topic.qdoc

Go to the documentation of this file.

// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page qtqml-typesystem-topic.html
\title The QML Type System
\brief Description of the QML type system
 
The types which may be used in the definition of an object hierarchy in a QML
document can come from various sources.  They may be:
 
\list
\li provided natively by the QML language
\li registered via C++ by QML modules
\li provided as QML documents by QML modules
\endlist
 
Furthermore, application developers can provide their own types, either by
registering C++ types directly, or by defining reusable components in QML
documents which can then be imported.
 
Wherever the type definitions come from, the engine will enforce type-safety
for properties and instances of those types.
 
 
\section1 QML Value Types
 
The QML language has built-in support for various primitive types including
integers, double-precision floating point numbers, strings, and boolean values.
Objects may have properties of these types, and values of these types may be
passed as arguments to methods of objects.
 
See the \l{qtqml-typesystem-valuetypes.html}{QML Value Types} documentation for
more information about value types.
 
\section1 QML Object Types
 
A QML object type is a type from which a QML object can be instantiated. QML
object types are derived from \l QtObject, and are provided by QML modules.
Applications can import these modules to use the object types they provide.
The \c QtQuick module provides the most common object types needed to create
user interfaces in QML.
 
Finally, every QML document implicitly defines a QML object type, which can be
re-used in other QML documents.  See the documentation about
\l{qtqml-typesystem-objecttypes.html}{object types in the QML type system} for
in-depth information about object types.
 
\section1 QML Sequence Types
 
Sequence types can be used to store sequences of values or objects.
 
See the documentation about
\l{qtqml-typesystem-sequencetypes.html}{sequence types in the QML type system}
for in-depth information about sequence types.
 
\section1 QML Namespaces
 
QML Namespaces can be used to expose enumerations from C++ namespaces.
 
See the documentation about
\l{qtqml-typesystem-namespaces.html}{namespaces in the QML type system}
for in-depth information about namespaces.
 
\section1 JavaScript Types
 
JavaScript objects and arrays are supported by the QML engine. Any standard
JavaScript type can be created and stored using the generic \l var type.
 
For example, the standard \c Date and \c Array types are available, as below:
 
\qml
import QtQuick
 
Item {
    property var theArray: []
    property var theDate: new Date()
 
    Component.onCompleted: {
        for (var i = 0; i < 10; i++)
            theArray.push("Item " + i)
        console.log("There are", theArray.length, "items in the array")
        console.log("The time is", theDate.toUTCString())
    }
}
\endqml
 
See \l {qtqml-javascript-expressions.html}{JavaScript Expressions in QML Documents} for more details.
 
 
*/