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// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page qtqml-typesystem-references.html
\title QML Value Type and Sequence References
\brief Description of QML value type and sequence references
 
\l{QML Value Types} and \l{QML Sequence Types} are necessarily passed by value.
In contrast to \l{QML Object Types} they have no identity of themselves, but can
only be accessed as properties of other objects or values, or as values returned
from methods. Each such access implicitly creates a copy. Yet, in JavaScript
everything is an object. There is no such concept as a value type in JavaScript.
For example, if you execute \c{font.bold = true} in JavaScript, we expect the \c bold
property of \c font to be set, no matter what \c font is. But consider the following
code snippet:
 
\qml
import QtQuick
Text {
    onSomethingHappened: font.bold = true
}
\endqml
 
In this case we know that \c font is a value type. Accessing it creates a local copy
by calling the getter of a \l{Q_PROPERTY}. We can then set the \c bold property on it,
but that would usually only affect the copy, not the original \l{Q_PROPERTY}.
 
To overcome this problem, QML offers the concept of references. When you retrieve
an instance of a value or sequence type from a property, the QML engine remembers
the property along with the value itself. If the value is modified, it is written
back to the property. This produces the illusion of an object with separate identity
and makes the above case, along with many others, just work.
 
This can be rather expensive, though. If a sequence is exposed as a Q_PROPERTY,
accessing any value in the sequence by index will cause the whole sequence data
to be read from the property. From this sequence data, a single element is then
retrieved. Similarly, modifying any value in the sequence causes the
sequence data to be read. Then the modification is performed and the modified
sequence is be written back to the property. A read operation can be relatively
cheap if the type in question is implicitly shared. A modification always incurs
at least one deep copy.
 
If you return an instance of a sequence or value type from a \l Q_INVOKABLE function
you avoid such overhead. Return values are not attached to any property and won't be
written back.
 
Sequences of object types are passed as \l{QQmlListProperty} by default.
\l{QQmlListProperty} is not an actual container, but only a view, or reference, to
some sequential storage. Therefore, \l{QQmlListProperty} is not affected by this
effect. You can, however, register other sequence types for objects using
\l{QML_SEQUENTIAL_CONTAINER}. Those will be affected.
 
*/