Implement single-selection mode for SelectionDelegate.

chrome/android/java/src/org/chromium/chrome/browser/widget/selection/SelectionDelegate.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.widget.selection;
 
 import org.chromium.base.ObserverList;
 
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
 /**
  * A generic delegate used to keep track of selected items.
  * @param <E> The type of the selectable items this delegate interacts with.
  */
 public class SelectionDelegate<E> {
+    // True if the SelectionDelegate should only support a single item being selected at a time.
+    private boolean mIsSingleSelection;
 
     /**
      * Observer interface to be notified of selection changes.
      */
     public interface SelectionObserver<E> {
         /**
          * Called when the set of selected items has changed.
          * @param selectedItems The list of currently selected items. An empty list indicates there
          *                      is no selection.
          */
-        public void onSelectionStateChange(List<E> selectedItems);
+        void onSelectionStateChange(List<E> selectedItems);
     }
 
     private Set<E> mSelectedItems = new HashSet<>();
     private ObserverList<SelectionObserver<E>> mObservers = new ObserverList<>();
 
     /**
+     * Sets the mode of this SelectionDelegate to single-selection.
+     */
+    public void setSingleSelectionMode() {
+        mIsSingleSelection = true;
+    }
+
+    /**
      * Toggles the selected state for the given item.
      * @param item The item to toggle.
      * @return Whether the item is selected.
      */
     public boolean toggleSelectionForItem(E item) {
-        if (mSelectedItems.contains(item)) mSelectedItems.remove(item);
-        else mSelectedItems.add(item);
+        if (mSelectedItems.contains(item)) {
+            mSelectedItems.remove(item);
+        } else {
+            if (mIsSingleSelection) mSelectedItems.clear();
+            mSelectedItems.add(item);
+        }
 
         notifyObservers();
 
         return isItemSelected(item);
     }
 
     /**
      * True if the item is selected. False otherwise.
      * @param item The item.
      * @return Whether the item is selected.
@@ skipping 39 matching lines @@
     public void removeObserver(SelectionObserver<E> observer) {
         mObservers.removeObserver(observer);
     }
 
     private void notifyObservers() {
         List<E> selectedItems = getSelectedItems();
         for (SelectionObserver<E> observer : mObservers) {
             observer.onSelectionStateChange(selectedItems);
         }
     }
-
 }