Extending QML - Inheritance and Coercion Example▲
This example builds on:
The Inheritance and Coercion Example shows how to use base classes to assign types of more than one type to a property. It specializes the Person type developed in the previous examples into two types - a Boy and a Girl.
BirthdayParty {
host: Boy {
name: "Bob Jones"
shoeSize: 12
}
guests: [
Boy { name: "Leo Hodges" },
Boy { name: "Jack Smith" },
Girl { name: "Anne Brown" }
]
}Declare Boy and Girl▲
class Boy : public Person
{
Q_OBJECT
public:
Boy(QObject * parent = nullptr);
};
class Girl : public Person
{
Q_OBJECT
public:
Girl(QObject * parent = nullptr);
};The Person class remains unaltered in this example and the Boy and Girl C++ classes are trivial extensions of it. As an example, the inheritance used here is a little contrived, but in real applications it is likely that the two extensions would add additional properties or modify the Person classes behavior.
Define People as a Base Class▲
The implementation of the People class itself has not changed since the previous example. However, as we have repurposed the People class as a common base for Boy and Girl, we want to prevent it from being instantiated from QML directly - an explicit Boy or Girl should be instantiated instead.
qmlRegisterType<Person>();While we want to disallow instantiating Person from within QML, it still needs to be registered with the QML engine, so that it can be used as a property type and other types can be coerced to it.
Define Boy and Girl▲
The implementation of Boy and Girl is trivial.
Boy::Boy(QObject * parent)
: Person(parent)
{
}
Girl::Girl(QObject * parent)
: Person(parent)
{
}All that is necessary is to implement the constructor, and to register the types and their QML name with the QML engine.
Running the Example▲
The BirthdayParty type has not changed since the previous example. The celebrant and guests property still use the People type.
Q_PROPERTY(Person *host READ host WRITE setHost)
Q_PROPERTY(QQmlListProperty<Person> guests READ guests)However, as all three types, Person, Boy and Girl, have been registered with the QML system, on assignment QML automatically (and type-safely) converts the Boy and Girl objects into a Person.
The main.cpp file in the example includes a simple shell application that loads and runs the QML snippet shown at the beginning of this page.