Qt 5.11 | Qt QML | C++ Classes | QQmlProperty

Qt 5.11.3 Reference Documentation

# QQmlProperty Class

The QQmlProperty class abstracts accessing properties on objects created from QML. More...

Header: | #include <QQmlProperty>
qmake: | QT += qml
Since: | Qt 5.0

- List of all members, including inherited members

## Public Types

enum | PropertyTypeCategory { InvalidCategory, List, Object, Normal }
enum | Type { Invalid, Property, SignalProperty }

## Public Functions

| QQmlProperty()
| QQmlProperty(QObject *obj)
| QQmlProperty(QObject *obj, QQmlContext *ctxt)
| QQmlProperty(QObject *obj, QQmlEngine *engine)
| QQmlProperty(QObject *obj, const QString &name)
| QQmlProperty(QObject *obj, const QString &name, QQmlContext *ctxt)
| QQmlProperty(QObject *obj, const QString &name, QQmlEngine *engine)
| QQmlProperty(const QQmlProperty &other)
bool | connectNotifySignal(QObject *dest, const char *slot) const
bool | connectNotifySignal(QObject *dest, int method) const
bool | hasNotifySignal() const
int | index() const
bool | isDesignable() const
bool | isProperty() const
bool | isResettable() const
bool | isSignalProperty() const
bool | isValid() const
bool | isWritable() const
QMetaMethod | method() const
QString | name() const
bool | needsNotifySignal() const
QObject * | object() const
QMetaProperty | property() const
int | propertyType() const
QQmlProperty::PropertyTypeCategory | propertyTypeCategory() const
const char * | propertyTypeName() const
QVariant | read() const
bool | reset() const
QQmlProperty::Type | type() const
bool | write(const QVariant &value) const
QQmlProperty & | operator=(const QQmlProperty &other)
bool | operator==(const QQmlProperty &other) const

## Static Public Members

QVariant | read(const QObject *object, const QString &name)
QVariant | read(const QObject *object, const QString &name, QQmlContext *ctxt)
QVariant | read(const QObject *object, const QString &name, QQmlEngine *engine)
bool | write(QObject *object, const QString &name, const QVariant &value)
bool | write(QObject *object, const QString &name, const QVariant &value, QQmlContext *ctxt)
bool | write(QObject *object, const QString &name, const QVariant &value, QQmlEngine *engine)

## Detailed Description

The QQmlProperty class abstracts accessing properties on objects created from QML.

As QML uses Qt's meta-type system all of the existing QMetaObject classes can be used to introspect and interact with objects created by QML. However, some of the new features provided by QML - such as type safety and attached properties - are most easily used through the QQmlProperty class that simplifies some of their natural complexity.

Unlike QMetaProperty which represents a property on a class type, QQmlProperty encapsulates a property on a specific object instance. To read a property's value, programmers create a QQmlProperty instance and call the read() method. Likewise to write a property value the write() method is used.

For example, for the following QML code:

      // MyItem.qml
      import QtQuick 2.0

      Text { text: "A bit of text" }

The Text object's properties could be accessed using QQmlProperty, like this:

      #include <QQmlProperty>
      #include <QGraphicsObject>

      ...

      QQuickView view(QUrl::fromLocalFile("MyItem.qml"));
      QQmlProperty property(view.rootObject(), "font.pixelSize");
      qWarning() << "Current pixel size:" << property.read().toInt();
      property.write(24);
      qWarning() << "Pixel size should now be 24:" << property.read().toInt();

The Qt Quick 1 version of this class was named QDeclarativeProperty.

## Member Type Documentation

### enum QQmlProperty::PropertyTypeCategory

This enum specifies a category of QML property.

Constant | Value | Description
--- | --- | ---
`QQmlProperty::InvalidCategory` | `0` | The property is invalid, or is a signal property.
`QQmlProperty::List` | `1` | The property is a QQmlListProperty list property
`QQmlProperty::Object` | `2` | The property is a QObject derived type pointer
`QQmlProperty::Normal` | `3` | The property is a normal value property.

### enum QQmlProperty::Type

This enum specifies a type of QML property.

Constant | Value | Description
--- | --- | ---
`QQmlProperty::Invalid` | `0` | The property is invalid.
`QQmlProperty::Property` | `1` | The property is a regular Qt property.
`QQmlProperty::SignalProperty` | `2` | The property is a signal property.

## Member Function Documentation

### QQmlProperty::QQmlProperty()

Create an invalid QQmlProperty.

### QQmlProperty::QQmlProperty(QObject *obj)

Creates a QQmlProperty for the default property of obj. If there is no default property, an invalid QQmlProperty will be created.

### QQmlProperty::QQmlProperty(QObject *obj, QQmlContext *ctxt)

Creates a QQmlProperty for the default property of obj using the context ctxt. If there is no default property, an invalid QQmlProperty will be created.

### QQmlProperty::QQmlProperty(QObject *obj, QQmlEngine *engine)

Creates a QQmlProperty for the default property of obj using the environment for instantiating QML components that is provided by engine. If there is no default property, an invalid QQmlProperty will be created.

### QQmlProperty::QQmlProperty(QObject *obj, const QString &name)

Creates a QQmlProperty for the property name of obj.

### QQmlProperty::QQmlProperty(QObject *obj, const QString &name, QQmlContext *ctxt)

Creates a QQmlProperty for the property name of obj using the context ctxt.

Creating a QQmlProperty without a context will render some properties - like attached properties - inaccessible.

### QQmlProperty::QQmlProperty(QObject *obj, const QString &name, QQmlEngine *engine)

Creates a QQmlProperty for the property name of obj using the environment for instantiating QML components that is provided by engine.

### QQmlProperty::QQmlProperty(const QQmlProperty &other)

Create a copy of other.

### bool QQmlProperty::connectNotifySignal(QObject *dest, const char *slot) const

Connects the property's change notifier signal to the specified slot of the dest object and returns true. Returns false if this metaproperty does not represent a regular Qt property or if it has no change notifier signal, or if the dest object does not have the specified slot.

Note: slot should be passed using the SLOT() macro so it is correctly identified.

### bool QQmlProperty::connectNotifySignal(QObject *dest, int method) const

Connects the property's change notifier signal to the specified method of the dest object and returns true. Returns false if this metaproperty does not represent a regular Qt property or if it has no change notifier signal, or if the dest object does not have the specified method.

### bool QQmlProperty::hasNotifySignal() const

Returns true if the property has a change notifier signal, otherwise false.

### int QQmlProperty::index() const

Return the Qt metaobject index of the property.

### bool QQmlProperty::isDesignable() const

Returns true if the property is designable, otherwise false.

### bool QQmlProperty::isProperty() const

Returns true if this QQmlProperty represents a regular Qt property.

### bool QQmlProperty::isResettable() const

Returns true if the property is resettable, otherwise false.

### bool QQmlProperty::isSignalProperty() const

Returns true if this QQmlProperty represents a QML signal property.

### bool QQmlProperty::isValid() const

Returns true if the QQmlProperty refers to a valid property, otherwise false.

### bool QQmlProperty::isWritable() const

Returns true if the property is writable, otherwise false.

### QMetaMethod QQmlProperty::method() const

Return the QMetaMethod for this property if it is a SignalProperty, otherwise returns an invalid QMetaMethod.

### QString QQmlProperty::name() const

Return the name of this QML property.

### bool QQmlProperty::needsNotifySignal() const

Returns true if the property needs a change notifier signal for bindings to remain upto date, false otherwise.

Some properties, such as attached properties or those whose value never changes, do not require a change notifier.

### QObject *QQmlProperty::object() const

Returns the QQmlProperty's QObject.

### QMetaProperty QQmlProperty::property() const

Returns the Qt property associated with this QML property.

### int QQmlProperty::propertyType() const

Returns the QVariant type of the property, or QVariant::Invalid if the property has no QVariant type.

### QQmlProperty::PropertyTypeCategory QQmlProperty::propertyTypeCategory() const

Returns the property category.

### const char *QQmlProperty::propertyTypeName() const

Returns the type name of the property, or 0 if the property has no type name.

### QVariant QQmlProperty::read() const

Returns the property value.

### `[static]` QVariant QQmlProperty::read(const QObject *object, const QString &name)

Return the name property value of object. This method is equivalent to:

      QQmlProperty p(object, name);
      p.read();

### `[static]` QVariant QQmlProperty::read(const QObject *object, const QString &name, QQmlContext *ctxt)

Return the name property value of object using the context ctxt. This method is equivalent to:

      QQmlProperty p(object, name, context);
      p.read();

### `[static]` QVariant QQmlProperty::read(const QObject *object, const QString &name, QQmlEngine *engine)

Return the name property value of object using the environment for instantiating QML components that is provided by engine. . This method is equivalent to:

      QQmlProperty p(object, name, engine);
      p.read();

### bool QQmlProperty::reset() const

Resets the property and returns true if the property is resettable. If the property is not resettable, nothing happens and false is returned.

### QQmlProperty::Type QQmlProperty::type() const

Returns the type of the property.

### bool QQmlProperty::write(const QVariant &value) const

Sets the property value to value. Returns `true` on success, or `false` if the property can't be set because the value is the wrong type, for example.

### `[static]` bool QQmlProperty::write(QObject *object, const QString &name, const QVariant &value)

Writes value to the name property of object. This method is equivalent to:

      QQmlProperty p(object, name);
      p.write(value);

Returns `true` on success, `false` otherwise.

### `[static]` bool QQmlProperty::write(QObject *object, const QString &name, const QVariant &value, QQmlContext *ctxt)

Writes value to the name property of object using the context ctxt. This method is equivalent to:

      QQmlProperty p(object, name, ctxt);
      p.write(value);

Returns `true` on success, `false` otherwise.

### `[static]` bool QQmlProperty::write(QObject *object, const QString &name, const QVariant &value, QQmlEngine *engine)

Writes value to the name property of object using the environment for instantiating QML components that is provided by engine. This method is equivalent to:

      QQmlProperty p(object, name, engine);
      p.write(value);

Returns `true` on success, `false` otherwise.

### QQmlProperty &QQmlProperty::operator=(const QQmlProperty &other)

Assign other to this QQmlProperty.

### bool QQmlProperty::operator==(const QQmlProperty &other) const

Returns true if other and this QQmlProperty represent the same property.