComponent, name->toQString(), requiredProperties);
         }
     }
 
     engine->hasException = false;
 }
 
+QQmlError QQmlComponentPrivate::unsetRequiredPropertyToQQmlError(const RequiredPropertyInfo &unsetRequiredProperty)
+{
+    QQmlError error;
+    QString description = QLatin1String("Required property %1 was not initialized").arg(unsetRequiredProperty.propertyName);
+    switch (unsetRequiredProperty.aliasesToRequired.size()) {
+    case 0:
+        break;
+    case 1: {
+        const auto info = unsetRequiredProperty.aliasesToRequired.first();
+        description += QLatin1String("\nIt can be set via the alias property %1 from %2\n").arg(info.propertyName, info.fileUrl.toString());
+        break;
+    }
+    default:
+        description += QLatin1String("\nIt can be set via one of the following alias properties:");
+        for (auto aliasInfo: unsetRequiredProperty.aliasesToRequired) {
+            description += QLatin1String("\n- %1 (%2)").arg(aliasInfo.propertyName, aliasInfo.fileUrl.toString());
+        }
+        description += QLatin1Char('\n');
+    }
+    error.setDescription(description);
+    error.setUrl(unsetRequiredProperty.fileUrl);
+    error.setLine(unsetRequiredProperty.location.line);
+    error.setColumn(unsetRequiredProperty.location.column);
+    return  error;
+}
+
 /*!
     \internal
 */
@@ -1370,7 +1469,17 @@ void QQmlComponent::createObject(QQmlV4Function *args)
 
     if (!valuemap->isUndefined()) {
         QV4::Scoped<QV4::QmlContext> qmlContext(scope, v4->qmlContext());
-        QQmlComponentPrivate::setInitialProperties(v4, qmlContext, object, valuemap);
+        QQmlComponentPrivate::setInitialProperties(v4, qmlContext, object, valuemap, d->requiredProperties(), rv);
+    }
+    if (!d->requiredProperties().empty()) {
+        QList<QQmlError> errors;
+        for (const auto &requiredProperty: d->requiredProperties()) {
+            errors.push_back(QQmlComponentPrivate::unsetRequiredPropertyToQQmlError(requiredProperty));
+        }
+        qmlWarning(rv, errors);
+        args->setReturnValue(QV4::Encode::null());
+        delete rv;
+        return;
     }
 
     d->completeCreate();
@@ -1502,7 +1611,7 @@ void QQmlComponent::incubateObject(QQmlV4Function *args)
 }
 
 // XXX used by QSGLoader
-void QQmlComponentPrivate::initializeObjectWithInitialProperties(QV4::QmlContext *qmlContext, const QV4::Value &valuemap, QObject *toCreate)
+void QQmlComponentPrivate::initializeObjectWithInitialProperties(QV4::QmlContext *qmlContext, const QV4::Value &valuemap, QObject *toCreate, RequiredProperties &requiredProperties)
 {
     QV4::ExecutionEngine *v4engine = engine->handle();
     QV4::Scope scope(v4engine);
@@ -1511,7 +1620,7 @@ void QQmlComponentPrivate::initializeObjectWithInitialProperties(QV4::QmlContext
     Q_ASSERT(object->as<QV4::Object>());
 
     if (!valuemap.isUndefined())
-        setInitialProperties(v4engine, qmlContext, object, valuemap);
+        setInitialProperties(v4engine, qmlContext, object, valuemap, requiredProperties, toCreate);
 }
 
 QQmlComponentExtension::QQmlComponentExtension(QV4::ExecutionEngine *v4)
@@ -1601,7 +1710,7 @@ void QV4::Heap::QmlIncubatorObject::destroy() {
     Object::destroy();
 }
 
-void QV4::QmlIncubatorObject::setInitialState(QObject *o)
+void QV4::QmlIncubatorObject::setInitialState(QObject *o, RequiredProperties &requiredProperties)
 {
     QQmlComponent_setQmlParent(o, d()->parent);
 
@@ -1610,7 +1719,7 @@ void QV4::QmlIncubatorObject::setInitialState(QObject *o)
         QV4::Scope scope(v4);
         QV4::ScopedObject obj(scope, QV4::QObjectWrapper::wrap(v4, o));
         QV4::Scoped<QV4::QmlContext> qmlCtxt(scope, d()->qmlContext);
-        QQmlComponentPrivate::setInitialProperties(v4, qmlCtxt, obj, d()->valuemap);
+        QQmlComponentPrivate::setInitialProperties(v4, qmlCtxt, obj, d()->valuemap, requiredProperties, o);
     }
 }
 
diff --git a/src/qml/qml/qqmlcomponent_p.h b/src/qml/qml/qqmlcomponent_p.h
index 2170646b89..8ae7672c19 100644
--- a/src/qml/qml/qqmlcomponent_p.h
+++ b/src/qml/qml/qqmlcomponent_p.h
@@ -86,8 +86,9 @@ public:
 
     QObject *beginCreate(QQmlContextData *);
     void completeCreate();
-    void initializeObjectWithInitialProperties(QV4::QmlContext *qmlContext, const QV4::Value &valuemap, QObject *toCreate);
-    static void setInitialProperties(QV4::ExecutionEngine *engine, QV4::QmlContext *qmlContext, const QV4::Value &o, const QV4::Value &v);
+    void initializeObjectWithInitialProperties(QV4::QmlContext *qmlContext, const QV4::Value &valuemap, QObject *toCreate, RequiredProperties &requiredProperties);
+    static void setInitialProperties(QV4::ExecutionEngine *engine, QV4::QmlContext *qmlContext, const QV4::Value &o, const QV4::Value &v, RequiredProperties &requiredProperties, QObject *createdComponent);
+    static QQmlError unsetRequiredPropertyToQQmlError(const RequiredPropertyInfo &unsetRequiredProperty);
 
     virtual void incubateObject(
             QQmlIncubator *incubationTask,
@@ -106,6 +107,7 @@ public:
     qreal progress;
 
     int start;
+    RequiredProperties& requiredProperties();
     QQmlRefPointer<QV4::ExecutableCompilationUnit> compilationUnit;
 
     struct ConstructionState {
@@ -134,6 +136,7 @@ public:
     static void completeDeferred(QQmlEnginePrivate *enginePriv, DeferredState *deferredState);
 
     static void complete(QQmlEnginePrivate *enginePriv, ConstructionState *state);
+    static QQmlProperty removePropertyFromRequired(QObject *createdComponent, const QString &name, RequiredProperties& requiredProperties, bool *wasInRequiredProperties = nullptr);
 
     QQmlEngine *engine;
     QQmlGuardedContextData creationContext;
diff --git a/src/qml/qml/qqmlincubator.cpp b/src/qml/qml/qqmlincubator.cpp
index bc06226cbf..7b3ae31c08 100644
--- a/src/qml/qml/qqmlincubator.cpp
+++ b/src/qml/qml/qqmlincubator.cpp
@@ -43,6 +43,7 @@
 
 #include "qqmlexpression_p.h"
 #include "qqmlobjectcreator_p.h"
+#include <private/qqmlcomponent_p.h>
 
 void QQmlEnginePrivate::incubate(QQmlIncubator &i, QQmlContextData *forContext)
 {
@@ -296,6 +297,20 @@ void QQmlIncubatorPrivate::incubate(QQmlInstantiationInterrupt &i)
         tresult = creator->create(subComponentToCreate, /*parent*/nullptr, &i);
         if (!tresult)
             errors = creator->errors;
+        else {
+           RequiredProperties& requiredProperties = creator->requiredProperties();
+           for (auto it = initialProperties.cbegin(); it != initialProperties.cend(); ++it) {
+               auto component = tresult;
+               auto name = it.key();
+               QQmlProperty prop = QQmlComponentPrivate::removePropertyFromRequired(component, name, requiredProperties);
+               if (!prop.isValid() || !prop.write(it.value())) {
+                   QQmlError error{};
+                   error.setUrl(compilationUnit->url());
+                   error.setDescription(QLatin1String("Could not set property %1").arg(name));
+                   errors.push_back(error);
+               }
+           }
+        }
         enginePriv->dereferenceScarceResources();
 
         if (watcher.hasRecursed())
@@ -312,8 +327,14 @@ void QQmlIncubatorPrivate::incubate(QQmlInstantiationInterrupt &i)
             ddata->indestructible = true;
             ddata->explicitIndestructibleSet = true;
             ddata->rootObjectInCreation = false;
-            if (q)
+            if (q) {
                 q->setInitialState(result);
+                if (!creator->requiredProperties().empty()) {
+                    const auto& unsetRequiredProperties = creator->requiredProperties();
+                    for (const auto& unsetRequiredProperty: unsetRequiredProperties)
+                        errors << QQmlComponentPrivate::unsetRequiredPropertyToQQmlError(unsetRequiredProperty);
+                }
+            }
         }
 
         if (watcher.hasRecursed())
@@ -657,6 +678,31 @@ QObject *QQmlIncubator::object() const
 }
 
 /*!
+Return a list of properties which are required but haven't been set yet.
+This list can be modified, so that subclasses which implement special logic
+setInitialProperties can mark properties set there as no longer required.
+
+\sa QQmlIncubator::setInitialProperties
+\since 5.15
+*/
+RequiredProperties &QQmlIncubatorPrivate::requiredProperties()
+{
+    return creator->requiredProperties();
+}
+
+/*!
+Stores a mapping from property names to initial values with which the incubated
+component will be initialized
+
+\sa QQmlComponent::setInitialProperties
+\since 5.15
+*/
+void QQmlIncubator::setInitialProperties(const QVariantMap &initialProperties)
+{
+    d->initialProperties = initialProperties;
+}
+
+/*!
 Called when the status of the incubator changes.  \a status is the new status.
 
 The default implementation does nothing.
diff --git a/src/qml/qml/qqmlincubator.h b/src/qml/qml/qqmlincubator.h
index e68f6e3c45..f075407e73 100644
--- a/src/qml/qml/qqmlincubator.h
+++ b/src/qml/qml/qqmlincubator.h
@@ -47,6 +47,9 @@ QT_BEGIN_NAMESPACE
 
 
 class QQmlEngine;
+class QQmlPropertyData;
+class QVariant;
+using QVariantMap = QMap<QString, QVariant>;
 
 class QQmlIncubatorPrivate;
 class Q_QML_EXPORT QQmlIncubator
@@ -84,6 +87,8 @@ public:
 
     QObject *object() const;
 
+    void setInitialProperties(const QVariantMap &initialProperties);
+
 protected:
     virtual void statusChanged(Status);
     virtual void setInitialState(QObject *);
diff --git a/src/qml/qml/qqmlincubator_p.h b/src/qml/qml/qqmlincubator_p.h
index 57ec8249cb..731db7aad3 100644
--- a/src/qml/qml/qqmlincubator_p.h
+++ b/src/qml/qml/qqmlincubator_p.h
@@ -61,8 +61,12 @@
 
 QT_BEGIN_NAMESPACE
 
+class QQmlPropertyData;
+struct RequiredPropertyInfo;
+using RequiredProperties = QHash<QQmlPropertyData*, RequiredPropertyInfo>;
+
 class QQmlIncubator;
-class QQmlIncubatorPrivate : public QQmlEnginePrivate::Incubator
+class Q_QML_PRIVATE_EXPORT QQmlIncubatorPrivate : public QQmlEnginePrivate::Incubator
 {
 public:
     QQmlIncubatorPrivate(QQmlIncubator *q, QQmlIncubator::IncubationMode m);
@@ -97,11 +101,13 @@ public:
     QIntrusiveList<QIPBase, &QIPBase::nextWaitingFor> waitingFor;
 
     QRecursionNode recursion;
+    QVariantMap initialProperties;
 
     void clear();
 
     void forceCompletion(QQmlInstantiationInterrupt &i);
     void incubate(QQmlInstantiationInterrupt &i);
+    RequiredProperties &requiredProperties();
 };
 
 QT_END_NAMESPACE
diff --git a/src/qml/qml/qqmlobjectcreator.cpp b/src/qml/qml/qqmlobjectcreator.cpp
index f89608cd5d..b723ddb381 100644
--- a/src/qml/qml/qqmlobjectcreator.cpp
+++ b/src/qml/qml/qqmlobjectcreator.cpp
@@ -783,6 +783,23 @@ void QQmlObjectCreator::setupBindings(bool applyDeferredBindings)
 
     const QV4::CompiledData::Binding *binding = _compiledObject->bindingTable();
     for (quint32 i = 0; i < _compiledObject->nBindings; ++i, ++binding) {
+        QQmlPropertyData *const property = propertyData.at(i);
+        if (property) {
+            QQmlPropertyData* targetProperty = property;
+            if (targetProperty->isAlias()) {
+                // follow alias
+                auto target = _bindingTarget;
+                QQmlPropertyIndex originalIndex(targetProperty->coreIndex(), _valueTypeProperty ? _valueTypeProperty->coreIndex() : -1);
+                QQmlPropertyIndex propIndex;
+                QQmlPropertyPrivate::findAliasTarget(target, originalIndex, &target, &propIndex);
+                QQmlData *data = QQmlData::get(target);
+                Q_ASSERT(data && data->propertyCache);
+                targetProperty = data->propertyCache->property(propIndex.coreIndex());
+            }
+            sharedState->requiredProperties.remove(targetProperty);
+        }
+
+
         if (binding->flags & QV4::CompiledData::Binding::IsCustomParserBinding)
             continue;
 
@@ -794,8 +811,6 @@ void QQmlObjectCreator::setupBindings(bool applyDeferredBindings)
                 continue;
         }
 
-        const QQmlPropertyData *property = propertyData.at(i);
-
         if (property && property->isQList()) {
             if (property->coreIndex() != currentListPropertyIndex) {
                 void *argv[1] = { (void*)&_currentList };
@@ -1504,10 +1519,42 @@ bool QQmlObjectCreator::populateInstance(int index, QObject *instance, QObject *
     if (_compiledObject->flags & QV4::CompiledData::Object::HasDeferredBindings)
         _ddata->deferData(_compiledObjectIndex, compilationUnit, context);
 
+    for (int propertyIndex = 0; propertyIndex != _compiledObject->propertyCount(); ++propertyIndex) {
+        const QV4::CompiledData::Property* property = _compiledObject->propertiesBegin() + propertyIndex;
+        QQmlPropertyData *propertyData = _propertyCache->property(_propertyCache->propertyOffset() + propertyIndex);
+        if (property->isRequired) {
+            sharedState->requiredProperties.insert(propertyData,
+                                                   RequiredPropertyInfo {compilationUnit->stringAt(property->nameIndex), compilationUnit->finalUrl(), property->location, {}});
+        }
+    }
+
     if (_compiledObject->nFunctions > 0)
         setupFunctions();
     setupBindings();
 
+    for (int aliasIndex = 0; aliasIndex != _compiledObject->aliasCount(); ++aliasIndex) {
+        const QV4::CompiledData::Alias* alias = _compiledObject->aliasesBegin() + aliasIndex;
+        const auto originalAlias = alias;
+        while (alias->aliasToLocalAlias)
+            alias = _compiledObject->aliasesBegin() + alias->localAliasIndex;
+        Q_ASSERT(alias->flags & QV4::CompiledData::Alias::Resolved);
+        if (!context->idValues->wasSet())
+            continue;
+        QObject *target = context->idValues[alias->targetObjectId].data();
+        if (!target)
+            continue;
+        QQmlData *targetDData = QQmlData::get(target, /*create*/false);
+        if (!targetDData)
+            continue;
+        int coreIndex = QQmlPropertyIndex::fromEncoded(alias->encodedMetaPropertyIndex).coreIndex();
+        QQmlPropertyData *const targetProperty = targetDData->propertyCache->property(coreIndex);
+        if (!targetProperty)
+            continue;
+        auto it = sharedState->requiredProperties.find(targetProperty);
+        if (it != sharedState->requiredProperties.end())
+            it->aliasesToRequired.push_back(AliasToRequiredInfo {compilationUnit->stringAt(originalAlias->nameIndex), compilationUnit->finalUrl()});
+    }
+
     qSwap(_vmeMetaObject, vmeMetaObject);
     qSwap(_bindingTarget, bindingTarget);
     qSwap(_ddata, declarativeData);
diff --git a/src/qml/qml/qqmlobjectcreator_p.h b/src/qml/qml/qqmlobjectcreator_p.h
index ecdbcc56dd..ee1d82d4e3 100644
--- a/src/qml/qml/qqmlobjectcreator_p.h
+++ b/src/qml/qml/qqmlobjectcreator_p.h
@@ -66,6 +66,28 @@ class QQmlAbstractBinding;
 class QQmlInstantiationInterrupt;
 class QQmlIncubatorPrivate;
 
+struct AliasToRequiredInfo {
+    QString propertyName;
+    QUrl fileUrl;
+};
+
+/*!
+\internal
+This struct contains information solely used for displaying error messages
+\variable aliasesToRequired allows us to give the user a way to know which (aliasing) properties
+can be set to set the required property
+\sa QQmlComponentPrivate::unsetRequiredPropertyToQQmlError
+*/
+struct RequiredPropertyInfo
+{
+    QString propertyName;
+    QUrl fileUrl;
+    QV4::CompiledData::Location location;
+    QVector<AliasToRequiredInfo> aliasesToRequired;
+};
+
+using RequiredProperties = QHash<QQmlPropertyData*, RequiredPropertyInfo>;
+
 struct QQmlObjectCreatorSharedState : public QSharedData
 {
     QQmlContextData *rootContext;
@@ -78,6 +100,7 @@ struct QQmlObjectCreatorSharedState : public QSharedData
     QList<QQmlEnginePrivate::FinalizeCallback> finalizeCallbacks;
     QQmlVmeProfiler profiler;
     QRecursionNode recursionNode;
+    RequiredProperties requiredProperties;
 };
 
 class Q_QML_PRIVATE_EXPORT QQmlObjectCreator
@@ -102,6 +125,8 @@ public:
     QQmlContextData *parentContextData() const { return parentContext.contextData(); }
     QFiniteStack<QPointer<QObject> > &allCreatedObjects() { return sharedState->allCreatedObjects; }
 
+    RequiredProperties &requiredProperties() {return sharedState->requiredProperties;}
+
 private:
     QQmlObjectCreator(QQmlContextData *contextData, const QQmlRefPointer<QV4::ExecutableCompilationUnit> &compilationUnit, QQmlObjectCreatorSharedState *inheritedSharedState);