Add Qt.uiLanguage and QJSEngine::uiLanguage properties

[ChangeLog][QtQml] Added Qt.uiLanguage and QJSEngine::uiLanguage properties

These properties mirror the same value in QML and C++ and can be used
freely. They also provide API symmetry to Qt for MCUs.
QQmlApplicationEngine binds to this property and applies translations
accordingly by constructing a QLocale with the value and using
QTranslator::load(locale).

Change-Id: Id87d6ee64679b07ff3cb47844594e8eeebd8c8b6
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Christian Kamm <mail@ckamm.de>

3 files changed, 38 insertions, 8 deletions

diff --git a/src/qml/jsapi/qjsengine.cpp b/src/qml/jsapi/qjsengine.cpp
index 065fbc1c0a..6946a64d11 100644
--- a/src/qml/jsapi/qjsengine.cpp
+++ b/src/qml/jsapi/qjsengine.cpp
@@ -71,11 +71,6 @@
 #include <private/qqmlglobal_p.h>
 #include <qqmlengine.h>
 
-#undef Q_D
-#undef Q_Q
-#define Q_D(blah)
-#define Q_Q(blah)
-
 Q_DECLARE_METATYPE(QList<int>)
 
 /*!
@@ -257,7 +252,7 @@ Q_DECLARE_METATYPE(QList<int>)
     \l installExtensions().
 
     \value TranslationExtension Indicates that translation functions (\c qsTr(),
-        for example) should be installed.
+        for example) should be installed. This also installs the Qt.uiLanguage property.
 
     \value ConsoleExtension Indicates that console functions (\c console.log(),
         for example) should be installed.
@@ -692,7 +687,6 @@ QJSValue QJSEngine::newArray(uint length)
 */
 QJSValue QJSEngine::newQObject(QObject *object)
 {
-    Q_D(QJSEngine);
     QV4::ExecutionEngine *v4 = m_v4Engine;
     QV4::Scope scope(v4);
     if (object) {
@@ -719,7 +713,6 @@ QJSValue QJSEngine::newQObject(QObject *object)
 */
 
 QJSValue QJSEngine::newQMetaObject(const QMetaObject* metaObject) {
-    Q_D(QJSEngine);
     QV4::ExecutionEngine *v4 = m_v4Engine;
     QV4::Scope scope(v4);
     QV4::ScopedValue v(scope, QV4::QMetaObjectWrapper::create(v4, metaObject));
@@ -1004,6 +997,35 @@ void QJSEngine::throwError(QJSValue::ErrorType errorType, const QString &message
     m_v4Engine->throwError(e);
 }
 
+/*!
+  \property QJSEngine::uiLanguage
+  \brief the language to be used for translating user interface strings
+  \since 5.15
+
+  This property holds the name of the language to be used for user interface
+  string translations. It is exposed for reading and writing as \c{Qt.uiLanguage} when
+  the QJSEngine::TranslationExtension is installed on the engine. It is always exposed
+  in instances of QQmlEngine.
+
+  You can set the value freely and use it in bindings. It is recommended to set it
+  after installing translators in your application. By convention, an empty string
+  means no translation from the language used in the source code is intended to occur.
+*/
+void QJSEngine::setUiLanguage(const QString &language)
+{
+    Q_D(QJSEngine);
+    if (language == d->uiLanguage)
+        return;
+    d->uiLanguage = language;
+    emit uiLanguageChanged();
+}
+
+QString QJSEngine::uiLanguage() const
+{
+    Q_D(const QJSEngine);
+    return d->uiLanguage;
+}
+
 QJSEnginePrivate *QJSEnginePrivate::get(QV4::ExecutionEngine *e)
 {
     return e->jsEngine()->d_func();
diff --git a/src/qml/jsapi/qjsengine.h b/src/qml/jsapi/qjsengine.h
index 31a4d68baa..31229e1f20 100644
--- a/src/qml/jsapi/qjsengine.h
+++ b/src/qml/jsapi/qjsengine.h
@@ -60,6 +60,7 @@ class Q_QML_EXPORT QJSEngine
     : public QObject
 {
     Q_OBJECT
+    Q_PROPERTY(QString uiLanguage READ uiLanguage WRITE setUiLanguage NOTIFY uiLanguageChanged)
 public:
     QJSEngine();
     explicit QJSEngine(QObject *parent);
@@ -121,6 +122,12 @@ public:
     void throwError(const QString &message);
     void throwError(QJSValue::ErrorType errorType, const QString &message = QString());
 
+    QString uiLanguage() const;
+    void setUiLanguage(const QString &language);
+
+Q_SIGNALS:
+    void uiLanguageChanged();
+
 private:
     QJSValue create(int type, const void *ptr);
 
diff --git a/src/qml/jsapi/qjsengine_p.h b/src/qml/jsapi/qjsengine_p.h
index 164a70d000..7866a5bdda 100644
--- a/src/qml/jsapi/qjsengine_p.h
+++ b/src/qml/jsapi/qjsengine_p.h
@@ -107,6 +107,7 @@ public:
     // Shared by QQmlEngine
     mutable QRecursiveMutex mutex;
 
+    QString uiLanguage;
 
     // These methods may be called from the QML loader thread
     inline QQmlPropertyCache *cache(QObject *obj, int minorVersion = -1);