Add more context information to criteriaScript in GridDropTargetConnector (#11321)

* Add more context information to criteriaScript in GridDropTargetConnector

When using drag and drop in Grids and TreeGrids a criteriaScript can be specified.
However, this criteriaScript only gets an event as input parameter.
It would be helpful if the criteriaScript would get more information:
- the DropLocation
- the targetElement which is used as a base for the DropLocation

This change provides a protected method in DropTargetExtensionConnector
which decides, if the drop is allowed by the criteriaScript.
This method is overriden in GridDropTargetConnector to
provide the both parameters dropLocation and targetElement.

* add Test UI for criteriaScript with targetElement and dropLocation

* add some description for the Test UI

2 files changed, 39 insertions, 6 deletions

diff --git a/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java b/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java
index 98c4100159..011b98654c 100644
--- a/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java
+++ b/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java
@@ -100,6 +100,25 @@ public class GridDropTargetConnector extends DropTargetExtensionConnector {
         super.extend(target);
     }
 
+    @Override
+    protected boolean isDropAllowedByCriteriaScript(NativeEvent event) {
+        final String criteriaScript = getState().criteriaScript;
+        if (criteriaScript == null) {
+            return true;
+        }
+        return executeScript(event,
+                getTargetElement(event.getEventTarget().cast()),
+                getDropLocation(getTargetElement(event.getEventTarget().cast()),
+                        event).name(),
+                criteriaScript);
+    }
+
+    private native boolean executeScript(NativeEvent event,
+            Element targetElement, String dropLocation, String script)
+    /*-{
+       return new Function('event', 'targetElement', 'dropLocation', script)(event, targetElement, dropLocation);
+    }-*/;
+
     /**
      * Inspects whether the current drop would happen on the whole grid instead
      * of specific row as the drop target. This is based on used drop mode,
diff --git a/client/src/main/java/com/vaadin/client/extensions/DropTargetExtensionConnector.java b/client/src/main/java/com/vaadin/client/extensions/DropTargetExtensionConnector.java
index 2bd0ae81f3..a2c221be44 100644
--- a/client/src/main/java/com/vaadin/client/extensions/DropTargetExtensionConnector.java
+++ b/client/src/main/java/com/vaadin/client/extensions/DropTargetExtensionConnector.java
@@ -326,13 +326,8 @@ public class DropTargetExtensionConnector extends AbstractExtensionConnector {
         // Currently Safari, Edge and IE don't follow the spec by allowing drop
         // if those don't match
 
-        // Allow by default when criteria not set
-        boolean allowed = true;
-
         // Execute criteria script
-        if (getState().criteriaScript != null) {
-            allowed = executeScript(event, getState().criteriaScript);
-        }
+        boolean allowed = isDropAllowedByCriteriaScript(event);
 
         // Execute criterion defined via API
         if (allowed && getState().criteria != null
@@ -366,6 +361,25 @@ public class DropTargetExtensionConnector extends AbstractExtensionConnector {
     }
 
     /**
+     * Checks if a criteria script exists and, if yes, executes it. This method
+     * is protected, so subclasses as e.g. GridDropTargetConnector can override
+     * it to add additional script parameters.
+     *
+     * @param event
+     *            browser event (dragEnter, dragOver, drop) that should be
+     *            evaluated by the criteria script
+     * @return {@code true} if no script was given or if the script returned
+     *         true, {@code false} otherwise.
+     */
+    protected boolean isDropAllowedByCriteriaScript(NativeEvent event) {
+        final String criteriaScript = getState().criteriaScript;
+        if (criteriaScript == null) {
+            return true;
+        }
+        return executeScript(event, criteriaScript);
+    }
+
+    /**
      * Tells if the given array of types contains files.
      * <p>
      * According to HTML specification, if any files are being dragged, {@code