Add ItemCollapseAllowedProvider interface

server/src/main/java/com/vaadin/data/provider/HierarchicalDataCommunicator.java

@@ -32,10 +32,10 @@
 import com.vaadin.data.provider.HierarchyMapper.TreeLevelQuery;
 import com.vaadin.data.provider.HierarchyMapper.TreeNode;
 import com.vaadin.server.SerializableConsumer;
-import com.vaadin.server.SerializablePredicate;
 import com.vaadin.shared.Range;
 import com.vaadin.shared.data.HierarchicalDataCommunicatorConstants;
 import com.vaadin.shared.extension.datacommunicator.HierarchicalDataCommunicatorState;
+import com.vaadin.ui.ItemCollapseAllowedProvider;
 
 import elemental.json.Json;
 import elemental.json.JsonArray;
@@ -67,7 +67,7 @@ public class HierarchicalDataCommunicator<T> extends DataCommunicator<T> {
     /**
      * Collapse allowed provider used to allow/disallow collapsing nodes.
      */
-    private SerializablePredicate<T> itemCollapseAllowedProvider = t -> true;
+    private ItemCollapseAllowedProvider<T> itemCollapseAllowedProvider = t -> true;
 
     /**
      * The captured client side cache size.
@@ -513,7 +513,7 @@ public Optional<String> collapseItem(T item) {
      *            the item collapse allowed provider, not {@code null}
      */
     public void setItemCollapseAllowedProvider(
-            SerializablePredicate<T> provider) {
+            ItemCollapseAllowedProvider<T> provider) {
         Objects.requireNonNull(provider, "Provider can't be null");
         itemCollapseAllowedProvider = provider;
 

server/src/main/java/com/vaadin/ui/ItemCollapseAllowedProvider.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2000-2016 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.ui;
+
+import com.vaadin.server.SerializablePredicate;
+
+/**
+ * A callback interface for resolving whether client-side collapsing should be
+ * allowed for an item in a listing component that displays hierarchical data,
+ * such as {@link TreeGrid}.
+ *
+ * @author Vaadin Ltd
+ * @since 8.1
+ *
+ * @see TreeGrid#setItemCollapseAllowedProvider(ItemCollapseAllowedProvider)
+ *
+ * @param <T>
+ *            item data type
+ */
+@FunctionalInterface
+public interface ItemCollapseAllowedProvider<T> extends SerializablePredicate<T> {
+
+    /**
+     * Returns whether collapsing is allowed for the given item.
+     *
+     * @param item
+     *            the item to test
+     * @return {@code true} if collapse is allowed for the given item,
+     *         {@code false} otherwise
+     */
+    @Override
+    boolean test(T item);
+}

server/src/main/java/com/vaadin/ui/TreeGrid.java

@@ -42,7 +42,6 @@
 import com.vaadin.event.CollapseEvent.CollapseListener;
 import com.vaadin.event.ExpandEvent;
 import com.vaadin.event.ExpandEvent.ExpandListener;
-import com.vaadin.server.SerializablePredicate;
 import com.vaadin.shared.Registration;
 import com.vaadin.shared.ui.treegrid.FocusParentRpc;
 import com.vaadin.shared.ui.treegrid.FocusRpc;
@@ -433,10 +432,10 @@ public void setHierarchyColumn(String id) {
      * @param provider
      *            the item collapse allowed provider, not {@code null}
      *
-     * @see HierarchicalDataCommunicator#setItemCollapseAllowedProvider(SerializablePredicate)
+     * @see HierarchicalDataCommunicator#setItemCollapseAllowedProvider(ItemCollapseAllowedProvider)
      */
     public void setItemCollapseAllowedProvider(
-            SerializablePredicate<T> provider) {
+            ItemCollapseAllowedProvider<T> provider) {
         getDataCommunicator().setItemCollapseAllowedProvider(provider);
     }