Add API support for setting server side drag data (#8915)

* Add API support for setting server side drag data

* Fix javadocs

* Add server side drag data API to ASCII doc

* Fix some javadocs and documentation details

2 files changed, 42 insertions, 0 deletions

diff --git a/server/src/main/java/com/vaadin/event/dnd/DragSourceExtension.java b/server/src/main/java/com/vaadin/event/dnd/DragSourceExtension.java
index f7089c9b9e..6a6a8a8c64 100644
--- a/server/src/main/java/com/vaadin/event/dnd/DragSourceExtension.java
+++ b/server/src/main/java/com/vaadin/event/dnd/DragSourceExtension.java
@@ -40,6 +40,12 @@ public class DragSourceExtension<T extends AbstractComponent> extends
     private Registration dragEndListenerHandle;
 
     /**
+     * Stores the server side drag data that is available for the drop target if
+     * it is in the same UI.
+     */
+    private Object dragData;
+
+    /**
      * Extends {@code target} component and makes it a drag source.
      *
      * @param target
@@ -146,6 +152,29 @@ public class DragSourceExtension<T extends AbstractComponent> extends
     }
 
     /**
+     * Set server side drag data. This data is available in the drop event and
+     * can be used to transfer data between drag source and drop target if they
+     * are in the same UI.
+     *
+     * @param data
+     *         Data to transfer to drop event.
+     */
+    public void setDragData(Object data) {
+        dragData = data;
+    }
+
+    /**
+     * Get server side drag data. This data is available in the drop event and
+     * can be used to transfer data between drag source and drop target if they
+     * are in the same UI.
+     *
+     * @return Server side drag data if set, otherwise {@literal null}.
+     */
+    public Object getDragData() {
+        return dragData;
+    }
+
+    /**
      * Attaches dragstart listener for the current drag source. {@link
      * DragStartListener#dragStart(DragStartEvent)} is called when dragstart
      * event happens on the client side.
diff --git a/server/src/main/java/com/vaadin/event/dnd/DropEvent.java b/server/src/main/java/com/vaadin/event/dnd/DropEvent.java
index 3fb655f176..c2b4943d0f 100644
--- a/server/src/main/java/com/vaadin/event/dnd/DropEvent.java
+++ b/server/src/main/java/com/vaadin/event/dnd/DropEvent.java
@@ -106,6 +106,19 @@ public class DropEvent<T extends AbstractComponent> extends Component.Event {
     }
 
     /**
+     * Gets the server side drag data. This data can be set during the drag
+     * start event on the server side and can be used to transfer data between
+     * drag source and drop target when they are in the same UI.
+     *
+     * @return Optional server side drag data if set and the drag source and the
+     * drop target are in the same UI, otherwise empty {@code Optional}.
+     * @see DragSourceExtension#setDragData(Object)
+     */
+    public Optional<Object> getDragData() {
+        return getDragSourceExtension().map(DragSourceExtension::getDragData);
+    }
+
+    /**
      * Returns the drop target component where the drop event occurred.
      *
      * @return Component on which a drag source was dropped.