qt5base-lts/doc/src/examples/pixelator.qdoc

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/*!
    \example itemviews/pixelator
    \title Pixelator Example

    The Pixelator example shows how delegates can be used to customize the way that
    items are rendered in standard item views.

    \image pixelator-example.png

    By default, QTreeView, QTableView, and QListView use a standard item delegate
    to display and edit a set of common data types that are sufficient for many
    applications. However, an application may need to represent items of data in a
    particular way, or provide support for rendering more specialized data types,
    and this often requires the use of a custom delegate.

    In this example, we show how to use custom delegates to modify the appearance
    of standard views. To do this, we implement the following components:

    \list
    \i A model which represents each pixel in an image as an item of data, where each
       item contains a value for the brightness of the corresponding pixel.
    \i A custom delegate that uses the information supplied by the model to represent
       each pixel as a black circle on a white background, where the radius of the
       circle corresponds to the darkness of the pixel.
    \endlist

    This example may be useful for developers who want to implement their own table
    models or custom delegates. The process of creating custom delegates for editing
    item data is covered in the \l{Spin Box Delegate Example}{Spin Box Delegate}
    example.

    \section1 ImageModel Class Definition

    The \c ImageModel class is defined as follows:

    \snippet examples/itemviews/pixelator/imagemodel.h 0

    Since we only require a simple, read-only table model, we only need to implement
    functions to indicate the dimensions of the image and supply data to other
    components.

    \section1 ImageModel Class Implementation

    The constructor is trivial:

    \snippet examples/itemviews/pixelator/imagemodel.cpp 0

    The \c setImage() function sets the image that will be used by the model:

    \snippet examples/itemviews/pixelator/imagemodel.cpp 1

    The QAbstractItemModel::reset() call tells the view(s) that the model
    has changed.

    The \c rowCount() and \c columnCount() functions return the height and width of
    the image respectively:

    \snippet examples/itemviews/pixelator/imagemodel.cpp 2
    \snippet examples/itemviews/pixelator/imagemodel.cpp 3

    Since the image is a simple two-dimensional structure, the \c parent arguments
    to these functions are unused. They both simply return the relevant size from
    the underlying image object.

    The \c data() function returns data for the item that corresponds to a given
    model index in a format that is suitable for a particular role:

    \snippet examples/itemviews/pixelator/imagemodel.cpp 4

    In this implementation, we only check that the model index is valid, and that
    the role requested is the \l{Qt::ItemDataRole}{DisplayRole}. If so, the function
    returns the grayscale value of the relevant pixel in the image; otherwise, a null
    model index is returned.

    This model can be used with QTableView to display the integer brightness values
    for the pixels in the image. However, we will implement a custom delegate to
    display this information in a more artistic way.

    The \c headerData() function is also reimplemented:

    \snippet examples/itemviews/pixelator/imagemodel.cpp 5

    We return (1, 1) as the size hint for a header item. If we
    didn't, the headers would default to a larger size, preventing
    us from displaying really small items (which can be specified
    using the \gui{Pixel size} combobox).

    \section1 PixelDelegate Class Definition

    The \c PixelDelegate class is defined as follows:

    \snippet examples/itemviews/pixelator/pixeldelegate.h 0

    This class provides only basic features for a delegate so, unlike the
    \l{Spin Box Delegate Example}{Spin Box Delegate} example, we subclass
    QAbstractItemDelegate instead of QItemDelegate.

    We only need to reimplement \l{QAbstractItemDelegate::paint()}{paint()} and
    \l{QAbstractItemDelegate::sizeHint()}{sizeHint()} in this class.
    However, we also provide a delegate-specific \c setPixelSize() function so
    that we can change the delegate's behavior via the signals and slots mechanism.

    \section1 PixelDelegate Class Implementation

    The \c PixelDelegate constructor is used to set up a default value for
    the size of each "pixel" that it renders. The base class constructor is
    also called to ensure that the delegate is set up with a parent object,
    if one is supplied:

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 0

    Each item is rendered by the delegate's
    \l{QAbstractItemDelegate::paint()}{paint()} function. The view calls this
    function with a ready-to-use QPainter object, style information that the
    delegate should use to correctly draw the item, and an index to the item in
    the model:

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 1

    The first task the delegate has to perform is to draw the item's background
    correctly. Usually, selected items appear differently to non-selected items,
    so we begin by testing the state passed in the style option and filling the
    background if necessary.

    The radius of each circle is calculated in the following lines of code:

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 3
    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 4

    First, the largest possible radius of the circle is determined by taking the
    smallest dimension of the style option's \c rect attribute.
    Using the model index supplied, we obtain a value for the brightness of the
    relevant pixel in the image. The radius of the circle is calculated by
    scaling the brightness to fit within the item and subtracting it from the
    largest possible radius.

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 5
    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 6
    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 7

    We save the painter's state, turn on antialiasing (to obtain smoother
    curves), and turn off the pen.

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 8
    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 9

    The foreground of the item (the circle representing a pixel) must be
    rendered using an appropriate brush. For unselected items, we will use a
    solid black brush; selected items are drawn using a predefined brush from
    the style option's palette.

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 10

    Finally, we paint the circle within the rectangle specified by the style
    option and we call \l{QPainter::}{restore()} on the painter.

    The \c paint() function does not have to be particularly complicated; it is
    only necessary to ensure that the state of the painter when the function
    returns is the same as it was when it was called. This usually
    means that any transformations applied to the painter must be preceded by
    a call to QPainter::save() and followed by a call to QPainter::restore().

    The delegate's \l{QAbstractItemDelegate::}{sizeHint()} function
    returns a size for the item based on the predefined pixel size, initially set
    up in the constructor:

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 11

    The delegate's size is updated whenever the pixel size is changed.
    We provide a custom slot to do this:

    \snippet examples/itemviews/pixelator/pixeldelegate.cpp 12

    \section1 Using The Custom Delegate

    In this example, we use a main window to display a table of data, using the
    custom delegate to render each cell in a particular way. Much of the
    \c MainWindow class performs tasks that are not related to item views. Here,
    we only quote the parts that are relevant. You can look at the rest of the
    implementation by following the links to the code at the top of this
    document.

    In the constructor, we set up a table view, turn off its grid, and hide its
    headers:

    \snippet examples/itemviews/pixelator/mainwindow.cpp 0
    \dots
    \snippet examples/itemviews/pixelator/mainwindow.cpp 1

    This enables the items to be drawn without any gaps between them. Removing
    the headers also prevents the user from adjusting the sizes of individual
    rows and columns. 

    We also set the minimum section size to 1 on the headers. If we
    didn't, the headers would default to a larger size, preventing
    us from displaying really small items (which can be specified
    using the \gui{Pixel size} combobox).

    The custom delegate is constructed with the main window as its parent, so
    that it will be deleted correctly later, and we set it on the table view.

    \snippet examples/itemviews/pixelator/mainwindow.cpp 2

    Each item in the table view will be rendered by the \c PixelDelegate
    instance.

    We construct a spin box to allow the user to change the size of each "pixel"
    drawn by the delegate:

    \snippet examples/itemviews/pixelator/mainwindow.cpp 3

    This spin box is connected to the custom slot we implemented in the
    \c PixelDelegate class. This ensures that the delegate always draws each
    pixel at the currently specified size:

    \snippet examples/itemviews/pixelator/mainwindow.cpp 4
    \dots
    \snippet examples/itemviews/pixelator/mainwindow.cpp 5

    We also connect the spin box to a slot in the \c MainWindow class. This
    forces the view to take into account the new size hints for each item;
    these are provided by the delegate in its \c sizeHint() function.

    \snippet examples/itemviews/pixelator/mainwindow.cpp 6

    We explicitly resize the columns and rows to match the
    \gui{Pixel size} combobox.
*/