Doc: Warn about slowness of createQmlObject()

Pick-to: 6.5
Change-Id: Ie00d0df9bc8ad6106559ce0a66c91e0aaac81cfd
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 85885624446558c2bf9fa27cb5bded9433943312)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 12 insertions, 1 deletions

diff --git a/src/qml/doc/src/javascript/dynamicobjectcreation.qdoc b/src/qml/doc/src/javascript/dynamicobjectcreation.qdoc
index 29a4a67460..4342d3bada 100644
--- a/src/qml/doc/src/javascript/dynamicobjectcreation.qdoc
+++ b/src/qml/doc/src/javascript/dynamicobjectcreation.qdoc
@@ -102,6 +102,12 @@ It is also possible to instantiate components without blocking via the
 
 \section2 Creating an Object from a String of QML
 
+\warning Creating objects from a string of QML is extremely slow since the engine has to compile the
+passed QML string every time you do it. Furthermore, it's very easy to produce invalid QML when
+programmatically constructing QML code. It's much better to keep your QML components as separate
+files and add properties and methods to customize their behavior than to produce new components by
+string manipulation.
+
 If the QML is not defined until runtime, you can create a QML object from
 a string of QML using the \l{QtQml::Qt::createQmlObject()}{Qt.createQmlObject()}
 function, as in the following example:
diff --git a/src/qml/qml/qqmlbuiltinfunctions.cpp b/src/qml/qml/qqmlbuiltinfunctions.cpp
index ebb5c301db..1bec4f869d 100644
--- a/src/qml/qml/qqmlbuiltinfunctions.cpp
+++ b/src/qml/qml/qqmlbuiltinfunctions.cpp
@@ -1300,10 +1300,15 @@ Each object in this array has the members \c lineNumber, \c columnNumber, \c fil
 For example, if the above snippet had misspelled color as 'colro' then the array would contain an object like the following:
 { "lineNumber" : 1, "columnNumber" : 32, "fileName" : "dynamicSnippet1", "message" : "Cannot assign to non-existent property \"colro\""}.
 
-Note that this function returns immediately, and therefore may not work if
+\note This function returns immediately, and therefore may not work if
 the \a qml string loads new components (that is, external QML files that have not yet been loaded).
 If this is the case, consider using \l{QtQml::Qt::createComponent()}{Qt.createComponent()} instead.
 
+\warning This function is extremely slow since it has to compile the passed QML string every time
+it is invoked. Furthermore, it's very easy to produce invalid QML when programmatically constructing
+QML code. It's much better to keep your QML components as separate files and add properties and
+methods to customize their behavior than to produce new components by string manipulation.
+
 See \l {Dynamic QML Object Creation from JavaScript} for more information on using this function.
 */
 QObject *QtObject::createQmlObject(const QString &qml, QObject *parent, const QUrl &url) const