path: root/doc/src/declarative/qdeclarativemodels.qdoc

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/*!
\page qdeclarativemodels.html
\target qmlmodels
\title Data Models

Some QML Items use Data Models to provide the data to be displayed.
These items typically require a \e delegate component that
creates an instance for each item in the model.  Models may be static, or
have items modified, inserted, removed or moved dynamically.

Data is provided to the delegate via named data roles which the
delegate may bind to.  The roles are exposed as properties of the
\e model context property, though this property is set as a default property
of the delegate so, unless there is a naming clash with a
property in the delegate, the roles are usually accessed unqualified.  The
example below would  have a clash between the \e color role of the model and
the \e color property of the Rectangle.  The clash is avoided by referencing
the \e color property of the model by its full name: \e model.color.

\code
ListModel {
    id: myModel
    ListElement { color: "red" }
    ListElement { color: "green" }
}

Component {
    id: myDelegate
    Rectangle {
        width: 20; height: 20
        color: model.color
    }
}
\endcode

A special \e index role containing the index of the item in the model
is also available.

\e Note: the index role will be set to -1 if the item is removed from
the model.  If you bind to the index role, be sure that the logic
accounts for the possibility of index being -1, i.e. that the item
is no longer valid.  Usually the item will shortly be destroyed, but
it is possible to delay delegate destruction in some views via a delayRemove
attached property.

Models that do not have named roles will have the data provided via
the \e modelData role.  The \e modelData role is also provided for
Models that have only one role.  In this case the \e modelData role
contains the same data as the named role.

There are a number of QML elements that operate using data models:

\list
\o ListView
\o GridView
\o PathView
\o \l Repeater
\endlist

QML supports several types of data model, which may be provided by QML
or C++ (via QDeclarativeContext::setContextProperty(), for example).

\section1 QML Data Models

\section2 ListModel

ListModel is a simple hierarchy of elements specified in QML.  The
available roles are specified by the \l ListElement properties.

\code
ListModel {
    id: fruitModel
    ListElement {
        name: "Apple"
        cost: 2.45
    }
    ListElement {
        name: "Orange"
        cost: 3.25
    }
    ListElement {
        name: "Banana"
        cost: 1.95
    }
}
\endcode

The above model has two roles, \e name and \e cost.  These can be bound
to by a ListView delegate, for example:

\code
Component {
    id: fruitDelegate
    Row {
        Text { text: "Fruit: " + name }
        Text { text: "Cost: $" + cost }
    }
}
ListView {
    model: fruitModel
    delegate: fruitDelegate
}
\endcode


\section2 XmlListModel

XmlListModel allows construction of a model from an XML data source. The roles
are specified via the \l XmlRole element.

The following model has three roles, \e title, \e link and \e description:
\code
XmlListModel {
     id: feedModel
     source: "http://rss.news.yahoo.com/rss/oceania"
     query: "/rss/channel/item"
     XmlRole { name: "title"; query: "title/string()" }
     XmlRole { name: "link"; query: "link/string()" }
     XmlRole { name: "description"; query: "description/string()" }
}
\endcode


\section2 VisualItemModel

VisualItemModel allows QML items to be provided as a model.  This model contains
both the data and delegate (its child items).  This model does not provide any roles.

\code
 VisualItemModel {
     id: itemModel
     Rectangle { height: 30; width: 80; color: "red" }
     Rectangle { height: 30; width: 80; color: "green" }
     Rectangle { height: 30; width: 80; color: "blue" }
 }

 ListView {
     anchors.fill: parent
     model: itemModel
 }
\endcode

Note that in the above example there is no delegate required.
The items of the model itself provide the visual elements that
will be positioned by the view.


\section1 C++ Data Models

\section2 QAbstractItemModel

QAbstractItemModel provides the roles set via the QAbstractItemModel::setRoleNames() method.
The default role names set by Qt are:

\table
\header
\o Qt Role
\o QML Role Name
\row
\o Qt::DisplayRole
\o display
\row
\o Qt::DecorationRole
\o decoration
\endtable

QAbstractItemModel presents a heirachy of tables.  Views currently provided by QML
can only display list data.  
In order to display child lists of a heirachical model
the VisualDataModel element provides several properties and functions for use
with models of type QAbstractItemModel:
\list
\o \e hasModelChildren role property to determine whether a node has child nodes.
\o \l VisualDataModel::rootIndex allows the root node to be specifed
\o \l VisualDataModel::modelIndex() returns a QModelIndex which can be assigned to VisualDataModel::rootIndex
\o \l VisualDataModel::parentModelIndex() returns a QModelIndex which can be assigned to VisualDataModel::rootIndex
\endlist


\section2 QStringList

QStringList provides the contents of the list via the \e modelData role:

\table
\row
\o
\code
// main.cpp
QStringList dataList;
dataList.append("Fred");
dataList.append("Ginger");
dataList.append("Skipper");

QDeclarativeContext *ctxt = view.rootContext();
ctxt->setContextProperty("myModel", QVariant::fromValue(dataList));
\endcode

\o
\code
// main.qml
ListView {
   width: 100
   height: 100
   anchors.fill: parent
   model: myModel
   delegate: Component {
       Rectangle {
            height: 25
            Text { text: modelData }
       }
   }
}
\endcode
\endtable

\note There is no way for the view to know that the contents of a QStringList
have changed.  If the QStringList is changed, it will be necessary to reset
the model by calling QDeclarativeContext::setContextProperty() again.


\section2 QList<QObject*>

QList<QObject*> provides the properties of the objects in the list as roles.

\code
class DataObject : public QObject
{
    Q_OBJECT

    Q_PROPERTY(QString name READ name WRITE setName)
    Q_PROPERTY(QString color READ color WRITE setColor)
...
};

QList<QObject*> dataList;
dataList.append(new DataObject("Item 1", "red"));
dataList.append(new DataObject("Item 2", "green"));
dataList.append(new DataObject("Item 3", "blue"));
dataList.append(new DataObject("Item 4", "yellow"));

QDeclarativeContext *ctxt = view.rootContext();
ctxt->setContextProperty("myModel", QVariant::fromValue(dataList));
\endcode

The QObject* is available as the \c modelData property.  As a convenience,
the properties of the object are also made available directly in the
delegate's context:

\code
ListView {
   width: 100
   height: 100
   anchors.fill: parent
   model: myModel
   delegate: Component {
       Rectangle {
            height: 25
            width: 100
            color: model.modelData.color
            Text { text: name }
       }
   }
}
\endcode

Note the use of the fully qualified access to the \c color property.  
The properties of the object are not replicated in the \c model
object, since they are easily available via the modelData
object.

Note: There is no way for the view to know that the contents of a QList
have changed.  If the QList is changed, it will be necessary to reset
the model by calling QDeclarativeContext::setContextProperty() again.


\section1 Other Data Models


\section2 An Integer

An Integer specifies a model containing the integer number of elements.
There are no data roles.

The following example creates a ListView with five elements:
\code
Item {
    width: 200
    height: 250

    Component {
        id: itemDelegate
        Text { text: "I am item number: " + index }
    }

    ListView {
        anchors.fill: parent
        model: 5
        delegate: itemDelegate
    }

}
\endcode


\section2 An Object Instance

An Object Instance specifies a model with a single Object element.  The
properties of the object are provided as roles.

The example below creates a list with one item, showing the color of the
\e myText text.  Note the use of the fully qualified \e model.color property
to avoid clashing with \e color property of the Text element in the delegate.

\code
Rectangle {
    Text {
        id: myText
        text: "Hello"
        color: "#dd44ee"
    }

    Component {
        id: myDelegate
        Text {
            text: model.color
        }
    }
    ListView {
        anchors.fill: parent
        anchors.topMargin: 30
        model: myText
        delegate: myDelegate
    }
}
\endcode

*/