Remove HasValue from Grid

Extracts grid single selection into separate class, which
is an extension like in V7. Using an extension makes it
possible to easily add multiselect and no-select modes back,
and support custom selection models.

Adds Grid:asSingleSelect() SingleSelect so that grid can be
used as a Select in a binder.

Removes all remaining references to SelectionModels in Listings.
Renames SingleSelectionChangeEvent to SingleSelectionEvent, because
then it is unified with selection listener and MultiSelectionEvent.

Fixes vaadin/framework8-issues#424
Fixes vaadin/framework8-issues#425

Change-Id: Ie22bef29cfd4336c3f65d4e63531c578b8dd76a3

1 files changed, 21 insertions, 0 deletions

diff --git a/server/src/main/java/com/vaadin/data/SelectionModel.java b/server/src/main/java/com/vaadin/data/SelectionModel.java
index eca43e6c8d..98a068993a 100644
--- a/server/src/main/java/com/vaadin/data/SelectionModel.java
+++ b/server/src/main/java/com/vaadin/data/SelectionModel.java
@@ -90,6 +90,11 @@ public interface SelectionModel<T> extends Serializable {
             return getSelectedItem().map(Collections::singleton)
                     .orElse(Collections.emptySet());
         }
+
+        @Override
+        default Optional<T> getFirstSelectedItem() {
+            return getSelectedItem();
+        }
     }
 
     /**
@@ -173,6 +178,11 @@ public interface SelectionModel<T> extends Serializable {
          *            the items to remove, not {@code null}
          */
         public void updateSelection(Set<T> addedItems, Set<T> removedItems);
+
+        @Override
+        default Optional<T> getFirstSelectedItem() {
+            return getSelectedItems().stream().findFirst();
+        }
     }
 
     /**
@@ -188,6 +198,17 @@ public interface SelectionModel<T> extends Serializable {
     public Set<T> getSelectedItems();
 
     /**
+     * Get first selected data item.
+     * <p>
+     * This is the same as {@link Single#getSelectedItem()} in case of single
+     * selection and the first selected item from
+     * {@link Multi#getSelectedItems()} in case of multiselection.
+     *
+     * @return the first selected item.
+     */
+    Optional<T> getFirstSelectedItem();
+
+    /**
      * Selects the given item. Depending on the implementation, may cause other
      * items to be deselected. If the item is already selected, does nothing.
      *