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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Drag and Drop</h1>

 
<p> 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.
<p> <ul>
<li><a href="#dragging">Dragging</a></li>
<li><a href="#dropping">Dropping</a></li>
<li><a href="#the-clipboard">The Clipboard</a></li>
<li><a href="#drag-and-drop-action">Drag and Drop Actions</a></li>
<li><a href="#adding-new-drag-and-drop-types">Adding New Drag and Drop
Types</a></li>
<li><a href="#advanced">Advanced Drag-and-Drop</a></li>
<li><a href="#inter-operating-with-other-applications">Inter-operating
with Other Applications</a></li> 
</ul>
<p> For drag and drop examples see (in increasing order of
sophistication): <tt>qt/examples/iconview/simple_dd</tt>, <tt>qt/examples/dragdrop</tt> and <tt>qt/examples/fileiconview</tt>. See also the
QMultiLineEdit widget source code.
<p> <h3><a name="dragging">Dragging</a></h3>
<p> To start a drag, for example in a <a href="qwidget.html#mouseMoveEvent">mouse motion event</a>, create an object of the
<a href="qdragobject.html">QDragObject</a> subclass appropriate for your media, such as
<a href="qtextdrag.html">QTextDrag</a> for text and <a href="qimagedrag.html">QImageDrag</a> for images. Then call
the drag() method. This is all you need for simple dragging
of existing types.
<p> For example, to start dragging some text from a widget:
<pre>
  void MyWidget::startDrag()
  {
    <a href="qdragobject.html">QDragObject</a> *d = new <a href="qtextdrag.html">QTextDrag</a>( myHighlightedText(), this );
    d-&gt;<a href="qdragobject.html#dragCopy">dragCopy</a>();
    // do NOT delete d.
  }
  </pre>
 
<p> Note that the QDragObject is not deleted after the drag.  The
QDragObject needs to persist after the drag is apparently finished -
it may still be communicating with another process.  Eventually Qt
will delete the object.  If the widget owning the drag object is
deleted before then, any pending drop will be cancelled and the drag
object deleted.  For this reason, you should be careful what the
object references.
<p> <h3><a name="dropping">Dropping</a></h3>
<p> To be able to receive media dropped on a widget, call
<a href="qwidget.html#setAcceptDrops">setAcceptDrops(TRUE)</a>
for the widget (e.g. in its constructor), and override the
event handler methods
<a href="qwidget.html#dragEnterEvent">dragEnterEvent()</a> and
<a href="qwidget.html#dropEvent">dropEvent()</a>.
For more sophisticated applications overriding
<a href="qwidget.html#dragMoveEvent">dragMoveEvent()</a> and
<a href="qwidget.html#dragLeaveEvent">dragLeaveEvent()</a> will also be
necessary.
<p> For example, to accept text and image drops:
<pre>
  MyWidget::MyWidget(...) :
    <a href="qwidget.html">QWidget</a>(...)
  {
    ...
    setAcceptDrops(TRUE);
  }

  void MyWidget::dragEnterEvent(QDragEnterEvent* event)
  {
    event-&gt;accept(
        QTextDrag::<a href="qtextdrag.html#canDecode">canDecode</a>(event) ||
        QImageDrag::<a href="qimagedrag.html#canDecode">canDecode</a>(event)
    );
  }

  void MyWidget::dropEvent(QDropEvent* event)
  {
    <a href="qimage.html">QImage</a> image;
    <a href="qstring.html">QString</a> text;

    if ( QImageDrag::<a href="qimagedrag.html#decode">decode</a>(event, image) ) {
      insertImageAt(image, event-&gt;pos());
    } else if ( QTextDrag::<a href="qtextdrag.html#decode">decode</a>(event, text) ) {
      insertTextAt(text, event-&gt;pos());
    }
  }
  </pre>
 
<p> <h3><a name="the-clipboard">The Clipboard</a></h3>
<p> The <a href="qdragobject.html">QDragObject</a>, <a href="qdragenterevent.html">QDragEnterEvent</a>, <a href="qdragmoveevent.html">QDragMoveEvent</a>, and <a href="qdropevent.html">QDropEvent</a>
classes are all subclasses of <a href="qmimesource.html">QMimeSource</a> - the class of objects
which provide
typed
information.  If you base your data
transfers on QDragObject, you not only get drag-and-drop, but you
also get traditional cut-and-paste for free - the <a href="qclipboard.html">QClipboard</a> has
two functions:
<pre> 
      setData(QMimeSource*)
      <a href="qmimesource.html">QMimeSource</a>* data()const
  </pre>
   
With these functions you can trivially put your drag-and-drop oriented
information on the clipboard:
<pre>
  void MyWidget::copy()
  {
    QApplication::<a href="qapplication.html#clipboard">clipboard</a>()-&gt;setData(
        new <a href="qtextdrag.html">QTextDrag</a>(myHighlightedText())
    );
  }

  void MyWidget::paste()
  {
    <a href="qstring.html">QString</a> text;
    if ( QTextDrag::<a href="qtextdrag.html#decode">decode</a>(QApplication::<a href="qapplication.html#clipboard">clipboard</a>()-&gt;data(), text) )
        insertText( text );
  }
  </pre>
 
You can even use <a href="qdragobject.html">QDragObject</a> subclasses as part of file IO.
For example,
if your application has a subclass of QDragObject that encodes
CAD designs in DXF format, your saving and loading code might be:
<pre>
  void MyWidget::save()
  {
    <a href="qfile.html">QFile</a> out(current_file_name);
    out.<a href="qfile.html#open">open</a>(IO_WriteOnly);
    MyCadDrag tmp(current_design);
    out.<a href="qiodevice.html#writeBlock">writeBlock</a>( tmp-&gt;encodedData( "image/x-dxf" ) );
  }

  void MyWidget::load()
  {
    <a href="qfile.html">QFile</a> in(current_file_name);
    in.<a href="qfile.html#open">open</a>(IO_ReadOnly);
    if ( !MyCadDrag::decode(in.<a href="qiodevice.html#readAll">readAll</a>(), current_design) ) {
        QMessageBox::<a href="qmessagebox.html#warning">warning</a>( this, "Format error",
            tr("The file \"%1\" is not in any supported format")
             .arg(current_file_name)
        );
    }
  }
  </pre>
 
Note how the QDragObject subclass is called "MyCadDrag", not
"MyDxfDrag" - because in the future you might extend it to
provide DXF, DWG, SVF, WMF, or even <a href="qpicture.html">QPicture</a> data to other
applications.
<p> <h3><a name="drag-and-drop-action">Drag and Drop Actions</a></h3>
<p> In the simpler cases, the target of a drag-and-drop receives
a copy of the data being dragged and the source decides whether
to delete the original.  This is the "Copy" action in <a href="qdropevent.html">QDropEvent</a>.
The target may also choose to understand other actions, specifically
the Move and Link actions.  If the target understands the Move action,
<em>the target</em>
is responsible for both the copy and delete operations and the source
will not attempt to delete the data itself.  If the target
understands the Link, it stores its own reference to the original
information, and again the source does not delete the original.
The most common use of drag-and-drop actions is when performing a Move
within the same widget - see the
<a href=#advanced>Advanced Drag-and-Drop</a> section below.
<p> 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.
<p> <h3><a name="adding-new-drag-and-drop-types">Adding New Drag and Drop
Types</a></h3>
<p> As suggested in the DXF example above,
drag-and-drop is not limited to text and images.  Any information
can be dragged and dropped.  To drag information between applications,
the two applications must be able to indicate to each other which data
formats they can accept and which they can produce. This is achieved using
<i><a href="http://www.rfc-editor.org/rfc/rfc1341.txt">MIME types</a></i>
- the drag source provides a list of MIME types that it can produce
(ordered from most appropriate to least appropriate), and the drop target
chooses which of those it can accept. For example, <a href="qtextdrag.html">QTextDrag</a> provides
support for the "<tt>text/plain</tt>" MIME type (ordinary unformatted
text), and the Unicode formats "<tt>text/utf16</tt>" and
"<tt>text/utf8</tt>"; <a href="qimagedrag.html">QImageDrag</a> provides for "<tt>image/</tt><tt>*</tt>",
where <tt>*</tt> is any image format that <a href="qimageio.html">QImageIO</a> supports; and the
<a href="quridrag.html">QUriDrag</a> subclass provides "<tt>text/uri-list</tt>", a standard format for
transferring a list of filenames (or URLs).
<p> To implement drag-and-drop of some type of information for which there
is no available <a href="qdragobject.html">QDragObject</a> subclass, the
first and most important step is to look for existing formats
that are appropriate - the Internet Assigned Numbers Authority
(<a href="http://www.iana.org">IANA</a>) provides a
<a href="http://www.isi.edu/in-notes/iana/assignments/media-types/">
hierarchical list of MIME media types</a>
at the Information Sciences Institute
(<a href="http://www.isi.edu">ISI</a>).
Using standard MIME types maximizes the inter-operability of your
application with other software now and in the future.
<p> To support an additional media type, subclass either QDragObject
or <a href="qstoreddrag.html">QStoredDrag</a>. Subclass QDragObject when you need to provide
support for multiple media types. Subclass the simpler QStoredDrag
when one type is sufficient.
<p> Subclasses of QDragObject will override the
<a href="qmimesource.html#format">const char* format(int i) const</a> and
<a href="qmimesource.html#encodedData">QByteArray encodedData(const char* mimetype) const</a>
members, and provide a set-method to encode
the media data and static members canDecode()
and decode() to decode incoming data, similar to
<a href="qimagedrag.html#canDecode">bool canDecode(QMimeSource*) const</a> and
<a href="qimagedrag.html#decode">QByteArray decode(QMimeSource*) const</a>
of <a href="qimagedrag.html">QImageDrag</a>.
Of course, you can
provide drag-only or drop-only support for a media type
by omitting some of these methods.
<p> Subclasses of QStoredDrag provide a set-method to encode
the media data and the same static members canDecode()
and decode() to decode incoming data.
<p> <h3><a name="advanced">Advanced Drag-and-Drop</a></h3>
<p> In the clipboard model, the user can <em>cut</em> or <em>copy</em> the source
information, then later paste it.  Similarly in the drag-and-drop
model, the user can drag a <em>copy</em> of the information or they can
drag the information itself to a new place (<em>moving</em> it).  The
drag-and-drop model however has an additional complication for
the programmer: the
program doesn't know whether the user wants to cut or copy until
the drop (paste) is done! For dragging between applications,
it makes no difference, but for dragging within an application,
the application must take a little extra care not to tread on
its own feet. For example, to
drag text around in a document, the drag start point and the
drop event might look like this:
<p> <pre>
  void MyEditor::startDrag()
  {
    <a href="qdragobject.html">QDragObject</a> *d = new <a href="qtextdrag.html">QTextDrag</a>(myHighlightedText(), this);
    if ( d-&gt;<a href="qdragobject.html#drag">drag</a>() &amp;&amp; d-&gt;<a href="qdragobject.html#target">target</a>() != this )
      cutMyHighlightedText();
  }

  void MyEditor::dropEvent(QDropEvent* event)
  {
    <a href="qstring.html">QString</a> text;

    if ( QTextDrag::<a href="qtextdrag.html#decode">decode</a>(event, text) ) {
      if ( event-&gt;source() == this &amp;&amp; event-&gt;action() == QDropEvent::Move ) {
        // Careful not to tread on my own feet
        event-&gt;acceptAction();
        moveMyHighlightedTextTo(event-&gt;pos());
      } else {
        pasteTextAt(text, event-&gt;pos());
      }
    }
  }
  </pre>
 
<p> Some widgets are more specific than just a "yes" or "no" response
when data is dragged onto them.  For example, a CAD program might
only accept drops of text onto text objects in the view.  In these
cases, the
<a href="qwidget.html#dragMoveEvent">dragMoveEvent()</a> is used
and an <em>area</em> is given for which the drag is accepted or ignored:
<pre>
  void MyWidget::dragMoveEvent(QDragMoveEvent* event)
  {
    if ( QTextDrag::<a href="qtextdrag.html#canDecode">canDecode</a>(event) ) {
      MyCadItem* item = findMyItemAt(event-&gt;pos());
      if ( item )
        event-&gt;accept();
    }
  }
  </pre>
 
If the computations to find objects are particularly slow,
you might achieve improved performance if you tell the system
an area for which you promise the acceptance persists:
<pre>
  void MyWidget::dragMoveEvent(QDragMoveEvent* event)
  {
    if ( QTextDrag::<a href="qtextdrag.html#canDecode">canDecode</a>(event) ) {
      MyCadItem* item = findMyItemAt(event-&gt;pos());
      if ( item ) {
        <a href="qrect.html">QRect</a> r = item-&gt;areaRelativeToMeClippedByAnythingInTheWay();
        if ( item-&gt;type() == MyTextType )
          event-&gt;accept( r );
        else
          event-&gt;ignore( r );
      }
    }
  }
  </pre>
 
<p> The dragMoveEvent() can also be used if you need to give
visual feedback as the drag progresses, to start timers, to
scroll the window, or whatever is appropriate (don't forget to
stop the scrolling and timers in a dragLeaveEvent() though).
<p> <h3><a name="inter-operating-with-other-applications">Inter-operating
with Other Applications</a></h3>
On X11, the public
<a class="r" href="http://www.cco.caltech.edu/~jafl/xdnd/">XDND protocol</a>
is used, while on Windows Qt uses the OLE standard, and Qt/Mac 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 has facilities
for translating proprietary clipboard formats to and from
MIME types.  This interface will be made public at some time, but
if you need to do such translations now, contact your Qt
Technical Support service.
<p> 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 &lt;koss@napri.sk&gt;
and Trolltech. Here is the original copyright notice:
<p> 
<p> Copyright 1996 Daniel Dardailler.
<p> 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.
<p> Modifications Copyright 1999 Matt Koss, under the same license as
above.

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<td>Copyright &copy; 2001 
<a href="http://www.trolltech.com">Trolltech</a><td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
<td align=right><div align=right>Qt version 3.0.2</div>
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