qt5base-lts/examples/widgets/doc/dropsite.qdoc

// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example draganddrop/dropsite
    \title Drop Site Example

    \brief The example shows how to distinguish the various MIME formats available
    in a drag and drop operation.

    \image dropsite-example.png Screenshot of the Drop Site example

    The Drop Site example accepts drops from other applications, and displays
    the MIME formats provided by the drag object.

    There are two classes, \c DropArea and \c DropSiteWindow, and a \c main()
    function in this example. A \c DropArea object is instantiated in
    \c DropSiteWindow; a \c DropSiteWindow object is then invoked in the
    \c main() function.

    \section1 DropArea Class Definition

    The \c DropArea class is a subclass of QLabel with a public slot,
    \c clear(), and a \c changed() signal.

    \snippet draganddrop/dropsite/droparea.h DropArea header part1

    In addition, \c DropArea contains reimplementations of four \l{QWidget}
    event handlers:

    \list 1
        \li \l{QWidget::dragEnterEvent()}{dragEnterEvent()}
        \li \l{QWidget::dragMoveEvent()}{dragMoveEvent()}
        \li \l{QWidget::dragLeaveEvent()}{dragLeaveEvent()}
        \li \l{QWidget::dropEvent()}{dropEvent()}
    \endlist

    These event handlers are further explained in the implementation of the
    \c DropArea class.

    \snippet draganddrop/dropsite/droparea.h DropArea header part2

    \section1 DropArea Class Implementation

    In the \c DropArea constructor, we set the \l{QWidget::setMinimumSize()}
    {minimum size} to 200x200 pixels, the \l{QFrame::setFrameStyle()}
    {frame style} to both QFrame::Sunken and QFrame::StyledPanel, and we align
    its contents to the center.

    \snippet draganddrop/dropsite/droparea.cpp DropArea constructor

    Also, we enable drop events in \c DropArea by setting the
    \l{QWidget::acceptDrops()}{acceptDrops} property to \c true. Then,
    we enable the \l{QWidget::autoFillBackground()}{autoFillBackground}
    property and invoke the \c clear() function.

    The \l{QWidget::dragEnterEvent()}{dragEnterEvent()} event handler is
    called when a drag is in progress and the mouse enters the \c DropArea
    object. For the \c DropSite example, when the mouse enters \c DropArea,
    we set its text to "<drop content>" and highlight its background.

    \snippet draganddrop/dropsite/droparea.cpp dragEnterEvent() function

    Then, we invoke \l{QDropEvent::acceptProposedAction()}
    {acceptProposedAction()} on \a event, setting the drop action to the one
    proposed. Lastly, we emit the \c changed() signal, with the data that was
    dropped and its MIME type information as a parameter.

    For \l{QWidget::dragMoveEvent()}{dragMoveEvent()}, we just accept the
    proposed QDragMoveEvent object, \a event, with
    \l{QDropEvent::acceptProposedAction()}{acceptProposedAction()}.

    \snippet draganddrop/dropsite/droparea.cpp dragMoveEvent() function

    The \c DropArea class's implementation of \l{QWidget::dropEvent()}
    {dropEvent()} extracts the \a{event}'s mime data and displays it
    accordingly.

    \snippet draganddrop/dropsite/droparea.cpp dropEvent() function part1

    The \c mimeData object can contain one of the following objects: an image,
    HTML text, Markdown text, plain text, or a list of URLs.

    \snippet draganddrop/dropsite/droparea.cpp dropEvent() function part2

    \list
        \li If \c mimeData contains an image, we display it in \c DropArea with
           \l{QLabel::setPixmap()}{setPixmap()}.
        \li If \c mimeData contains HTML, we display it with
           \l{QLabel::setText()}{setText()} and set \c{DropArea}'s text format
           as Qt::RichText.
        \li If \c mimeData contains Markdown, we display it with
           \l{QLabel::setText()}{setText()} and set \c{DropArea}'s text format
           as Qt::MarkdownText.
        \li If \c mimeData contains plain text, we display it with
           \l{QLabel::setText()}{setText()} and set \c{DropArea}'s text format
           as Qt::PlainText. In the event that \c mimeData contains URLs, we
           iterate through the list of URLs to display them on individual
           lines.
        \li If \c mimeData contains other types of objects, we set
           \c{DropArea}'s text, with \l{QLabel::setText()}{setText()} to
           "Cannot display data" to inform the user.
    \endlist

    We then set \c{DropArea}'s \l{QWidget::backgroundRole()}{backgroundRole} to
    QPalette::Dark and we accept \c{event}'s proposed action.

    \snippet draganddrop/dropsite/droparea.cpp dropEvent() function part3

    The \l{QWidget::dragLeaveEvent()}{dragLeaveEvent()} event handler is
    called when a drag is in progress and the mouse leaves the widget.

    \snippet draganddrop/dropsite/droparea.cpp dragLeaveEvent() function

    For \c{DropArea}'s implementation, we clear invoke \c clear() and then
    accept the proposed event.

    The \c clear() function sets the text in \c DropArea to "<drop content>"
    and sets the \l{QWidget::backgroundRole()}{backgroundRole} to
    QPalette::Dark. Lastly, it emits the \c changed() signal.

    \snippet draganddrop/dropsite/droparea.cpp clear() function

    \section1 DropSiteWindow Class Definition

    The \c DropSiteWindow class contains a constructor and a public slot,
    \c updateFormatsTable().

    \snippet draganddrop/dropsite/dropsitewindow.h DropSiteWindow header

    The class also contains a private instance of \c DropArea, \c dropArea,
    QLabel, \c abstractLabel, QTableWidget, \c formatsTable, QDialogButtonBox,
    \c buttonBox, and two QPushButton objects, \c clearButton and
    \c quitButton.

    \section1 DropSiteWindow Class Implementation

    In the constructor of \c DropSiteWindow, we instantiate \c abstractLabel
    and set its \l{QLabel::setWordWrap()}{wordWrap} property to \c true. We
    also call the \l{QLabel::adjustSize()}{adjustSize()} function to adjust
    \c{abstractLabel}'s size according to its contents.

    \snippet draganddrop/dropsite/dropsitewindow.cpp constructor part1

    Then we instantiate \c dropArea and connect its \c changed() signal to
    \c{DropSiteWindow}'s \c updateFormatsTable() slot.

    \snippet draganddrop/dropsite/dropsitewindow.cpp constructor part2

    We now set up the QTableWidget object, \c formatsTable. Its
    horizontal header is set using a QStringList object, \c labels. The number
    of columms are set to two and the table is not editable. Also, the
    \c{formatTable}'s horizontal header is formatted to ensure that its second
    column stretches to occupy additional space available.

    \snippet draganddrop/dropsite/dropsitewindow.cpp constructor part3

    Three QPushButton objects, \c clearButton, \c copyButton, and \c quitButton,
    are instantiated and added to \c buttonBox - a QDialogButtonBox object. We
    use QDialogButtonBox here to ensure that the push buttons are presented in a
    layout that conforms to the platform's style.

    \snippet draganddrop/dropsite/dropsitewindow.cpp constructor part4

    The \l{QPushButton::clicked()}{clicked()} signals for \c copyButton,
    \c clearButton, and \c quitButton are connected to \c copy(),
    \c clear() and \l{QWidget::close()}{close()}, respectively.

    For the layout, we use a QVBoxLayout, \c mainLayout, to arrange our widgets
    vertically. We also set the window title to "Drop Site" and the minimum
    size to 350x500 pixels.

    \snippet draganddrop/dropsite/dropsitewindow.cpp constructor part5

    We move on to the \c updateFormatsTable() function. This function updates
    the \c formatsTable, displaying the MIME formats of the object dropped onto
    the \c DropArea object. First, we set \l{QTableWidget}'s
    \l{QTableWidget::setRowCount()}{rowCount} property to 0. Then, we validate
    to ensure that the QMimeData object passed in is a valid object.

    \snippet draganddrop/dropsite/dropsitewindow.cpp updateFormatsTable() part1

    Once we are sure that \c mimeData is valid, we iterate through its
    supported formats.

    \note The \l{QMimeData::formats()}{formats()} function returns a
    QStringList object, containing all the formats supported by the
    \c mimeData.

    \snippet draganddrop/dropsite/dropsitewindow.cpp updateFormatsTable() part2

    Within each iteration, we create a QTableWidgetItem, \c formatItem and we
    set its \l{QTableWidgetItem::setFlags()}{flags} to Qt::ItemIsEnabled, and
    its \l{QTableWidgetItem::setTextAlignment()}{text alignment} to Qt::AlignTop
    and Qt::AlignLeft.

    A QString object, \c text, is customized to display data according to the
    contents of \c format. We invoke \l{QString}'s \l{QString::simplified()}
    {simplified()} function on \c text, to obtain a string that has no
    additional space before, after or in between words.

    \snippet draganddrop/dropsite/dropsitewindow.cpp updateFormatsTable() part3

    If \c format contains a list of URLs, we iterate through them, using spaces
    to separate them. On the other hand, if \c format contains an image, we
    display the data by converting the text to hexadecimal.

    \snippet draganddrop/dropsite/dropsitewindow.cpp updateFormatsTable() part4

    Once \c text has been customized to contain the appropriate data, we insert
    both \c format and \c text into \c formatsTable with
    \l{QTableWidget::setItem()}{setItem()}. Lastly, we invoke
    \l{QTableView::resizeColumnToContents()}{resizeColumnToContents()} on
    \c{formatsTable}'s first column.

    \section1 The main() Function

    Within the \c main() function, we instantiate \c DropSiteWindow and invoke
    its \l{QWidget::show()}{show()} function.

    \snippet draganddrop/dropsite/main.cpp main() function
*/