source: trunk/doc/src/frameworks-technologies/dnd.qdoc

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/*!
    \page dnd.html
    \title Drag and Drop
    \brief An overview of the drag and drop system provided by Qt.

    \ingroup qt-gui-concepts

    Drag and drop provides a simple visual mechanism which users can use
    to transfer information between and within applications. (In the
    literature this is referred to as a "direct manipulation model".) Drag
    and drop is similar in function to the clipboard's cut and paste
    mechanism.

    \tableofcontents

    This document describes the basic drag and drop mechanism and
    outlines the approach used to enable it in custom widgets. Drag
    and drop operations are also supported by Qt's item views and by
    the graphics view framework. More information is available in
    \l{Using drag and drop with item views} and \l{Graphics View
    Framework}.

    \section1 Drag and Drop Classes

    These classes deal with drag and drop and the necessary mime type
    encoding and decoding.

    \annotatedlist draganddrop

    \section1 Configuration

    The QApplication object provides some properties that are related
    to drag and drop operations:

    \list
    \i \l{QApplication::startDragTime} describes the amount of time in
       milliseconds that the user must hold down a mouse button over an
       object before a drag will begin.
    \i \l{QApplication::startDragDistance} indicates how far the user has to
       move the mouse while holding down a mouse button before the movement
       will be interpreted as dragging. Use of high values for this quantity
       prevents accidental dragging when the user only meant to click on an
       object.
    \endlist

    These quantities provide sensible default values for you to use if you
    provide drag and drop support in your widgets.

    \section1 Dragging

    To start a drag, create a QDrag object, and call its
    exec() function. In most applications, it is a good idea to begin a drag
    and drop operation only after a mouse button has been pressed and the
    cursor has been moved a certain distance. However, the simplest way to
    enable dragging from a widget is to reimplement the widget's
    \l{QWidget::mousePressEvent()}{mousePressEvent()} and start a drag
    and drop operation:

    \snippet doc/src/snippets/dragging/mainwindow.cpp 0
    \dots 8
    \snippet doc/src/snippets/dragging/mainwindow.cpp 2

    Although the user may take some time to complete the dragging operation,
    as far as the application is concerned the exec() function is a blocking
    function that returns with \l{Qt::DropActions}{one of several values}.
    These indicate how the operation ended, and are described in more detail
    below.

    Note that the exec() function does not block the main event loop.

    For widgets that need to distinguish between mouse clicks and drags, it
    is useful to reimplement the widget's
    \l{QWidget::mousePressEvent()}{mousePressEvent()} function to record to
    start position of the drag:

    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 6

    Later, in \l{QWidget::mouseMoveEvent()}{mouseMoveEvent()}, we can determine
    whether a drag should begin, and construct a drag object to handle the
    operation:

    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 7
    \dots
    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 8

    This particular approach uses the \l QPoint::manhattanLength() function
    to get a rough estimate of the distance between where the mouse click
    occurred and the current cursor position. This function trades accuracy
    for speed, and is usually suitable for this purpose.

    \section1 Dropping

    To be able to receive media dropped on a widget, call
    \l{QWidget::setAcceptDrops()}{setAcceptDrops(true)} for the widget,
    and reimplement the \l{QWidget::dragEnterEvent()}{dragEnterEvent()} and
    \l{QWidget::dropEvent()}{dropEvent()} event handler functions.

    For example, the following code enables drop events in the constructor of
    a QWidget subclass, making it possible to usefully implement drop event
    handlers:

    \snippet doc/src/snippets/dropevents/window.cpp 0
    \dots
    \snippet doc/src/snippets/dropevents/window.cpp 1
    \snippet doc/src/snippets/dropevents/window.cpp 2

    The dragEnterEvent() function is typically used to inform Qt about the
    types of data that the widget accepts.
    You must reimplement this function if you want to receive either
    QDragMoveEvent or QDropEvent in your reimplementations of
    \l{QWidget::dragMoveEvent()}{dragMoveEvent()} and
    \l{QWidget::dropEvent()}{dropEvent()}.

    The following code shows how \l{QWidget::dragEnterEvent()}{dragEnterEvent()}
    can be reimplemented to
    tell the drag and drop system that we can only handle plain text:

    \snippet doc/src/snippets/dropevents/window.cpp 3

    The \l{QWidget::dropEvent()}{dropEvent()} is used to unpack dropped data
    and handle it in way that is suitable for your application.

    In the following code, the text supplied in the event is passed to a
    QTextBrowser and a QComboBox is filled with the list of MIME types that
    are used to describe the data:

    \snippet doc/src/snippets/dropevents/window.cpp 4

    In this case, we accept the proposed action without checking what it is.
    In a real world application, it may be necessary to return from the
    \l{QWidget::dropEvent()}{dropEvent()} function without accepting the
    proposed action or handling
    the data if the action is not relevant. For example, we may choose to
    ignore Qt::LinkAction actions if we do not support
    links to external sources in our application.

    \section2 Overriding Proposed Actions

    We may also ignore the proposed action, and perform some other action on
    the data. To do this, we would call the event object's
    \l{QDropEvent::setDropAction()}{setDropAction()} with the preferred
    action from Qt::DropAction before calling \l{QEvent::}{accept()}.
    This ensures that the replacement drop action is used instead of the
    proposed action.

    For more sophisticated applications, reimplementing
    \l{QWidget::dragMoveEvent()}{dragMoveEvent()} and
    \l{QWidget::dragLeaveEvent()}{dragLeaveEvent()} will let you make
    certain parts of your widgets sensitive to drop events, and give you more
    control over drag and drop in your application.

    \section2 Subclassing Complex Widgets

    Certain standard Qt widgets provide their own support for drag and drop.
    When subclassing these widgets, it may be necessary to reimplement
    \l{QWidget::dragMoveEvent()}{dragMoveEvent()} in addition to
    \l{QWidget::dragEnterEvent()}{dragEnterEvent()} and
    \l{QWidget::dropEvent()}{dropEvent()} to prevent the base class from
    providing default drag and drop handling, and to handle any special
    cases you are interested in.

    \section1 Drag and Drop Actions

    In the simplest case, the target of a drag and drop action receives a
    copy of the data being dragged, and the source decides whether to
    delete the original. This is described by the \c CopyAction action.
    The target may also choose to handle other actions, specifically the
    \c MoveAction and \c LinkAction actions. If the source calls
    QDrag::exec(), and it returns \c MoveAction, the source is responsible
    for deleting any original data if it chooses to do so. The QMimeData
    and QDrag objects created by the source widget \e{should not be deleted}
    - they will be destroyed by Qt. The target is responsible for taking
    ownership of the data sent in the drag and drop operation; this is
    usually done by keeping references to the data.

    If the target understands the \c LinkAction action, it should
    store its own reference to the original information; the source
    does not need to perform any further processing on the data. The
    most common use of drag and drop actions is when performing a
    Move within the same widget; see the section on \l{Drop Actions}
    for more information about this feature.

    The other major use of drag actions is when using a reference type
    such as text/uri-list, where the dragged data are actually references
    to files or objects.

    \section1 Adding New Drag and Drop Types

    Drag and drop is not limited to text and images. Any type of information
    can be transferred in a drag and drop operation. To drag information
    between applications, the applications must be able to indicate to each
    other which data formats they can accept and which they can produce.
    This is achieved using
    \l{http://www.rfc-editor.org/rfc/rfc1341.txt}{MIME types}. The QDrag
    object constructed by the source contains a list of MIME types that it
    uses to represent the data (ordered from most appropriate to least
    appropriate), and the drop target uses one of these to access the data.
    For common data types, the convenience functions handle the MIME types
    used transparently but, for custom data types, it is necessary to
    state them explicitly.

    To implement drag and drop actions for a type of information that is
    not covered by the QDrag convenience functions, the first and most
    important step is to look for existing formats that are appropriate:
    The Internet Assigned Numbers Authority (\l{http://www.iana.org}{IANA})
    provides a
    \l{http://www.iana.org/assignments/media-types/}{hierarchical
    list of MIME media types} at the Information Sciences Institute
    (\l{http://www.isi.edu}{ISI}).
    Using standard MIME types maximizes the interoperability of
    your application with other software now and in the future.

    To support an additional media type, simply set the data in the QMimeData
    object with the \l{QMimeData::setData()}{setData()} function, supplying
    the full MIME type and a QByteArray containing the data in the appropriate
    format. The following code takes a pixmap from a label and stores it
    as a Portable Network Graphics (PNG) file in a QMimeData object:

    \snippet doc/src/snippets/separations/finalwidget.cpp 0

    Of course, for this case we could have simply used
    \l{QMimeData::setImageData()}{setImageData()} instead to supply image data
    in a variety of formats:

    \snippet doc/src/snippets/separations/finalwidget.cpp 1

    The QByteArray approach is still useful in this case because it provides
    greater control over the amount of data stored in the QMimeData object.

    Note that custom datatypes used in item views must be declared as
    \l{QMetaObject}{meta objects} and that stream operators for them
    must be implemented.

    \section1 Drop Actions

    In the clipboard model, the user can \e cut or \e copy the source
    information, then later paste it. Similarly in the drag and drop
    model, the user can drag a \e copy of the information or they can drag
    the information itself to a new place (\e moving it). The
    drag and drop model has an additional complication for the programmer:
    The program doesn't know whether the user wants to cut or copy the
    information until the operation is complete. This often makes no
    difference when dragging information between applications, but within
    an application it is important to check which drop action was used.

    We can reimplement the mouseMoveEvent() for a widget, and start a drag
    and drop operation with a combination of possible drop actions. For
    example, we may want to ensure that dragging always moves objects in
    the widget:

    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 7
    \dots
    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 8

    The action returned by the exec() function may default to a
    \c CopyAction if the information is dropped into another application
    but, if it is dropped in another widget in the same application, we
    may obtain a different drop action.

    The proposed drop actions can be filtered in a widget's dragMoveEvent()
    function. However, it is possible to accept all proposed actions in
    the dragEnterEvent() and let the user decide which they want to accept
    later:

    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 0

    When a drop occurs in the widget, the dropEvent() handler function is
    called, and we can deal with each possible action in turn. First, we
    deal with drag and drop operations within the same widget:

    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 1

    In this case, we refuse to deal with move operations. Each type of drop
    action that we accept is checked and dealt with accordingly:

    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 2
    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 3
    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 4
    \dots
    \snippet doc/src/snippets/draganddrop/dragwidget.cpp 5

    Note that we checked for individual drop actions in the above code.
    As mentioned above in the section on
    \l{#Overriding Proposed Actions}{Overriding Proposed Actions}, it is
    sometimes necessary to override the proposed drop action and choose a
    different one from the selection of possible drop actions.
    To do this, you need to check for the presence of each action in the value
    supplied by the event's \l{QDropEvent::}{possibleActions()}, set the drop
    action with \l{QDropEvent::}{setDropAction()}, and call
    \l{QEvent::}{accept()}.

    \section1 Drop Rectangles

    The widget's dragMoveEvent() can be used to restrict drops to certain parts
    of the widget by only accepting the proposed drop actions when the cursor
    is within those areas. For example, the following code accepts any proposed
    drop actions when the cursor is over a child widget (\c dropFrame):

    \snippet doc/src/snippets/droprectangle/window.cpp 0

    The dragMoveEvent() can also be used if you need to give visual
    feedback during a drag and drop operation, to scroll the window, or
    whatever is appropriate.

    \section1 The Clipboard

    Applications can also communicate with each other by putting data on
    the clipboard. To access this, you need to obtain a QClipboard object
    from the QApplication object:

    \snippet examples/widgets/charactermap/mainwindow.cpp 3

    The QMimeData class is used to represent data that is transferred to and
    from the clipboard. To put data on the clipboard, you can use the
    setText(), setImage(), and setPixmap() convenience functions for common
    data types. These functions are similar to those found in the QMimeData
    class, except that they also take an additional argument that controls
    where the data is stored: If \l{QClipboard::Mode}{Clipboard} is
    specified, the data is placed on the clipboard; if
    \l{QClipboard::Mode}{Selection} is specified, the data is placed in the
    mouse selection (on X11 only). By default, data is put on the clipboard.

    For example, we can copy the contents of a QLineEdit to the clipboard
    with the following code:

    \snippet examples/widgets/charactermap/mainwindow.cpp 11

    Data with different MIME types can also be put on the clipboard.
    Construct a QMimeData object and set data with setData() function in
    the way described in the previous section; this object can then be
    put on the clipboard with the
    \l{QClipboard::setMimeData()}{setMimeData()} function.

    The QClipboard class can notify the application about changes to the
    data it contains via its \l{QClipboard::dataChanged()}{dataChanged()}
    signal. For example, we can monitor the clipboard by connecting this
    signal to a slot in a widget:

    \snippet doc/src/snippets/clipboard/clipwindow.cpp 0

    The slot connected to this signal can read the data on the clipboard
    using one of the MIME types that can be used to represent it:

    \snippet doc/src/snippets/clipboard/clipwindow.cpp 1
    \dots
    \snippet doc/src/snippets/clipboard/clipwindow.cpp 2

    The \l{QClipboard::selectionChanged()}{selectionChanged()} signal can
    be used on X11 to monitor the mouse selection.

    \section1 Examples

    \list
    \o \l{draganddrop/draggableicons}{Draggable Icons}
    \o \l{draganddrop/draggabletext}{Draggable Text}
    \o \l{draganddrop/dropsite}{Drop Site}
    \o \l{draganddrop/fridgemagnets}{Fridge Magnets}
    \o \l{draganddrop/puzzle}{Drag and Drop Puzzle}
    \endlist

    \section1 Interoperating with Other Applications

    On X11, the public \l{http://www.newplanetsoftware.com/xdnd/}{XDND
    protocol} is used, while on Windows Qt uses the OLE standard, and
    Qt for Mac OS X uses the Carbon Drag Manager. On X11, XDND uses MIME,
    so no translation is necessary. The Qt API is the same regardless of
    the platform. On Windows, MIME-aware applications can communicate by
    using clipboard format names that are MIME types. Already some
    Windows applications use MIME naming conventions for their
    clipboard formats. Internally, Qt uses QWindowsMime and
    QMacPasteboardMime for translating proprietary clipboard formats
    to and from MIME types.

    On X11, Qt also supports drops via the Motif Drag & Drop Protocol. The
    implementation incorporates some code that was originally written by
    Daniel Dardailler, and adapted for Qt by Matt Koss <koss@napri.sk>
    and Nokia. Here is the original copyright notice:

    \legalese
    Copyright 1996 Daniel Dardailler.
    Copyright 1999 Matt Koss

    Permission to use, copy, modify, distribute, and sell this software
    for any purpose is hereby granted without fee, provided that the above
    copyright notice appear in all copies and that both that copyright
    notice and this permission notice appear in supporting documentation,
    and that the name of Daniel Dardailler not be used in advertising or
    publicity pertaining to distribution of the software without specific,
    written prior permission. Daniel Dardailler makes no representations
    about the suitability of this software for any purpose. It is
    provided "as is" without express or implied warranty.
    \endlegalese
    \omit NOTE: The original version of this copyright notice can be found
    in qmotifdnd_x11.cpp. \endomit

    \note The Motif Drag \& Drop Protocol only allows receivers to
    request data in response to a QDropEvent. If you attempt to
    request data in response to e.g. a QDragMoveEvent, an empty
    QByteArray is returned.
*/