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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit -  Drag and Drop</title><style type="text/css"><!--
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<h1 align=center> Drag and Drop</h1><br clear="all">
  Drag-and-drop is a direct-manipulation model for allowing the user to
  transfer information between and within applications. It is similar
  in function to the cut-and-paste clipboard model.
<p>
  <h3>Dragging</h3>
<p>
  To start a drag, for example in a <a href="qwidget.html#a51d92">mouse motion event</a>, create an object of the
  QDragObject subclass appropriate for your media, such as
  QTextDrag for text and QImageDrag 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::startDragging()
  {
    <a href="qdragobject.html">QDragObject</a> *d = new <a href="qtextdrag.html">QTextDrag</a>( myHighlightedText(), this );
    d-&gt;<a href="qdragobject.html#9d27cf">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>Dropping</h3>
<p>
  To be able to receive media dropped on a widget, call
  <a href="qwidget.html#8169cb">setAcceptDrops(TRUE)</a>
  for the widget (eg. in its constructor), and override the
  event handler methods
  <a href="qwidget.html#c8639f">dragEnterEvent()</a>,
  <a href="qwidget.html#3b47df">dragMoveEvent()</a>,
  <a href="qwidget.html#fdd48c">dragLeaveEvent()</a>, and
  <a href="qwidget.html#e9b4c3">dropEvent()</a>.
<p>
  For example, to accept text and image drops:
  <pre>  MyWidget::MyWidget(...) :
    <a href="qwidget.html">QWidget</a>(...)
  {
    ...
    setAcceptDrops(TRUE);
  }

  void MyWidget::dragEnterEvent(<a href="qdragenterevent.html">QDragEnterEvent</a>* event)
  {
    event-&gt;<a href="qdropevent.html#da0c5a">accept</a>(
        <a href="qtextdrag.html#48ca8d">QTextDrag::canDecode</a>(event) ||
        <a href="qimagedrag.html#c35128">QImageDrag::canDecode</a>(event)
    );
  }

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

    if ( <a href="qimagedrag.html#037fd2">QImageDrag::decode</a>(event, image) ) {
      insertImageAt(image, event-&gt;<a href="qdropevent.html#b3ba77">pos</a>());
    } else if ( <a href="qtextdrag.html#b02128">QTextDrag::decode</a>(event, text) ) {
      insertTextAt(text, event-&gt;<a href="qdropevent.html#b3ba77">pos</a>());
    }
  }
</pre>
<p>
  <h3>The Clipboard</h3>
<p>
  The QDragObject, QDragEnterEvent, QDragMoveEvent, and QDropEvent
  classes are all subclasses of QMimeSource - 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 QClipboard has
  two functions:
  <pre> setData(<a href="qmimesource.html">QMimeSource</a>*)</pre>
 and
  <pre> <a href="qmimesource.html">QMimeSource</a>* data()const
</pre>
.  With these, you can trivially
  put your drag-and-drop oriented information on the clipboard:
  <pre>  void MyWidget::copy()
  {
    <a href="qapplication.html#1c95b5">QApplication::clipboard</a>()-&gt;setData(
        new QTextDrag(myHighlightedText())
    );
  }

  void MyWidget::paste()
  {
    <a href="qstring.html">QString</a> text;
    if ( <a href="qtextdrag.html#b02128">QTextDrag::decode</a>(<a href="qapplication.html#1c95b5">QApplication::clipboard</a>()-&gt;data(), text) )
        insertText( text );
  }
</pre>
<p>
  You can even use QDragObject 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#255995">open</a>(IO_WriteOnly);
    MyCadDrag tmp(current_design);
    out.<a href="qfile.html#a44b6a">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#255995">open</a>(IO_ReayOnly);
    if ( !MyCadDrag::decode(in.<a href="qiodevice.html#c272a4">readAll</a>(), current_design) ) {
        <a href="qmessagebox.html#63edba">QMessageBox::warning</a>( this, "Format error",
            tr("The file \"%1\" is not in any supported format")
             .arg(current_file_name)
        );
    }
  }
</pre>
<p>
  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 QPicture data to other
  applications.
<p>
  <h3>Drag-and-drop action</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 QDropEvent.
  The target may also choose to understand other actions, specifically
  the Move and Link actions.  If the target understands the Move action,
  <em>it</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 some kind of 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 is actually references
  to files or objects.
<p>
  <h3>Adding new drag-and-drop types</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 need a way to agree on the type of information
  they both understand.
  The way they do it is 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 handle. For example, QTextDrag 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>"; QImageDrag provides for "<tt>image/</tt><tt>*</tt>",
  where <tt>*</tt> is any image format that <a href="qimageio.html">QImageIO</a> supports; and the
  QUriDrag 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 QDragObject 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>).
  This maximizes inter-operability of your software with other
  software now and in the future.
<p>
  To support an additional media type, subclass either QDragObject
  or QStoredDrag. 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#17ce96">const char* format(int i) const</a> and
  <a href="qmimesource.html#81afea">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#c35128">bool canDecode(QMimeSource*) const</a> and
  <a href="qimagedrag.html#037fd2">QByteArray decode(QMimeSource*) const</a>
  of QImageDrag.
  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>
  <a name=advanced><h3>Advanced Drag-and-Drop</h3></a>
<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 want 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::startDragging()
  {
    <a href="qdragobject.html">QDragObject</a> *d = new <a href="qtextdrag.html">QTextDrag</a>(myHighlightedText(), this);
    if ( d-&gt;<a href="qdragobject.html#61c2fe">drag</a>() &amp;&amp; d-&gt;<a href="qdragobject.html#cdb79f">target</a>() != this )
      cutMyHighlightedText();
  }

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

    if ( <a href="qtextdrag.html#b02128">QTextDrag::decode</a>(event, text) ) {
      if ( event-&gt;<a href="qdropevent.html#00626c">source</a>() == this &amp;&amp; event-&gt;<a href="qdropevent.html#d9f8b7">action</a>() == QDropEvent::Move ) {
        // Careful not to tread on my own feet
        event-&gt;<a href="qdropevent.html#d37a0e">acceptAction</a>();
        moveMyHighlightedTextTo(event-&gt;<a href="qdropevent.html#b3ba77">pos</a>());
      } else {
        pasteTextAt(text, event-&gt;<a href="qdropevent.html#b3ba77">pos</a>());
      }
    }
  }
</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#3b47df">dragMoveEvent()</a> is used
  and an <em>area</em> is given for which the drag is accepted or ignored:
  <pre>  void MyWidget::dragMoveEvent(<a href="qdragmoveevent.html">QDragMoveEvent</a>* event)
  {
    if ( <a href="qtextdrag.html#48ca8d">QTextDrag::canDecode</a>(event) ) {
      MyCadItem* item = findMyItemAt(event-&gt;<a href="qdropevent.html#b3ba77">pos</a>());
      if ( item )
        event-&gt;<a href="qdragmoveevent.html#250cc4">accept</a>();
    }
  }
</pre>
<p>
  If the computations to find objects are particularly slow,
  you might find improved performance if you tell the system
  an area for which you promise the acceptance persists:
  <pre>  void MyWidget::dragMoveEvent(<a href="qdragmoveevent.html">QDragMoveEvent</a>* event)
  {
    if ( <a href="qtextdrag.html#48ca8d">QTextDrag::canDecode</a>(event) ) {
      MyCadItem* item = findMyItemAt(event-&gt;<a href="qdropevent.html#b3ba77">pos</a>());
      if ( item ) {
        <a href="qrect.html">QRect</a> r = item-&gt;areaRelativeToMeClippedByAnythingInTheWay();
        if ( item-&gt;type() == MyTextType )
          event-&gt;<a href="qdragmoveevent.html#250cc4">accept</a>( r );
        else
          event-&gt;<a href="qdragmoveevent.html#2a033b">ignore</a>( 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 you need (don't forget to
  stop scrolling timers in a dragLeaveEvent() though).
<p>
  For a complete example of drag and drop, examine the
  <code>dragdrop</code> example program, or the QMultiLineEdit widget
  source code.
<p>
  <h3>Inter-operating with other applications</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.  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>
  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.

<p><address><hr><div align="center">
<table width="100%" cellspacing="0" border="0"><tr>
<td>Copyright  2001 Trolltech<td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
<td align="right"><div align="right">Qt version 2.3.2</div>
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