For provided file systems; a) directory tree only navigates open nodes. b) Tree closes child nodes …

ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

Index: ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
index 11f2dc607c59841cb31589b6fcbcfe40cba82023..3cd331ef1656cd4bcbf89f9d6ae84a7d703302b0 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
@@ -92,7 +92,6 @@ var MENU_TREE_ITEM_INNER_HTML =
 function DirectoryItem(label, tree) {
   var item = new cr.ui.TreeItem();
   item.__proto__ = DirectoryItem.prototype;
-
   item.parentTree_ = tree;
   item.directoryModel_ = tree.directoryModel;
   item.fileFilter_ = tree.directoryModel.getFileFilter();
@@ -100,6 +99,15 @@ function DirectoryItem(label, tree) {
   item.innerHTML = TREE_ITEM_INNER_HTML;
   item.addEventListener('expand', item.onExpand_.bind(item), false);
 
+  // Listen for collapse because for the delayed expansion case all
+  // children are also collapsed.
+  item.addEventListener('collapse', item.onCollapse_.bind(item), false);
+
+  // Default delayExpansion to false. Volumes will set it to true for
+  // provided file systems. SubDirectories will inherit from their
+  // parent.
+  item.delayExpansion = false;
+
   // Sets hasChildren=false tentatively. This will be overridden after
   // scanning sub-directories in updateSubElementsFromList().
   item.hasChildren = false;
@@ -155,8 +163,19 @@ DirectoryItem.prototype.updateSubElementsFromList = function(recursive) {
       index++;
     } else if (util.isSameEntry(currentEntry, currentElement.entry)) {
       currentElement.updateSharedStatusIcon();
-      if (recursive && this.expanded)
-        currentElement.updateSubDirectories(true /* recursive */);
+      if (recursive && this.expanded) {
+        if (this.delayExpansion) {
+          // Only update deeper on expanded children.
+          if (currentElement.expanded) {
+            currentElement.updateSubDirectories(true /* recursive */);
+          }
+
+          // Show the expander even without knowing if there are children.
+          currentElement.mayHaveChildren_ = true;
+        } else {
+          currentElement.updateSubDirectories(true /* recursive */);
+        }
+      }
       index++;
     } else if (currentEntry.toURL() < currentElement.entry.toURL()) {
       var item = new SubDirectoryItem(label, currentEntry, this, tree);
@@ -224,6 +243,18 @@ DirectoryItem.prototype.remove = function(child) {
     this.hasChildren = false;
 };
 
+
+/**
+ * Removes the has-children attribute which allows returning
+ * to the ambiguous may-have-children state.
+ */
+DirectoryItem.prototype.clearHasChildren = function() {
+  var rowItem = this.firstElementChild;
+  this.removeAttribute('has-children');
+  rowItem.removeAttribute('has-children');
+};
+
+
 /**
  * Invoked when the item is being expanded.
  * @param {!Event} e Event.
@@ -240,6 +271,30 @@ DirectoryItem.prototype.onExpand_ = function(e) {
   e.stopPropagation();
 };
 
+
+/**
+ * Invoked when the item is being collapsed.
+ * @param {!Event} e Event.
+ * @private
+ */
+DirectoryItem.prototype.onCollapse_ = function(e) {
+  if (this.delayExpansion) {
+    // For file systems where it is performance intensive
+    // to update recursively when items expand this proactively
+    // collapses all children to avoid having to traverse large
+    // parts of the tree when reopened.
+    for (var i = 0; i < this.items.length; i++) {
+      var item = this.items[i];
+
+      if (item.expanded) {
+        item.expanded = false;
+      }
+    }
+  }
+
+  e.stopPropagation();
+};
+
 /**
  * Invoked when the tree item is clicked.
  *
@@ -389,6 +444,12 @@ function SubDirectoryItem(label, dirEntry, parentDirItem, tree) {
   item.__proto__ = SubDirectoryItem.prototype;
 
   item.entry = dirEntry;
+  item.delayExpansion = parentDirItem.delayExpansion;
+
+  if (item.delayExpansion) {
+    item.clearHasChildren();
+    item.mayHaveChildren_ = true;
+  }
 
   // Sets up icons of the item.
   var icon = item.querySelector('.icon');
@@ -461,6 +522,10 @@ function VolumeItem(modelItem, tree) {
   item.modelItem_ = modelItem;
   item.volumeInfo_ = modelItem.volumeInfo;
 
+  // Provided volumes should delay the expansion of child nodes
+  // for performance reasons.
+  item.delayExpansion = (item.volumeInfo.volumeType === 'provided');
+
   // Set helper attribute for testing.
   if (window.IN_TEST)
     item.setAttribute('volume-type-for-testing', item.volumeInfo_.volumeType);