[MD Bookmarks] Auto expand folders on drag hover.

chrome/browser/resources/md_bookmarks/dnd_manager.js

 // Copyright 2017 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.
 
 cr.define('bookmarks', function() {
   /**
    * @param {BookmarkElement} element
    * @return {boolean}
    */
   function isBookmarkItem(element) {
@@ skipping 95 matching lines @@
         parents[parentId] = true;
         parentId = nodes[parentId].parentId;
       }
 
       return !!this.dragData && this.dragData.elements.some(function(node) {
         return parents[node.id];
       });
     },
   };
 
+
+  /**
+   * Manages auto expanding of sidebar folders on hover while dragging.
+   * @constructor
+   */
+  function AutoExpander() {
+    /** @const {number} */
+    this.EXPAND_FOLDER_DELAY = 400;
+
+    /** @private {number} */
+    this.lastTimestamp_ = 0;
+
+    /** @private {BookmarkElement|null} */
+    this.lastElement_ = null;
+
+    /** @private {number} */
+    this.testTimestamp_ = 0;
+  }
+
+  AutoExpander.prototype = {
+    /**
+     * @param {Event} e
+     * @param {?BookmarkElement} overElement
+     */
+    update: function(e, overElement) {
+      var eventTimestamp = this.testTimestamp_ || e.timeStamp;
+      var itemId = overElement ? overElement.itemId : null;
+      var store = bookmarks.Store.getInstance();
+
+      // If hovering over the same folder as last update, open the folder after
+      // the delay has passed.
+      if (overElement && overElement == this.lastElement_) {
+        if (eventTimestamp - this.lastTimestamp_ < this.EXPAND_FOLDER_DELAY)
+          return;
+
+        var action = bookmarks.actions.changeFolderOpen(itemId, true);
+        store.handleAction(action);
+      } else if (
+          overElement && isBookmarkFolderNode(overElement) &&
+          bookmarks.util.hasChildFolders(itemId, store.data.nodes) &&
+          store.data.closedFolders.has(itemId)) {
+        // Since this is a closed folder node that has children, set the auto
+        // expander to this element.
+        this.lastTimestamp_ = eventTimestamp;
+        this.lastElement_ = overElement;
+        return;
+      }
+
+      // If the folder has been expanded or we have moved to a different
+      // element, reset the auto expander.
+      this.lastTimestamp_ = 0;
+      this.lastElement_ = null;
+    },
+  };
+
   /**
    * Encapsulates the behavior of the drag and drop indicator which puts a line
    * between items or highlights folders which are valid drop targets.
    * @constructor
    */
   function DropIndicator() {
     /**
      * @private {number|null} Timer id used to help minimize flicker.
      */
     this.removeDropIndicatorTimer_ = null;
@@ skipping 97 matching lines @@
     this.dropIndicator_ = null;
 
     /** @private {Object<string, function(!Event)>} */
     this.documentListeners_ = null;
   }
 
   DNDManager.prototype = {
     init: function() {
       this.dragInfo_ = new DragInfo();
       this.dropIndicator_ = new DropIndicator();
+      this.autoExpander_ = new AutoExpander();
 
       this.documentListeners_ = {
         'dragstart': this.onDragStart_.bind(this),
         'dragenter': this.onDragEnter_.bind(this),
         'dragover': this.onDragOver_.bind(this),
         'dragleave': this.onDragLeave_.bind(this),
         'drop': this.onDrop_.bind(this),
         'dragend': this.clearDragData_.bind(this),
         'mouseup': this.clearDragData_.bind(this),
         // TODO(calamity): Add touch support.
@@ skipping 133 matching lines @@
         return;
 
       // The default operation is to allow dropping links etc to do
       // navigation. We never want to do that for the bookmark manager.
       e.preventDefault();
 
       if (!this.dragInfo_.isDragValid())
         return;
 
       var overElement = getBookmarkElement(e.path);
+      this.autoExpander_.update(e, overElement);
       if (!overElement)
         return;
 
-      // TODO(calamity): open folders on hover.
 
       // Now we know that we can drop. Determine if we will drop above, on or
       // below based on mouse position etc.
       this.dropDestination_ =
           this.calculateDropDestination_(e.clientY, overElement);
       if (!this.dropDestination_)
         return;
 
       if (e.dataTransfer) {
         e.dataTransfer.dropEffect =
@@ skipping 130 matching lines @@
       return !this.dragInfo_.isDraggingChildBookmark(overElement.itemId)
     },
   };
 
   return {
     DNDManager: DNDManager,
     DragInfo: DragInfo,
     DropIndicator: DropIndicator,
   };
 });