[Interest] Porting custom QML plugin to the Qt6

[Mitch Curtis]

See also https://bugreports.qt.io/browse/QTBUG-89274 and some of the related tasks.

> -----Original Message-----
> From: Interest <interest-bounces at qt-project.org> On Behalf Of Jakub
> Narolewski
> Sent: Wednesday, 28 April 2021 9:54 PM
> To: interest at qt-project.org
> Subject: [Interest] Porting custom QML plugin to the Qt6
> 
> Lately, mostly for fame and glory, I decided to port some of my stuff from
> Qt5 to Qt6.
> Most of it went ok'ish - after some CMakeLists.txt hacking and 'modernizing'
> the code I was able to compile two of my dynamic libraries under Qt6.
> 
> Problems started with my custom QML plugin. Here after compiling and
> importing my module in my example app I got:
> "qrc:/src/QML/MainWindow.qml:5:1: module "IzLibrary" plugin
> "IzLibraryPlugin" not found"
> 
> Well oopsie doopsie, some more work is needed. After quick look at:
> https://doc.qt.io/qt-6/qtqml-modules-cppplugins.html
> 
> I spotted, I believe, the most important changes. QQmlExtensionPlugin being
> changed to QmlEngineExtensionPlugin, QML_ELEMENT added in Qt 5.15.X
> and removal of QQmlExtensionPlugin::registerTypes.
> 
> Then I noticed stuff about QML_IMPORT_NAME,
> QML_IMPORT_MAJOR_VERSION and 'CONFIG += qmltypes' that are
> supposed to be added to the .pro project files. And, although, docs are silent
> about CMake, there is handy example located at:
> https://code.qt.io/cgit/qt/qtdeclarative.git/tree/examples/qml/qmlextensio
> nplugins/CMakeLists.txt?h=6.0
> 
> I added to my CMakeLists.txt:
> 
> set_target_properties(
>      IzLibraryPlugin
> PROPERTIES
>      QT_QML_MODULE_VERSION 1.0
>      QT_QML_MODULE_URI IzLibrary
> )
> 
> omitting, for the moment, the "QT_QMLTYPES_FILENAME
> imports/TimeExample/plugins.qmltypes"
> as I was typically generating .qmltypes manually using the supplied
> qmlplugindump and some associated CMake hackery.
> 
> I also added the:
> 
> qt6_qml_type_registration(IzLibraryPlugin)
> 
> And this is when first build error occurred. qt6_qml_type_registration seems
> to be trying to automate some of the QML type registration machinery and
> generates the "izlibraryplugin_qmltyperegistrations.cpp" file. In my case it
> looks like this:
> 
> #include <QtQml/qqml.h>
> #include <QtQml/qqmlmoduleregistration.h>
> 
> #include <CountdownTimer.h>
> #include <DoubleValidator.h>
> #include <IntValidator.h>
> 
> void qml_register_types_IzLibrary()
> {
>      qmlRegisterTypesAndRevisions<IzLibrary::CountdownTimer>("IzLibrary",
> 1);
>      qmlRegisterTypesAndRevisions<IzLibrary::DoubleValidator>("IzLibrary",
> 1);
>      qmlRegisterTypesAndRevisions<IzLibrary::IntValidator>("IzLibrary", 1);
>      QMetaType::fromType<QDoubleValidator *>().id();
>      QMetaType::fromType<QIntValidator *>().id();
>      qmlRegisterModule("IzLibrary", 1, 0); }
> 
> static const QQmlModuleRegistration registration("IzLibrary",
> qml_register_types_IzLibrary);
> 
> 
> But! My includes are not available under <SomeBeautifulHeader.h> but
> rather under "IzLibrary/SomeBeautifulHeader.h" as denoted by my
> target_include_directories:
> 
> # include directories for target
> target_include_directories(
>      IzLibraryPlugin
> PUBLIC
>      $<INSTALL_INTERFACE:include>
>      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
> PRIVATE
>      ${CMAKE_CURRENT_SOURCE_DIR}/private
> )
> 
> So, my questions (for now :P) are:
> Is this the intended behavior or have I fubared something and not added
> "something, somewhere" in my CMakeLists.txt?
> Is there a documentation for the qt6_* cmake functions?
> Is the stuff "qt6_add_qml_module" important or can I skip it?
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