- Qt 5.15
- Qt QML
- C++ Classes
- QQmlPropertyMap
- Qt 5.15.19 Reference Documentation

# QQmlPropertyMap Class

The QQmlPropertyMap class allows you to set key-value pairs that can be used in QML bindings. More...

Header: | #include <QQmlPropertyMap>
qmake: | QT += qml
Inherits: | QObject

- List of all members, including inherited members

## Public Functions

| QQmlPropertyMap(QObject *parent = nullptr)
virtual | ~QQmlPropertyMap() override
void | clear(const QString &key)
bool | contains(const QString &key) const
int | count() const
void | insert(const QString &key, const QVariant &value)
bool | isEmpty() const
QStringList | keys() const
int | size() const
QVariant | value(const QString &key) const
QVariant & | operator[](const QString &key)
QVariant | operator[](const QString &key) const

## Signals

void | valueChanged(const QString &key, const QVariant &value)

## Protected Functions

| QQmlPropertyMap(DerivedType *derived, QObject *parent)
virtual QVariant | updateValue(const QString &key, const QVariant &input)

## Detailed Description

QQmlPropertyMap provides a convenient way to expose domain data to the UI layer. The following example shows how you might declare data in C++ and then access it in QML.

In the C++ file:

     // create our data
     QQmlPropertyMap ownerData;
     ownerData.insert("name", QVariant(QString("John Smith")));
     ownerData.insert("phone", QVariant(QString("555-5555")));

     // expose it to the UI layer
     QQuickView view;
     QQmlContext *ctxt = view.rootContext();
     ctxt->setContextProperty("owner", &ownerData);

     view.setSource(QUrl::fromLocalFile("main.qml"));
     view.show();

Then, in `main.qml`:

     Text { text: owner.name + " " + owner.phone }

The binding is dynamic - whenever a key's value is updated, anything bound to that key will be updated as well.

To detect value changes made in the UI layer you can connect to the valueChanged() signal. However, note that valueChanged() is NOT emitted when changes are made by calling insert() or clear() - it is only emitted when a value is updated from QML.

Note: It is not possible to remove keys from the map; once a key has been added, you can only modify or clear its associated value.

Note: When deriving a class from QQmlPropertyMap, use the protected two-argument constructor which ensures that the class is correctly registered with the Qt Meta-Object System.

Note: The QMetaObject of a QQmlPropertyMap is dynamically generated and modified. Operations on that meta object are not thread safe, so applications need to take care to explicitly synchronize access to the meta object.

## Member Function Documentation

### `[protected]` template <typename DerivedType> QQmlPropertyMap::QQmlPropertyMap(DerivedType *derived, QObject *parent)

Constructs a bindable map with parent object parent. Use this constructor in classes derived from QQmlPropertyMap.

The type of derived is used to register the property map with the Meta-Object System, which is necessary to ensure that properties of the derived class are accessible. This type must be derived from QQmlPropertyMap.

### QQmlPropertyMap::QQmlPropertyMap(QObject *parent = nullptr)

Constructs a bindable map with parent object parent.

### `[signal]` void QQmlPropertyMap::valueChanged(const QString &key, const QVariant &value)

This signal is emitted whenever one of the values in the map is changed. key is the key corresponding to the value that was changed.

Note: valueChanged() is NOT emitted when changes are made by calling insert() or clear() - it is only emitted when a value is updated from QML.

### `[override virtual]` QQmlPropertyMap::~QQmlPropertyMap()

Destroys the bindable map.

### void QQmlPropertyMap::clear(const QString &key)

Clears the value (if any) associated with key.

### bool QQmlPropertyMap::contains(const QString &key) const

Returns true if the map contains key.

See also size().

### int QQmlPropertyMap::count() const

This is an overloaded function.

Same as size().

### void QQmlPropertyMap::insert(const QString &key, const QVariant &value)

Sets the value associated with key to value.

If the key doesn't exist, it is automatically created.

### bool QQmlPropertyMap::isEmpty() const

Returns true if the map contains no keys; otherwise returns false.

See also size().

### QStringList QQmlPropertyMap::keys() const

Returns the list of keys.

Keys that have been cleared will still appear in this list, even though their associated values are invalid QVariants.

Note: This function can be invoked via the meta-object system and from QML. See Q_INVOKABLE.

### int QQmlPropertyMap::size() const

Returns the number of keys in the map.

See also isEmpty() and count().

### `[virtual protected]` QVariant QQmlPropertyMap::updateValue(const QString &key, const QVariant &input)

Returns the new value to be stored for the key key. This function is provided to intercept updates to a property from QML, where the value provided from QML is input.

Override this function to manipulate the property value as it is updated. Note that this function is only invoked when the value is updated from QML.

### QVariant QQmlPropertyMap::value(const QString &key) const

Returns the value associated with key.

If no value has been set for this key (or if the value has been cleared), an invalid QVariant is returned.

### QVariant &QQmlPropertyMap::operator[](const QString &key)

Returns the value associated with the key key as a modifiable reference.

If the map contains no item with key key, the function inserts an invalid QVariant into the map with key key, and returns a reference to it.

See also insert() and value().

### QVariant QQmlPropertyMap::operator[](const QString &key) const

This is an overloaded function.

Same as value().