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Drag and Drop · Vaadin
Vaadin Framework - Advanced Topics - Drag and Drop
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Drag and Drop

This feature is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.alpha1.

Dragging an object from one location to another by grabbing it with mouse, holding the mouse button pressed, and then releasing the button to "drop" it to the other location is a common way to move, copy, or associate objects. For example, most operating systems allow dragging and dropping files between folders or dragging a document on a program to open it. Framework version 8.1 adds support for HTML5 drag and drop features. This makes it possible to set components as drag sources that user can drag and drop, or to set them as drop targets to drop things on.

Drag Source

Any component can be made a drag source that has textual data that is transferred when it is dragged and dropped.

To make a component a drag source, you apply the DragSourceExtension to it. Then you can define the text to transfer, and the allowed drag effect.

Label draggableLabel = new Label("You can grab and drag me");
DragSourceExtension<Label> dragSource = new DragSourceExtension<>(draggableLabel);

// set the allowed effect
dragSource.setEffectAllowed(EffectAllowed.MOVE);
// set the text to transfer
dragSource.setDataTransferText("hello receiver");
// set other data to transfer (in this case HTML)
dragSource.setDataTransferData("text/html", "<label>hello receiver</label>");

The effect allowed specifies the allowed effects that must match the drop effect of the drop target. If these don’t match, the drop event is never fired on the target. If multiple effects are allowed, the user can use the modifier keys to switch between the desired effects. The default effect and the modifier keys are system and browser dependent.

The data transfer text is textual data, that the drop target will receive in the drop event.

The data transfer data is any data of the given type, that the drop target will also receive in the drop event. The order, in which the data is set for the drag source, is preserved, which helps the drop target finding the most preferred and supported data.

Note that "text" is the only cross browser supported data type. If your application supports IE11, pleas only use the setDataTransferText() method.

The DragStartEvent is fired when the drag has started, and the DragEndEvent event when the drag has ended, either in a drop or a cancel.

dragSource.addDragStartListener(event ->
    Notification.show("Drag event started");
);
dragSource.addDragEndListener(event -> {
    if (event.isCanceled()) {
        Notification.show("Drag event was canceled");
    } else {
        Notification.show("Drag event finished");
    }
});

You can check whether the drag was canceled using the isCanceled() method.

It is possible to transfer any Object as server side data to the drop target if both the drag source and drop target are placed in the same UI. This data is available in the drop event via the DropEvent.getDragData() method.

dragSource.addDragStartListener(event ->
    dragSource.setDragData(myObject);
);
dragSource.addDragEndListener(event ->
    dragSource.setDragData(null);
};

CSS Style Rules

The drag source element, additional to it’s primary style name, have a style name with the -dragsource suffix. For example, a Label component would have the style name v-label-dragsource when the drag source extension is applied to it. Additionally, the elements also have the v-draggable style name that is independent of the component’s primary style.

While being dragged, the element also gets the style name with suffix -dragged. An example for this is v-label-dragged in case of a Label component. This style name is added during the drag start and removed during the drag end event.

The browsers allow the user to select and drag and drop text, which could cause issues with components that have text. The Framework tries to prevent this by automatically adding the following style to all v-draggable elements. It is included by the sass mixin valo-drag-element.

.v-draggable {
    -moz-user-select: none !important;
    -ms-user-select: none !important;
    -webkit-user-select: none !important;
    user-select: none !important;
}

Drop Target

The drag operations end when the mouse button is released on a valid drop target. It is then up to the target to react to the drop event and the data associated with the drag, set by the drag source.

To make a component be a drop target, you apply the DropTargetExtension to it. The extension allows you to control when the drop is acceptable and then react to the drop event.

VerticalLayout dropTargetLayout = new VerticalLayout();
dropTargetLayout.setCaption("Drop things inside me");
dropTargetLayout.addStyleName(ValoTheme.LAYOUT_CARD);

// make the layout accept drops
DropTargetExtension<VerticalLayout> dropTarget = new DropTargetExtension<>(dropTargetLayout);

// the drop effect must match the allowed effect in the drag source for a successful drop
dropTarget.setDropEffect(DropEffect.MOVE);

// catch the drops
dropTarget.addDropListener(event -> {
    // if the drag source is in the same UI as the target
    Optional<AbstractComponent> dragSource = event.getDragSourceComponent();
    if (dragSource.isPresent() && dragSource.get() instanceof Label) {
        // move the label to the layout
        dropTargetLayout.addComponent(dragSource.get());

        // get possible transfer data
        String message = event.getDataTransferData("text/html");
        if (message != null) {
            Notification.show("DropEvent with data transfer html: " + message);
        } else {
            // get transfer text
            message = event.getDataTransferText();
            Notification.show("DropEvent with data transfer text: " + message);
        }

        // handle possible server side drag data, if the drag source was in the same UI
        event.getDragData().ifPresent(data -> handleMyDragData((MyObject) data));
    }
});

When data is dragged over a drop target, the v-drag-over class name is applied to the root element of the drop target component automatically.

Controlling When The Drop is Acceptable

The drop effect allows you to specify the desired drop effect, and for a succesful drop it must match the allowed effect that has been set for the drag source. Note that you can allow multiple effects, and that you should not rely on the default effect since it may vary between browsers.

The drop criteria allows you to determine whether the current drag data can be dropped on the drop target. It is executed on dragenter, dragover and drop events. The script gets the current event as a parameter named event. Returning false will prevent the drop and no drop event is fired on the server side.

CSS Style Rules

Each drop target element have an applied style name, the primary style name with -droptarget suffix, e.g. v-label-droptarget, to indicate that it is a potential target for data to be dropped onto it.

When dragging data over a drop target and the drag over criteria passes, a style name is applied to indicate that the element accepts the drop. This style name is the primary style name with -drag-center suffix, e.g. v-label-drag-center.

Mobile Drag And Drop Support

The HTML 5 Drag and Drop API is not yet supported by mobile browsers. To enable HTML5 DnD support on mobile devices, we have included an external Polyfill. Please note that this Polyfill is under the BSD 2 License.

By default, the mobile DnD support is disabled, but you can enable it any time for a UI. Starting from the request where the support was enabled, all the added DragSourceExtension, DropTargetExtension and their subclasses will also work on mobile devices for that UI. The Polyfill is only loaded when the user is using a touch device.

Drag and Drop is mutually exclusive with context click on mobile devices.

public class MyUI extends UI {
    protected void init(VaadinRequest request) {
        setMobileHtml5DndEnabled(true);
    }
}

When disabling the support, you need to also remove all the DragSourceExtension, DropTargetExtension and their subclasses that were added when the mobile DnD support was enabled.

CSS Style Rules

The Polyfill allows you to apply custom styling to enhance the user experience on touch devices. It is important to remember that these customizations are only used when the polyfill is loaded, and not possible for desktop DnD operations.

The drag image can be customized using the dnd-poly-drag-image class name. You must NOT wrap the class rule with e.g. .valo, since that is not applied to the drag image element. The following styling can be used to make the drag image opaque and "snap back" when the user did not drop to a valid dropzone:

dnd-poly-drag-image {
    opacity: .5 !important;
}
dnd-poly-drag-image.dnd-poly-snapback {
    transition-property: transform, -webkit-transform !important;
    transition-duration: 200ms !important;
    transition-timing-function: ease-out !important;
}

More details can be found from the Polyfill website.

Drag and Drop Rows in Grid

It is possible to drag and drop the rows of a Grid component. This allows reordering of rows, dragging rows between different Grids, dragging rows outside of a Grid or dropping data onto rows.

Grid as a Drag Source

A Grid component’s rows can be made draggable by applying GridDragSource extension to the component. The extended Grid’s rows become draggable, meaning that each row can be grabbed and moved by the mouse individually. When the Grid’s selection mode is SelectionMode.MULTI and multiple rows are selected, it is possible to drag all the visible selected rows by grabbing one of them. However, when the grabbed row is not selected, only that one row will be dragged.

It is important to note that when dragging multiple rows, only the visible selected rows will be set as dragged data.

By default, the drag data of type "text" will contain the content of each column separated by a tabulator character ("\t"). When multiple rows are dragged, the generated data is combined into one String separated by new line characters ("\n"). You can override the default behaviour and provide a custom drag data for each item by setting a custom drag data generator for the "text" type. The generator is executed for each item and returns a String containing the data to be transferred for that item.

The following example shows how you can define the allowed drag effect and customize the drag data by setting a drag data generator.

Grid<Person> grid = new Grid<>();
// ...
GridDragSource<Person> dragSource = new GridDragSource<>(grid);

// set allowed effects
dragSource.setEffectAllowed(EffectAllowed.MOVE);

// add a drag data generator
dragSource.setDragDataGenerator("text", person -> {
    return person.getFirstName() + " " + person.getLastName() +
           "\t" +  // tabulator character
           person.getAddress().getCity();
});

It is possible to set multiple generators with the setDragDataGenerator(type, generator) method. The generated data will be set as data transfer data with the given type and can then be accessed during drop from the drop event’s getDataTransferData(type) method.

The GridDragStartEvent is fired when dragging a row has started, and the GridDragEndEvent when the drag has ended, either in a drop or a cancel.

dragSource.addGridDragStartListener(event ->
    // Keep reference to the dragged items
    draggedItems = event.getDraggedItems()
);

// Add drag end listener
dragSource.addGridDragEndListener(event -> {
    // If drop was successful, remove dragged items from source Grid
    if (event.getDropEffect() == DropEffect.MOVE) {
        ((ListDataProvider<Person>) grid.getDataProvider()).getItems()
                .removeAll(draggedItems);
        grid.getDataProvider().refreshAll();

        // Remove reference to dragged items
        draggedItems = null;
    }
});

The dragged rows can be accessed from both events using the getDraggedItems() method.

CSS Style Rules

A drag source Grid’s rows have the v-grid-row-dragsource and the v-draggable style names applied to indicate that the rows are draggable.

Additionally, the style name v-grid-row-dragged is applied to all the dragged rows during the drag start event and removed during the drag end event.

Grid as a Drop Target

To make a Grid component’s rows accept a drop event, apply the GridDropTarget extension to the component. When creating the extension, you need to specify where the transferred data can be dropped on.

Grid<Person> grid = new Grid<>();
// ...
GridDropTarget<Person> dropTarget = new GridDropTarget<>(grid, DropMode.BETWEEN);
dropTarget.setDropEffect(DropEffect.MOVE);

The drop mode specifies the behaviour of the row when an element is dragged over or dropped onto it. Use DropMode.ON_TOP when you want to drop elements on top of a row and DropMode.BETWEEN when you want to drop elements between rows. DropMode_ON_TOP_OR_BETWEEN allows to drop on between or top rows.

The GridDropEvent is fired when data is dropped onto one of the Grid’s rows. The following example shows how you can insert items into the Grid at the drop position. If the drag source is another Grid, you can access the generated drag data with the event’s getDataTransferText() method. If the drag source Grid uses a custom generator for a different type than "text", you can access it’s generated data using the getDataTransferData(type) method. You can also check all the received data transfer data by fetching the type-to-data map with the getDataTransferData() method.

dropTarget.addGridDropListener(event -> {
    // Accepting dragged items from another Grid in the same UI
    event.getDragSourceExtension().ifPresent(source -> {
        if (source instanceof GridDragSource) {
            // Get the target Grid's items
            ListDataProvider<Person> dataProvider = (ListDataProvider<Person>)
                    event.getComponent().getDataProvider();
            List<Person> items = (List<Person>) dataProvider.getItems();

            // Calculate the target row's index
            int index = items.size();
            if (event.getDropTargetRow().isPresent()) {
                index = items.indexOf(event.getDropTargetRow().get()) + (
                    event.getDropLocation() == DropLocation.BELOW ? 1 : 0);
            }

            // Add dragged items to the target Grid
            items.addAll(index, draggedItems);
            dataProvider.refreshAll();

            // Show the dropped data
            Notification.show("Dropped row data: " + event.getDataTransferText());
        }
    });
});

The drop location property in the GridDropEvent specifies the dropped location in relative to grid row the drop happened on and depends on the used DropMode. When the drop happened on top of a row, the possible options for the location are ON_TOP, ABOVE and BELOW.

If the grid is empty or if the drop was on empty space after the last row in grid, and the DropMode.ON_TOP was used, then the drop location EMPTY will be used. If the drop modes DropMode.BETWEEN or DropMode.ON_TOP_OR_BETWEEN are used, then the location can be EMPTY only when the grid was empty; otherwise the drop happened BELOW the last visible row. When the drop location is EMPTY, the getDropTargetRow method will also return an empty optional.

When dropping on top of the grid’s header or footer, the drop location will be EMPTY if there are no rows in the grid or if DropMode.ON_TOP was used. If there are rows in the grid, dropping on top of the header will set the drop location to ABOVE and the dropped row will be the first currently visible row in grid. Similarly, if dropping on top of the footer, the drop location will be BELOW and the dropped row will be the last visible row in the grid.

CSS Style Rules

A drop target Grid’s body has the style name v-grid-body-droptarget to indicate that it is a potential target for data to be dropped.

When dragging data over a drop target Grid’s row, depending on the drop mode and the mouse position relative to the row, a style name is applied to the row or to the grid body to indicate the drop location. When dragging on top of a row, v-grid-row-drag-center indicates ON_TOP, v-grid-row-drag-top indicates ABOVE and v-grid-row-drag-bottom indicates BELOW locations. When dragging on top of an empty grid, or when the drop location is ON_TOP and dragged below the last row in grid (and there is empty space visible), the v-grid-body-body-drag-top style is applied to the v-grid-tablewrapper element which surrounds the grid header, body and footer.

Drag and Drop Rows in TreeGrid

To make the rows of a TreeGrid component draggable or to make them a drop target, apply TreeGridDragSource or TreeGridDropTarget extensions to the component, respectively. In addition to the drag and drop features for Grid above, TreeGridDropEvent provides information about the status of the node (expanded or collapsed) and its depth in the hierarchy.

Drag and Drop Files

Files can be uploaded to the server by dropping them onto a file drop target. To make a component a file drop target, apply the FileDropTarget extension to it by creating a new instance and passing the component as first constructor parameter to it.

You can handle the dropped files with the FileDropHandler that you add as the second constructor parameter. The FileDropEvent, received by the handler, contains information about the dropped files such as file name, file size and mime type. In the handler you can decide if you would like to upload each of the dropped files.

To start uploading a file, set a StreamVariable to it. The stream variable provides an output stream where the file will be written and has callback methods for all the stages of the upload process.

Label dropArea = new Label("Drop files here");
FileDropTarget<Label> dropTarget = new FileDropTarget<>(dropArea, event -> {

    List<Html5File> files = event.getFiles();
    files.forEach(file -> {
        // Max 1 MB files are uploaded
        if (file.getFileSize() <= 1024 * 1024) {
            file.setStreamVariable(new StreamVariable() {

                // Output stream to write the file to
                @Override
                public OutputStream getOutputStream() {
                    return new FileOutputStream("/path/to/files/"
                        + file.getFileName());
                }

                // Returns whether onProgress() is called during upload
                @Override
                public boolean listenProgress() {
                    return true;
                }

                // Called periodically during upload
                @Override
                public void onProgress(StreamingProgressEvent event) {
                    Notification.show("Progress, bytesReceived="
                        + event.getBytesReceived());
                }

                // Called when upload started
                @Override
                public void streamingStarted(StreamingStartEvent event) {
                    Notification.show("Stream started, fileName="
                        + event.getFileName());
                }

                // Called when upload finished
                @Override
                public void streamingFinished(StreamingEndEvent event) {
                    Notification.show("Stream finished, fileName="
                        + event.getFileName());
                }

                // Called when upload failed
                @Override
                public void streamingFailed(StreamingErrorEvent event) {
                    Notification.show("Stream failed, fileName="
                        + event.getFileName());
                }
            });
        }
    }
});