Make it possible to reset single select values

Change-Id: I60c5d9f183d1955fbbd3341e3ac9f93aae755f80

3 files changed, 74 insertions, 0 deletions

diff --git a/server/src/main/java/com/vaadin/ui/AbstractSingleSelect.java b/server/src/main/java/com/vaadin/ui/AbstractSingleSelect.java
index 6510b5305a..81126f3189 100644
--- a/server/src/main/java/com/vaadin/ui/AbstractSingleSelect.java
+++ b/server/src/main/java/com/vaadin/ui/AbstractSingleSelect.java
@@ -39,6 +39,8 @@ import com.vaadin.ui.declarative.DesignContext;
 import com.vaadin.ui.declarative.DesignException;
 import com.vaadin.util.ReflectTools;
 
+import elemental.json.Json;
+
 /**
  * An abstract base class for listing components that only support single
  * selection and no lazy loading of data items.
@@ -241,6 +243,12 @@ public abstract class AbstractSingleSelect<T> extends AbstractListing<T>
         }
 
         doSetSelectedKey(key);
+
+        // Update diffstate so that a change will be sent to the client if the
+        // selection is changed to its original value
+        updateDiffstate("selectedItemKey",
+                key == null ? Json.createNull() : Json.create(key));
+
         fireEvent(new SingleSelectionEvent<>(AbstractSingleSelect.this, true));
     }
 
diff --git a/server/src/main/java/com/vaadin/ui/ComboBox.java b/server/src/main/java/com/vaadin/ui/ComboBox.java
index 22e6a698eb..b958ca9c32 100644
--- a/server/src/main/java/com/vaadin/ui/ComboBox.java
+++ b/server/src/main/java/com/vaadin/ui/ComboBox.java
@@ -52,6 +52,7 @@ import com.vaadin.ui.declarative.DesignAttributeHandler;
 import com.vaadin.ui.declarative.DesignContext;
 import com.vaadin.ui.declarative.DesignFormatter;
 
+import elemental.json.Json;
 import elemental.json.JsonObject;
 
 /**
@@ -670,4 +671,21 @@ public class ComboBox<T> extends AbstractSingleSelect<T>
         // ComboBox.
         return (DataCommunicator<T, String>) super.getDataCommunicator();
     }
+
+    @Override
+    protected void setSelectedFromClient(String key) {
+        super.setSelectedFromClient(key);
+
+        /*
+         * The client side for combo box always expects a state change for
+         * selectedItemKey after it has sent a selection change. This means that
+         * we must store a value in the diffstate that guarantees that a new
+         * value will be sent, regardless of what the value actually is at the
+         * time when changes are sent.
+         *
+         * Keys are always strings (or null), so using a non-string type will
+         * always trigger a diff mismatch and a resend.
+         */
+        updateDiffstate("selectedItemKey", Json.create(0));
+    }
 }
diff --git a/server/src/test/java/com/vaadin/ui/ComboBoxTest.java b/server/src/test/java/com/vaadin/ui/ComboBoxTest.java
new file mode 100644
index 0000000000..36b29fdd7a
--- /dev/null
+++ b/server/src/test/java/com/vaadin/ui/ComboBoxTest.java
@@ -0,0 +1,48 @@
+/*
+ * 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 org.junit.Test;
+
+import com.vaadin.shared.data.selection.SelectionServerRpc;
+import com.vaadin.tests.util.MockUI;
+
+public class ComboBoxTest {
+
+    @Test
+    public void testResetValue() {
+        ComboBox<String> comboBox = new ComboBox<>();
+        comboBox.setItems("one", "two");
+
+        // Reset value whenever it changes (in a real case, this listener would
+        // do something with the selected value before discarding it)
+        comboBox.addValueChangeListener(e -> comboBox.setValue(null));
+
+        // "Attach" the component and initialize diffstate
+        new MockUI().setContent(comboBox);
+        ComponentTest.syncToClient(comboBox);
+
+        // Emulate selection of "one"
+        String oneKey = comboBox.getDataCommunicator().getKeyMapper()
+                .key("one");
+        ComponentTest.getRpcProxy(comboBox, SelectionServerRpc.class)
+                .select(oneKey);
+
+        ComponentTest.assertEncodedStateProperties(comboBox,
+                "Selection change done by the listener should be sent to the client",
+                "selectedItemKey");
+    }
+}