WebUI TaskManager: Remember column settings

chrome/browser/resources/task_manager/preload.js

 // Copyright (c) 2012 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.
 
 // Defines global variables.
 var commands = TaskManagerCommands;
 var taskmanager = undefined;  // This will be instantiated in main.js.
 
 /**
  * Invoked when a range of items has changed.
  */
 function taskChanged(start, length, tasks) {
   var task = {type: 'change', start: start, length: length, tasks: tasks};
 
   if (taskmanager) taskmanager.processTaskChange(task);
 }
 
+// Cached list of enabled columns to prevent frequent access to localStorage.
+var cachedEnabledColumns;
+
+/**
+ * Returns the dictionary which contains the list of columns and whether each
+ * column is enabled or not.
+ */
+function getEnabledColumns() {
+  // Use the cache after the second time since access to localStorage is slow.
+  if (cachedEnabledColumns)
+    return cachedEnabledColumns;
+
+  var json = window.localStorage.getItem(ENABLED_COLUMNS_KEY);
+  try {
+    cachedEnabledColumns = JSON.parse(json) || {};
+  } catch (e) {
+    cachedEnabledColumns = {};
+  }
+  for (var i = 0; i < DEFAULT_COLUMNS.length; i++) {
+    if (typeof(cachedEnabledColumns[DEFAULT_COLUMNS[i][0]]) == 'undefined')
+      cachedEnabledColumns[DEFAULT_COLUMNS[i][0]] = DEFAULT_COLUMNS[i][3];
+  }
+  return cachedEnabledColumns;
+}
+
+/**
+ * Returns whether the given column (at |columnId|) is enabled or not.
+ */
+function isColumnEnabled(columnId) {
+  return (getEnabledColumns())[columnId];
+}
+
+/**
+ * Sets whether the given column is enabled or not.
+ */
+function setColumnEnabled(columnId, newChecked) {
+  commands.setUpdateColumn(columnId, newChecked);
+  cachedEnabledColumns[columnId] = newChecked;
+  var json = JSON.stringify(cachedEnabledColumns);
+  window.localStorage.setItem(ENABLED_COLUMNS_KEY, json);
+}
+
 // Enable the taskmanager model before the loading of scripts.
 (function () {
   for (var i = 0; i < DEFAULT_COLUMNS.length; i++) {
-    if (DEFAULT_COLUMNS[i][3])
-      commands.setUpdateColumn(DEFAULT_COLUMNS[i][0], true);
+    var columnId = DEFAULT_COLUMNS[i][0];
+    if (isColumnEnabled(columnId))
+      commands.setUpdateColumn(columnId, true);
   }
   commands.enableTaskManager();
   commands.setUpdateColumn('canInspect', true);
   commands.setUpdateColumn('canActivate', true);
 })();