Make it possible for grid drop target to accept dragged data when grid is empty (#9332)

* Make it possible for grid drop target to accept dragged data when grid is empty (#9068)
* Make return type of getDropTargetRow() optional

client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java

     }
 
     @Override
-    protected void addTargetClassIndicator(NativeEvent event) {
+    protected void addDragOverStyle(NativeEvent event) {
         getTargetRow(((Element) event.getEventTarget().cast()))
                 .ifPresent(target -> {
 
     }
 
     @Override
-    protected void removeTargetClassIndicator(NativeEvent event) {
+    protected void removeDragOverStyle(NativeEvent event) {
 
         // Remove all possible style names
         getTargetRow((Element) event.getEventTarget().cast()).ifPresent(e -> {

client/src/main/java/com/vaadin/client/extensions/DropTargetExtensionConnector.java

      */
     protected static final String STYLE_SUFFIX_DRAG_BOTTOM = "-drag-bottom";
 
+    /**
+     * Style name suffix for indicating that the element is drop target.
+     */
+    protected static final String STYLE_SUFFIX_DROPTARGET = "-droptarget";
+
     // Create event listeners
     private final EventListener dragEnterListener = this::onDragEnter;
     private final EventListener dragOverListener = this::onDragOver;
         addDropListeners(getDropTargetElement());
 
         ((AbstractComponentConnector) target).onDropTargetAttached();
+
+        // Add drop target indicator to the drop target element
+        addDropTargetStyle();
     }
 
     /**
 
         removeDropListeners(getDropTargetElement());
         ((AbstractComponentConnector) getParent()).onDropTargetDetached();
+
+        // Remove drop target indicator
+        removeDropTargetStyle();
     }
 
     /**
                 + STYLE_SUFFIX_DRAG_CENTER;
 
         if (isDropAllowed(nativeEvent)) {
-            addTargetClassIndicator(nativeEvent);
+            addDragOverStyle(nativeEvent);
 
             setDropEffect(nativeEvent);
 
         if (isDropAllowed(nativeEvent)) {
             setDropEffect(nativeEvent);
 
-            // Add drop target indicator in case the element doesn't have one
-            addTargetClassIndicator(nativeEvent);
+            // Add drag over indicator in case the element doesn't have one
+            addDragOverStyle(nativeEvent);
 
             // Prevent default to allow drop
             nativeEvent.preventDefault();
             nativeEvent.getDataTransfer()
                     .setDropEffect(DataTransfer.DropEffect.NONE);
 
-            // Remove drop target indicator
-            removeTargetClassIndicator(nativeEvent);
+            // Remove drag over indicator
+            removeDragOverStyle(nativeEvent);
         }
     }
 
      *            browser event to be handled
      */
     protected void onDragLeave(Event event) {
-        removeTargetClassIndicator((NativeEvent) event);
+        removeDragOverStyle((NativeEvent) event);
     }
 
     /**
                     .getDropEffect(nativeEvent.getDataTransfer()), nativeEvent);
         }
 
-        removeTargetClassIndicator(nativeEvent);
+        removeDragOverStyle(nativeEvent);
     }
 
     private boolean isDropAllowed(NativeEvent event) {
     }
 
     /**
-     * Add class that indicates that the component is a target.
+     * Add class name for the drop target element indicating that data can be
+     * dropped onto it. The class name has the following format:
+     * <pre>
+     *     [primaryStyleName]-droptarget
+     * </pre>
+     * The added class name is update
+     * automatically by the framework when the primary style name changes.
+     */
+    protected void addDropTargetStyle() {
+        getDropTargetElement().addClassName(
+                getStylePrimaryName(getDropTargetElement())
+                        + STYLE_SUFFIX_DROPTARGET);
+    }
+
+    /**
+     * Remove class name from the drop target element indication that data can
+     * be dropped onto it.
+     */
+    protected void removeDropTargetStyle() {
+        getDropTargetElement().removeClassName(
+                getStylePrimaryName(getDropTargetElement())
+                        + STYLE_SUFFIX_DROPTARGET);
+    }
+
+    /**
+     * Add class that indicates that the component is a target while data is
+     * being dragged over it.
      * <p>
      * This is triggered on {@link #onDragEnter(Event) dragenter} and
      * {@link #onDragOver(Event) dragover} events pending if the drop is
      * @param event
      *            the dragenter or dragover event that triggered the indication.
      */
-    protected void addTargetClassIndicator(NativeEvent event) {
+    protected void addDragOverStyle(NativeEvent event) {
         getDropTargetElement().addClassName(styleDragCenter);
     }
 
     /**
-     * Remove the drag target indicator class name from the target element.
+     * Remove the drag over indicator class name from the target element.
      * <p>
      * This is triggered on {@link #onDrop(Event) drop},
      * {@link #onDragLeave(Event) dragleave} and {@link #onDragOver(Event)
      * @param event
      *            the event that triggered the removal of the indicator
      */
-    protected void removeTargetClassIndicator(NativeEvent event) {
+    protected void removeDragOverStyle(NativeEvent event) {
         getDropTargetElement().removeClassName(styleDragCenter);
     }
 
         return new Function('event', script)(event);
     }-*/;
 
+    private native boolean getStylePrimaryName(Element element)
+    /*-{
+        return @com.google.gwt.user.client.ui.UIObject::getStylePrimaryName(Lcom/google/gwt/dom/client/Element;)(element);
+    }-*/;
+
     @Override
     public DropTargetState getState() {
         return (DropTargetState) super.getState();

documentation/advanced/advanced-dragndrop.asciidoc

 
 === CSS Style Rules
 
-When dragging data over a drop target and the drag over criteria passes, a style name is applied to indicate that the element accepts drops. This style name is the primary style name with `-drag-center` suffix, e.g. `v-label-drag-center`.
+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`.
 
 
 ////
             List<Person> items = (List<Person>) dataProvider.getItems();
 
             // Calculate the target row's index
-            int index = items.indexOf(event.getDropTargetRow()) + (
+            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);
 
 ==== 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 to indicate the drop location.
 `v-grid-row-drag-center` indicates ON_TOP, `v-grid-row-drag-top` indicates ABOVE and `v-grid-row-drag-bottom` indicates BELOW locations.
 

server/src/main/java/com/vaadin/ui/components/grid/GridDropEvent.java

 package com.vaadin.ui.components.grid;
 
 import java.util.Map;
+import java.util.Optional;
 
 import com.vaadin.shared.ui.dnd.DropEffect;
 import com.vaadin.shared.ui.grid.DropLocation;
     /**
      * Get the row item source of this event.
      *
-     * @return The row item this event was originated from.
+     * @return The optional row item if the event was originated from a row,
+     * otherwise an empty optional.
      */
-    public T getDropTargetRow() {
-        return dropTargetRow;
+    public Optional<T> getDropTargetRow() {
+        return Optional.ofNullable(dropTargetRow);
     }
 
     /**

themes/src/main/themes/VAADIN/themes/valo/components/_grid.scss

       background: darken($v-grid-drag-indicator-color, 10%);
     }
   }
+
+  // Expand Grid's body to cover the whole Grid
+  .#{$primary-stylename}-body-droptarget {
+    width: 100%;
+    height: 100%;
+  }
 }
 
 

uitest/src/main/java/com/vaadin/tests/components/grid/GridDragAndDrop.java

         GridDragSource<Person> dragSource = applyDragSource(left);
 
         // Drop target Grid
-        Grid<Person> right = createGridAndFillWithData(5);
+        Grid<Person> right = createGridAndFillWithData(0);
         GridDropTarget<Person> dropTarget = applyDropTarget(right);
 
         // Layout the two grids
                     List<Person> items = (List<Person>) dataProvider.getItems();
 
                     // Calculate the target row's index
-                    int index = items.indexOf(event.getDropTargetRow())
-                            + (event.getDropLocation() == DropLocation.BELOW ? 1
-                                    : 0);
+                    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);
                             + ", dragDataJson="
                             + event.getDataTransferData("application/json")
                             + ", target="
-                            + event.getDropTargetRow().getFirstName() + " "
-                            + event.getDropTargetRow().getLastName()
+                            + (event.getDropTargetRow().isPresent() ?
+                            event.getDropTargetRow().get().getFirstName() + " "
+                                    + event.getDropTargetRow().get()
+                                    .getLastName() : "[BODY]")
                             + ", location=" + event.getDropLocation());
                 }
             });