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Drag and Drop

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.

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 the Using Drag and Drop with Item Views and The Graphics View Framework documents.

Drag and Drop Classes

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

QDragEnterEventEvent which is sent to a widget when a drag and drop action enters it
QDragLeaveEventEvent that is sent to a widget when a drag and drop action leaves it
QDragMoveEventEvent which is sent while a drag and drop action is in progress
QDropEventEvent which is sent when a drag and drop action is completed
QMacPasteboardMimeConverts between a MIME type and a Uniform Type Identifier (UTI) format
QWindowsMimeMaps open-standard MIME to Window Clipboard formats

Configuration

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

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

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 mousePressEvent() and start a drag and drop operation:

 void MainWindow::mousePressEvent(QMouseEvent *event)
 {
     if (event->button() == Qt::LeftButton
         && iconLabel->geometry().contains(event->pos())) {

         QDrag *drag = new QDrag(this);
         QMimeData *mimeData = new QMimeData;

         mimeData->setText(commentEdit->toPlainText());
         drag->setMimeData(mimeData);
         drag->setPixmap(iconPixmap);

         Qt::DropAction dropAction = drag->exec();
         ...
     }
 }

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 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 mousePressEvent() function to record to start position of the drag:

 void DragWidget::mousePressEvent(QMouseEvent *event)
 {
     if (event->button() == Qt::LeftButton)
         dragStartPosition = event->pos();
 }

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

 void DragWidget::mouseMoveEvent(QMouseEvent *event)
 {
     if (!(event->buttons() & Qt::LeftButton))
         return;
     if ((event->pos() - dragStartPosition).manhattanLength()
          < QApplication::startDragDistance())
         return;

     QDrag *drag = new QDrag(this);
     QMimeData *mimeData = new QMimeData;

     mimeData->setData(mimeType, data);
     drag->setMimeData(mimeData);

     Qt::DropAction dropAction = drag->exec(Qt::CopyAction | Qt::MoveAction);
     ...
 }

This particular approach uses the 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.

Dropping

To be able to receive media dropped on a widget, call setAcceptDrops(true) for the widget, and reimplement the dragEnterEvent() and 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:

 Window::Window(QWidget *parent)
     : QWidget(parent)
 {
     ...
     setAcceptDrops(true);
 }

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 dragMoveEvent() and dropEvent().

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

 void Window::dragEnterEvent(QDragEnterEvent *event)
 {
     if (event->mimeData()->hasFormat("text/plain"))
         event->acceptProposedAction();
 }

The 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:

 void Window::dropEvent(QDropEvent *event)
 {
     textBrowser->setPlainText(event->mimeData()->text());
     mimeTypeCombo->clear();
     mimeTypeCombo->addItems(event->mimeData()->formats());

     event->acceptProposedAction();
 }

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 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.

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 setDropAction() with the preferred action from Qt::DropAction before calling accept(). This ensures that the replacement drop action is used instead of the proposed action.

For more sophisticated applications, reimplementing dragMoveEvent() and 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.

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 dragMoveEvent() in addition to dragEnterEvent() and dropEvent() to prevent the base class from providing default drag and drop handling, and to handle any special cases you are interested in.

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 CopyAction action. The target may also choose to handle other actions, specifically the MoveAction and LinkAction actions. If the source calls QDrag::exec(), and it returns 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 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 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 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.

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 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 (IANA) provides a hierarchical list of MIME media types at the Information Sciences Institute (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 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:

     QByteArray output;
     QBuffer outputBuffer(&output);
     outputBuffer.open(QIODevice::WriteOnly);
     imageLabel->pixmap()->toImage().save(&outputBuffer, "PNG");
     mimeData->setData("image/png", output);

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

     mimeData->setImageData(QVariant(*imageLabel->pixmap()));

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 meta objects and that stream operators for them must be implemented.

Drop Actions

In the clipboard model, the user can cut or copy the source information, then later paste it. Similarly in the drag and drop model, the user can drag a copy of the information or they can drag the information itself to a new place (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:

 void DragWidget::mouseMoveEvent(QMouseEvent *event)
 {
     if (!(event->buttons() & Qt::LeftButton))
         return;
     if ((event->pos() - dragStartPosition).manhattanLength()
          < QApplication::startDragDistance())
         return;

     QDrag *drag = new QDrag(this);
     QMimeData *mimeData = new QMimeData;

     mimeData->setData(mimeType, data);
     drag->setMimeData(mimeData);

     Qt::DropAction dropAction = drag->exec(Qt::CopyAction | Qt::MoveAction);
     ...
 }

The action returned by the exec() function may default to a 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:

 void DragWidget::dragEnterEvent(QDragEnterEvent *event)
 {
     event->acceptProposedAction();
 }

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:

 void DragWidget::dropEvent(QDropEvent *event)
 {
     if (event->source() == this && event->possibleActions() & Qt::MoveAction)
         return;

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

     if (event->proposedAction() == Qt::MoveAction) {
         event->acceptProposedAction();
         // Process the data from the event.
     } else if (event->proposedAction() == Qt::CopyAction) {
         event->acceptProposedAction();
         // Process the data from the event.
     } else {
         // Ignore the drop.
         return;
     }
     ...
 }

Note that we checked for individual drop actions in the above code. As mentioned above in the section on 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 possibleActions(), set the drop action with setDropAction(), and call accept().

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 (dropFrame):

 void Window::dragMoveEvent(QDragMoveEvent *event)
 {
     if (event->mimeData()->hasFormat("text/plain")
         && event->answerRect().intersects(dropFrame->geometry()))

         event->acceptProposedAction();
 }

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.

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:

     clipboard = QApplication::clipboard();

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 Clipboard is specified, the data is placed on the clipboard; if 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:

     clipboard->setText(lineEdit->text(), QClipboard::Clipboard);

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 setMimeData() function.

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

     connect(clipboard, SIGNAL(dataChanged()), this, SLOT(updateClipboard()));

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:

 void ClipWindow::updateClipboard()
 {
     QStringList formats = clipboard->mimeData()->formats();
     QByteArray data = clipboard->mimeData()->data(format);
     ...
 }

The selectionChanged() signal can be used on X11 to monitor the mouse selection.

Examples

Interoperating with Other Applications

On X11, the public 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:

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.

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.


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Qt 4.6.3