Class DragSource

  • public class DragSource
    extends Widget
    DragSource defines the source object for a drag and drop transfer.

    IMPORTANT: This class is not intended to be subclassed.

    A drag source is the object which originates a drag and drop operation. For the specified widget, it defines the type of data that is available for dragging and the set of operations that can be performed on that data. The operations can be any bit-wise combination of DND.MOVE, DND.COPY or DND.LINK. The type of data that can be transferred is specified by subclasses of Transfer such as TextTransfer or FileTransfer. The type of data transferred can be a predefined system type or it can be a type defined by the application. For instructions on how to define your own transfer type, refer to ByteArrayTransfer.

    You may have several DragSources in an application but you can only have one DragSource per Control. Data dragged from this DragSource can be dropped on a site within this application or it can be dropped on another application such as an external Text editor.

    The application supplies the content of the data being transferred by implementing the DragSourceListener and associating it with the DragSource via DragSource#addDragListener.

    When a successful move operation occurs, the application is required to take the appropriate action to remove the data from its display and remove any associated operating system resources or internal references. Typically in a move operation, the drop target makes a copy of the data and the drag source deletes the original. However, sometimes copying the data can take a long time (such as copying a large file). Therefore, on some platforms, the drop target may actually move the data in the operating system rather than make a copy. This is usually only done in file transfers. In this case, the drag source is informed in the DragEnd event that a DROP_TARGET_MOVE was performed. It is the responsibility of the drag source at this point to clean up its displayed information. No action needs to be taken on the operating system resources.

    The following example shows a Label widget that allows text to be dragged from it.

            // Enable a label as a Drag Source
            Label label = new Label(shell, SWT.NONE);
            // This example will allow text to be dragged
            Transfer[] types = new Transfer[] {TextTransfer.getInstance()};
            // This example will allow the text to be copied or moved to the drop target
            int operations = DND.DROP_MOVE | DND.DROP_COPY;
            DragSource source = new DragSource(label, operations);
            source.addDragListener(new DragSourceListener() {
                    public void dragStart(DragSourceEvent e) {
                            // Only start the drag if there is actually text in the
                            // label - this text will be what is dropped on the target.
                            if (label.getText().length() == 0) {
                                    event.doit = false;
                    public void dragSetData(DragSourceEvent event) {
                            // A drop has been performed, so provide the data of the
                            // requested type.
                            // (Checking the type of the requested data is only
                            // necessary if the drag source supports more than
                            // one data type but is shown here as an example).
                            if (TextTransfer.getInstance().isSupportedType(event.dataType)){
                           = label.getText();
                    public void dragFinished(DragSourceEvent event) {
                            // A Move operation has been performed so remove the data
                            // from the source
                            if (event.detail == DND.DROP_MOVE)
    DND.DragStart, DND.DragSetData, DND.DragEnd
    See Also:
    Drag and Drop snippets, SWT Example: DNDExample, Sample code and further information
    This class is not intended to be subclassed by clients.
    • Constructor Detail

      • DragSource

        public DragSource​(Control control,
                          int style)
        Creates a new DragSource to handle dragging from the specified Control. Creating an instance of a DragSource may cause system resources to be allocated depending on the platform. It is therefore mandatory that the DragSource instance be disposed when no longer required.
        control - the Control that the user clicks on to initiate the drag
        style - the bitwise OR'ing of allowed operations; this may be a combination of any of DND.DROP_NONE, DND.DROP_COPY, DND.DROP_MOVE, DND.DROP_LINK
        SWTException -
        • ERROR_THREAD_INVALID_ACCESS - if not called from the thread that created the parent
        • ERROR_INVALID_SUBCLASS - if this class is not an allowed subclass
        SWTError -
        • ERROR_CANNOT_INIT_DRAG - unable to initiate drag source; this will occur if more than one drag source is created for a control or if the operating system will not allow the creation of the drag source

        NOTE: ERROR_CANNOT_INIT_DRAG should be an SWTException, since it is a recoverable error, but can not be changed due to backward compatibility.

        See Also:
        Widget.dispose(), checkSubclass(), DND.DROP_NONE, DND.DROP_COPY, DND.DROP_MOVE, DND.DROP_LINK
    • Method Detail

      • addDragListener

        public void addDragListener​(DragSourceListener listener)
        Adds the listener to the collection of listeners who will be notified when a drag and drop operation is in progress, by sending it one of the messages defined in the DragSourceListener interface.
        • dragStart is called when the user has begun the actions required to drag the widget. This event gives the application the chance to decide if a drag should be started.
        • dragSetData is called when the data is required from the drag source.
        • dragFinished is called when the drop has successfully completed (mouse up over a valid target) or has been terminated (such as hitting the ESC key). Perform cleanup such as removing data from the source side on a successful move operation.
        listener - the listener which should be notified
        IllegalArgumentException -
        • ERROR_NULL_ARGUMENT - if the listener is null
        SWTException -
        • ERROR_WIDGET_DISPOSED - if the receiver has been disposed
        • ERROR_THREAD_INVALID_ACCESS - if not called from the thread that created the receiver
        See Also:
        DragSourceListener, getDragListeners(), removeDragListener(org.eclipse.swt.dnd.DragSourceListener), DragSourceEvent
      • checkSubclass

        protected void checkSubclass()
        Description copied from class: Widget
        Checks that this class can be subclassed.

        The SWT class library is intended to be subclassed only at specific, controlled points (most notably, Composite and Canvas when implementing new widgets). This method enforces this rule unless it is overridden.

        IMPORTANT: By providing an implementation of this method that allows a subclass of a class which does not normally allow subclassing to be created, the implementer agrees to be fully responsible for the fact that any such subclass will likely fail between SWT releases and will be strongly platform specific. No support is provided for user-written classes which are implemented in this fashion.

        The ability to subclass outside of the allowed SWT classes is intended purely to enable those not on the SWT development team to implement patches in order to get around specific limitations in advance of when those limitations can be addressed by the team. Subclassing should not be attempted without an intimate and detailed understanding of the hierarchy.

        checkSubclass in class Widget
      • getControl

        public Control getControl()
        Returns the Control which is registered for this DragSource. This is the control that the user clicks in to initiate dragging.
        the Control which is registered for this DragSource
      • getDragSourceEffect

        public DragSourceEffect getDragSourceEffect()
        Returns the drag effect that is registered for this DragSource. This drag effect will be used during a drag and drop operation.
        the drag effect that is registered for this DragSource
      • getTransfer

        public Transfer[] getTransfer()
        Returns the list of data types that can be transferred by this DragSource.
        the list of data types that can be transferred by this DragSource
      • setDragSourceEffect

        public void setDragSourceEffect​(DragSourceEffect effect)
        Specifies the drag effect for this DragSource. This drag effect will be used during a drag and drop operation.
        effect - the drag effect that is registered for this DragSource
      • setTransfer

        public void setTransfer​(Transfer... transferAgents)
        Specifies the list of data types that can be transferred by this DragSource. The application must be able to provide data to match each of these types when a successful drop has occurred.
        transferAgents - a list of Transfer objects which define the types of data that can be dragged from this source