Class DragEvent

Drag and drop gesture can be started by calling startDragAndDrop() (on a node or scene) inside of a DRAG_DETECTED event handler. The data to be transfered to drop target are placed to a dragBoard at this moment.

Drag entered/exited events behave similarly to mouse entered/exited events, please see MouseEvent overview.

Drag sources: initiating a drag and drop gesture

The default drag detection mechanism uses mouse movements with a pressed button in combination with hysteresis. This behavior can be augmented by the application. Each MOUSE_PRESSED and MOUSE_DRAGGED event has a dragDetect flag that determines whether a drag gesture has been detected. The default value of this flag depends on the default detection mechanism and can be modified by calling setDragDetect() inside of an event handler. When processing of one of these events ends with the dragDetect flag set to true, a DRAG_DETECTED MouseEvent is sent to the potential gesture source (the object on which a mouse button has been pressed). This event notifies about the gesture detection.

Inside a DRAG_DETECTED event handler, if the startDragAndDrop() method is called on a node or scene and a dragged data is made available to the returned Dragboard, the object on which startDragAndDrop() has been called is considred a gesture source and the drag and drop gesture is started. The Dragboard has system clipboard functionality but is specifically used for drag and drop data transfer.

The startDragAndDrop() method takes a set of TransferModes supported by the gesture source. For instance passing only TransferMode.COPY indicates that the gesture source allows only copying of the data, not moving or referencing.

Following example shows a simple drag and drop source:

Rectangle rect = new Rectangle(100, 100);
rect.setOnDragDetected(new EventHandler<MouseEvent>() {
    @Override public void handle(MouseEvent event) {
        Dragboard db = startDragAndDrop(TransferMode.ANY);
        ClipboardContent content = new ClipboardContent();
        content.putString("Hello!");
        db.setContent(content);
        event.consume();
    }
});
 

Potential drop targets

After the drag and drop gesture has been started, any object (Node, Scene) over which the mouse is dragged is a potential drop target.

When the mouse is dragged into the boundaries of potential drop target, the potential target gets a DRAG_ENTERED event. When the mouse is dragged outside of the potential target's bounds, it gets a DRAG_EXITED event. There are also the bubbling DRAG_ENTERED_TARGET and DRAG_EXITED_TARGET variants. They behave similarly to mouse entered/exited events, please see MouseEvent overview.

A potential drop target can decide to change its appearance to let the user know that the dragged data can be dropped on it. This can be done in a DRAG_OVER event handler, based on the position of the mouse. Another option is to change the potential target's appearance in a DRAG_ENTERED and DRAG_EXITED handlers.

In DRAG_OVER event handler a potential drop target has the ability to make it known that it is an actual target. This is done by calling acceptTransferModes(TransferMode...) on the event, passing transfer modes it is willing to accept. If it is not called during the event delivery or if none of the passed transfer modes is supported by gesture source, then the potential drop target is not considered to be an actual drop target.

When deciding weather to accept the event by calling acceptTransferModes(TransferMode...), the type of data available on the Dragboard should be considered. Access to the Dragboard is provided by the getDragboard() method.

When accepting an event, the potential gesture target decides which TransferMode is accepted for the operation. To make the decision, DragBoard.getTransferModes() (set of transfer modes supported by the gesture source) and DragEvent.getTransferMode() (default transfer mode issued by platform, driven by key modifiers) can be used. It is poosible to pass more transfer modes into the acceptTransferModes(TransferMode...) method. In this case it makes the decision in behalf of the application (it chooses the default mode if it's supported by gesture source and accepted by gesture target, otherwise it chooses the most common mode of the supported and accepted ones). The DRAG_DROPPED event's getTransferMode() later reports the transfer mode accepted by the DRAG_OVER event handler.

A drag and drop gesture ends when the mouse button is released. If this happens over a gesture target that accepted previous DRAG_OVER events with a transfer mode supported by gesture source, a DRAG_DROPPED event is sent to the gesture target. In its handler, the gesture target can access the data on the dragboard. After data has been transferred (or decided not to transfer), the gesture needs to be completed by calling setDropCompleted(Boolean) on the event. The Boolean argument indicates if the data has been transferred successfully or not. If it is not called, the gesture is considered unsuccessful.

Following example shows a simple drag and drop target for text data:

Rectangle rect = new Rectangle(100, 100);

rect.setOnDragOver(new EventHandler<DragEvent>() {
    @Override public void handle(DragEvent event) {
        Dragboard db = event.getDragboard();
        if (db.hasString()) {
            event.acceptTransferModes(TransferMode.COPY_OR_MOVE);
        }
        event.consume();
    }
});

rect.setOnDragDropped(new EventHandler<DragEvent>() {
    @Override public void handle(DragEvent event) {
        Dragboard db = event.getDragboard();
        boolean success = false;
        if (db.hasString()) {
            System.out.println("Dropped: " + db.getString());
            success = true;
        }
        event.setDropCompleted(success);
        event.consume();
    }
});
 

Drag sources: finalizing drag and drop gesture

After the gesture has been finished, whether by successful or unsuccessful data transfer or being canceled, the DRAG_DONE event is sent to the gesture source. The getTransferMode() method of the event indicates to the gesture source how the transfer of data was completed. If the transfer mode has the value MOVE, then this allows the source to clear out its data. Clearing the source's data gives the appropriate appearance to a user that the data has been moved by the drag and drop gesture. If it has the value null, then the drag and drop gesture ended without any data being transferred. This could happen as a result of a mouse release event over a node that is not a drop target, or the user pressing the ESC key to cancel the drag and drop gesture, or by the gesture target reporting an unsuccessful data transfer.