'How to simulate C-style "ifdef" macro in QML?

I need to switch off some parts of QML code, because this code has been done for demo purposes and it will be removed in final release. But the product will be used with these demo features for long, so I can't use a separate branch with demo features and constantly merge all new features to that branch - it's just not convenient. So it'd be nice to have this code running but easy to switch off and/or remove when needed. In C and C++ I use ifdef macro for that, but is it possible to do the same in QML?

qtmacrosqmlqt5


Solution 1:[1]

If you can refactor those parts into their own components, you have two options:

  1. File selectors
  2. Loader

The first option is perhaps a bit closer to a C-style #ifdef macro than using Loader, as it works at the file level. As long as you don't overuse Loader, though (e.g. as a delegate in a large view), they should both do the job fine.

Solution 2:[2]

It's not the same as an #ifdef because the decision making all happens at runtime, but I sometimes use this approach, which gives a similar usage pattern.

main.cpp

    QQmlApplicationEngine engine;
#ifdef DEMO_MODE
    engine.rootContext()->setContextProperty("DEMO_MODE", QVariant(true));
#else
    engine.rootContext()->setContextProperty("DEMO_MODE", QVariant(false));
#endif
    engine.load(QUrl("qrc:/ui/MainQmlFile.qml"));

then, from any .qml file in the project

if (DEMO_MODE) {

} else {

}

Solution 3:[3]

#ifdef is a preprocessor instructions, i.e. something processed at build time before the C or C++ compiler gets to see the code. You could do the same using your text manipulation language of choice, processing the QML files at build time before they get processed e.g. by the Qt resource compiler.

Solution 4:[4]

You can also use class DebugC with function

Q_INVOKABLE bool isDebuggingEnable()
{
  #ifdef QT_DEBUG
    return true;
  #else
    return false;
  #endif
}

and then register this class in qml

viewer->rootContext()->setContextProperty("stName", DebugCObj)

now you can easily call stName.isDebuggingEnable() method for checking debugging is enable or not in your QML file. its just a trick

Solution 5:[5]

QML is not compiled but interpreted at run time, so preprocessor directives can't be used directly by QML. But this is not a limitation, because it is possible to introduce properties to QML from c++ side, Thus directives can be placed on the C++ side to staff those properties, while decision making based on those properties, normal if/then, with Javascript can be used in QML side.

Solution 6:[6]

In addition to some of the other answers here (dynamically adding QML code from javascript based on a property, or using file selectors), another approach that may work if what you want is to have different QML component implementations, or disable a QML component, based on a compile time condition or setting, is to configure CMake to include alternate QML files at build time, and set QML_RESOURCE_ALIAS to the desired QML component name (as referenced by other QML files) in the built executable's resources. For example:

  1. Create two versions of a QML component, e.g. MyComponent_enabled.qml which defines a component and MyComponent_disabled.qml which is an empty item (just import QtQuick ; Item {}.)

  2. In CMakeLists, choose which one to use. E.g.

option(ENABLE_MY_COMPONENT "Enable MyComponent")
if(ENABLE_MY_COMPONENT)
  set(MYCOMPONENT_QML_FILE MyComponent_enabled.qml)
else()
  set(MYCOMPONENT_QML_FILE MyComponent_disabled.qml)
endif()
set_source_file_properties(${MYCOMPONENT_QML_FILE} PROPERTIES QT_RESOURCE_ALIAS MyComponent.qml)
qml_add_qml_module(yourapp
  ... other stuff ...
  QML_FILES
    ... other QML files ...
    ${MYCOMPONENT_QML_FILE}
  ...
)
  1. Use MyComponent {...} to instantiate MyComponent in other QML files.

(This will of course baffle QtCreator a bit -- it will only show one or the other QML file in the file list etc. There may be ways to get QtCreator or other IDEs to know about both files but that's another topic.)

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Mitch
Solution 2 Mark Ch
Solution 3 Kevin Krammer
Solution 4 rightaway717
Solution 5 Mohammad Kanan
Solution 6 Reed Hedges

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