QQmlContext Class

The QQmlContext class defines a context within a QML engine. More...

Header: #include <QQmlContext>
CMake: find_package(Qt6 REQUIRED COMPONENTS Qml)
target_link_libraries(mytarget PRIVATE Qt6::Qml)
qmake: QT += qml
Inherits: QObject

Public Types

struct PropertyPair

Public Functions

QQmlContext(QQmlEngine *engine, QObject *parent = nullptr)
QQmlContext(QQmlContext *parentContext, QObject *parent = nullptr)
virtual ~QQmlContext() override
QUrl baseUrl() const
QObject *contextObject() const
QVariant contextProperty(const QString &name) const
QQmlEngine *engine() const
bool isValid() const
QString nameForObject(const QObject *object) const
QObject *objectForName(const QString &name) const
QQmlContext *parentContext() const
QUrl resolvedUrl(const QUrl &src) const
void setBaseUrl(const QUrl &baseUrl)
void setContextObject(QObject *object)
void setContextProperties(const QList<QQmlContext::PropertyPair> &properties)
void setContextProperty(const QString &name, QObject *value)
void setContextProperty(const QString &name, const QVariant &value)

Detailed Description

Contexts hold the objects identified by id in a QML document. You can use {nameForObject()} and objectForName() to retrieve them.

Note: It is the responsibility of the creator to delete any QQmlContext it constructs. If a QQmlContext is no longer needed, it must be destroyed explicitly. The simplest way to ensure this is to give the QQmlContext a parent.

The Context Hierarchy

Contexts form a hierarchy. The root of this hierarchy is the QML engine's root context. Each QML component creates its own context when instantiated and some QML elements create extra contexts for themselves.

While QML objects instantiated in a context are not strictly owned by that context, their bindings are. If a context is destroyed, the property bindings of outstanding QML objects will stop evaluating.

Context Properties

Contexts also allow data to be exposed to the QML components instantiated by the QML engine. Such data is invisible to any tooling, including the Qt Quick Compiler and to future readers of the QML documents in question. It will only be exposed if the QML component is instantiated in the specific C++ context you are envisioning. In other places, different context data may be exposed instead.

Instead of using the QML context to expose data to your QML components, you should either create additional object properties to hold the data or use singletons. See Exposing C++ State to QML for a detailed explanation.

Each QQmlContext contains a set of properties, distinct from its QObject properties, that allow data to be explicitly bound to a context by name. The context properties can be defined and updated by calling QQmlContext::setContextProperty().

To simplify binding and maintaining larger data sets, a context object can be set on a QQmlContext. All the properties of the context object are available by name in the context, as though they were all individually added through calls to QQmlContext::setContextProperty(). Changes to the property's values are detected through the property's notify signal. Setting a context object is both faster and easier than manually adding and maintaining context property values.

All properties added explicitly by QQmlContext::setContextProperty() take precedence over the context object's properties.

Child contexts inherit the context properties of their parents; if a child context sets a context property that already exists in its parent, the new context property overrides that of the parent.

Warning: Setting the context object or adding new context properties after an object has been created in that context is an expensive operation (essentially forcing all bindings to re-evaluate). Thus, if you need to use context properties, you should at least complete the "setup" of the context before using it to create any objects.

See also Exposing Attributes of C++ Types to QML.

Member Function Documentation

QQmlContext::QQmlContext(QQmlEngine *engine, QObject *parent = nullptr)

Create a new QQmlContext as a child of engine's root context, and the QObject parent.

QQmlContext::QQmlContext(QQmlContext *parentContext, QObject *parent = nullptr)

Create a new QQmlContext with the given parentContext, and the QObject parent.

[override virtual noexcept] QQmlContext::~QQmlContext()

Destroys the QQmlContext.

Any expressions, or sub-contexts dependent on this context will be invalidated, but not destroyed (unless they are parented to the QQmlContext object).

QUrl QQmlContext::baseUrl() const

Returns the base url of the component, or the containing component if none is set.

See also setBaseUrl().

QObject *QQmlContext::contextObject() const

Return the context object, or nullptr if there is no context object.

See also setContextObject().

QVariant QQmlContext::contextProperty(const QString &name) const

Returns the value of the name property for this context as a QVariant. If you know that the property you're looking for is a QObject assigned using a QML id in the current context, objectForName() is more convenient and faster. In contrast to objectForName() and nameForObject(), this method does traverse the context hierarchy and searches in parent contexts if the name is not found in the current one. It also considers any contextObject() you may have set.

See also setContextProperty(), objectForName(), nameForObject(), and contextObject().

QQmlEngine *QQmlContext::engine() const

Return the context's QQmlEngine, or nullptr if the context has no QQmlEngine or the QQmlEngine was destroyed.

bool QQmlContext::isValid() const

Returns whether the context is valid.

To be valid, a context must have a engine, and it's contextObject(), if any, must not have been deleted.

QString QQmlContext::nameForObject(const QObject *object) const

Returns the name of object in this context, or an empty string if object is not named in the context. Objects are named by setContextProperty(), or as properties of a context object, or by ids in the case of QML created contexts.

If the object has multiple names, the first is returned.

In contrast to contextProperty(), this method does not traverse the context hierarchy. If the name is not found in the current context, an empty String is returned.

See also contextProperty() and objectForName().

[since 6.2] QObject *QQmlContext::objectForName(const QString &name) const

Returns the object for a given name in this context. Returns nullptr if name is not available in the context or if the value associated with name is not a QObject. Objects are named by setContextProperty(), or as properties of a context object, or by ids in the case of QML created contexts. In contrast to contextProperty(), this method does not traverse the context hierarchy. If the name is not found in the current context, nullptr is returned.

This function was introduced in Qt 6.2.

See also contextProperty() and nameForObject().

QQmlContext *QQmlContext::parentContext() const

Return the context's parent QQmlContext, or nullptr if this context has no parent or if the parent has been destroyed.

QUrl QQmlContext::resolvedUrl(const QUrl &src) const

Resolves the URL src relative to the URL of the containing component.

See also QQmlEngine::baseUrl() and setBaseUrl().

void QQmlContext::setBaseUrl(const QUrl &baseUrl)

Explicitly sets the url resolvedUrl() will use for relative references to baseUrl.

Calling this function will override the url of the containing component used by default.

See also baseUrl() and resolvedUrl().

void QQmlContext::setContextObject(QObject *object)

Set the context object.

Note: You should not use context objects to inject values into your QML components. Use singletons or regular object properties instead.

See also contextObject().

void QQmlContext::setContextProperties(const QList<QQmlContext::PropertyPair> &properties)

Set a batch of properties on this context.

Setting all properties in one batch avoids unnecessary refreshing expressions, and is therefore recommended instead of calling setContextProperty() for each individual property.

Note: You should not use context properties to inject values into your QML components. Use singletons or regular object properties instead.

See also QQmlContext::setContextProperty().

void QQmlContext::setContextProperty(const QString &name, QObject *value)

Set the value of the name property on this context.

QQmlContext does not take ownership of value.

Note: You should not use context properties to inject values into your QML components. Use singletons or regular object properties instead.

See also contextProperty().

void QQmlContext::setContextProperty(const QString &name, const QVariant &value)

Set a the value of the name property on this context.

Note: You should not use context properties to inject values into your QML components. Use singletons or regular object properties instead.

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