qt5base-lts/examples/widgets/doc/src/spinboxdelegate.qdoc

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/*!
    \example itemviews/spinboxdelegate
    \title Spin Box Delegate Example
    \ingroup examples-itemviews
    \brief The Spin Box Delegate example shows how to create an editor for a custom delegate in
    the model/view framework by reusing a standard Qt editor widget.

    The model/view framework provides a standard delegate that is used by default
    with the standard view classes. For most purposes, the selection of editor
    widgets available through this delegate is sufficient for editing text, boolean
    values, and other simple data types. However, for specific data types, it is
    sometimes necessary to use a custom delegate to either display the data in a
    specific way, or allow the user to edit it with a custom control.

    \image spinboxdelegate-example.png

    This concepts behind this example are covered in the
    \l{Model/View Programming#Delegate Classes}{Delegate Classes} chapter
    of the \l{Model/View Programming} overview.

    \section1 SpinBoxDelegate Class Definition

    The definition of the delegate is as follows:

    \snippet itemviews/spinboxdelegate/delegate.h 0

    The delegate class declares only those functions that are needed to
    create an editor widget, display it at the correct location in a view,
    and communicate with a model. Custom delegates can also provide their
    own painting code by reimplementing the \c paintEvent() function.
    Furthermore it is also possible to reuse (and avoid deleting) the editor
    widget by reimplementing the \a destroyEditor() function. A reused widget
    could be a mutable member created in the constructor and deleted in
    the destructor.

    \section1 SpinBoxDelegate Class Implementation

    Delegates are often stateless. The constructor only needs to
    call the base class's constructor with the parent QObject as its
    argument:

    \snippet itemviews/spinboxdelegate/delegate.cpp 0

    Since the delegate is a subclass of QStyledItemDelegate, the data it retrieves
    from the model is displayed in a default style, and we do not need to
    provide a custom \c paintEvent().

    The \c createEditor() function returns an editor widget, in this case a
    spin box that restricts values from the model to integers from 0 to 100
    inclusive.

    \snippet itemviews/spinboxdelegate/delegate.cpp 1

    We install an event filter on the spin box to ensure that it behaves in
    a way that is consistent with other delegates. The implementation for
    the event filter is provided by the base class.

    The \c setEditorData() function reads data from the model, converts it
    to an integer value, and writes it to the editor widget.

    \snippet itemviews/spinboxdelegate/delegate.cpp 2

    Since the view treats delegates as ordinary QWidget instances, we have
    to use a static cast before we can set the value in the spin box.

    The \c setModelData() function reads the contents of the spin box, and
    writes it to the model.

    \snippet itemviews/spinboxdelegate/delegate.cpp 3

    We call \l{QSpinBox::interpretText()}{interpretText()} to make sure that
    we obtain the most up-to-date value in the spin box.

    The \c updateEditorGeometry() function updates the editor widget's
    geometry using the information supplied in the style option. This is the
    minimum that the delegate must do in this case.

    \snippet itemviews/spinboxdelegate/delegate.cpp 4

    More complex editor widgets may divide the rectangle available in
    \c{option.rect} between different child widgets if required.

    \section1 The Main Function

    This example is written in a slightly different way to many of the
    other examples supplied with Qt. To demonstrate the use of a custom
    editor widget in a standard view, it is necessary to set up a model
    containing some arbitrary data and a view to display it.

    We set up the application in the normal way, construct a standard item
    model to hold some data, set up a table view to use the data in the
    model, and construct a custom delegate to use for editing:

    \snippet itemviews/spinboxdelegate/main.cpp 0

    The table view is informed about the delegate, and will use it to
    display each of the items. Since the delegate is a subclass of
    QStyledItemDelegate, each cell in the table will be rendered using standard
    painting operations.

    We insert some arbitrary data into the model for demonstration purposes:

    \snippet itemviews/spinboxdelegate/main.cpp 1
    \snippet itemviews/spinboxdelegate/main.cpp 2

    Finally, the table view is displayed with a window title, and we start
    the application's event loop:

    \snippet itemviews/spinboxdelegate/main.cpp 3

    Each of the cells in the table can now be edited in the usual way, but
    the spin box ensures that the data returned to the model is always
    constrained by the values allowed by the spin box delegate.
*/