diff --git a/blockly_uncompressed.js b/blockly_uncompressed.js
index 47f400da5..5c109ce3c 100644
--- a/blockly_uncompressed.js
+++ b/blockly_uncompressed.js
@@ -25,7 +25,7 @@ goog.addDependency('../../core/block.js', ['Blockly.Block'], ['Blockly.ASTNode',
 goog.addDependency('../../core/block_animations.js', ['Blockly.blockAnimations'], ['Blockly.utils.dom'], {});
 goog.addDependency('../../core/block_drag_surface.js', ['Blockly.BlockDragSurfaceSvg'], ['Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.dom'], {});
 goog.addDependency('../../core/block_dragger.js', ['Blockly.BlockDragger'], ['Blockly.Events', 'Blockly.Events.BlockMove', 'Blockly.Events.Ui', 'Blockly.InsertionMarkerManager', 'Blockly.blockAnimations', 'Blockly.utils.Coordinate', 'Blockly.utils.dom'], {});
-goog.addDependency('../../core/block_events.js', ['Blockly.Events.BlockBase', 'Blockly.Events.BlockChange', 'Blockly.Events.BlockCreate', 'Blockly.Events.BlockDelete', 'Blockly.Events.BlockMove', 'Blockly.Events.Change', 'Blockly.Events.Create', 'Blockly.Events.Delete', 'Blockly.Events.Move'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.utils.Coordinate', 'Blockly.utils.object', 'Blockly.utils.xml'], {});
+goog.addDependency('../../core/block_events.js', ['Blockly.Events.BlockBase', 'Blockly.Events.BlockChange', 'Blockly.Events.BlockCreate', 'Blockly.Events.BlockDelete', 'Blockly.Events.BlockMove', 'Blockly.Events.Change', 'Blockly.Events.Create', 'Blockly.Events.Delete', 'Blockly.Events.Move'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.registry', 'Blockly.utils.Coordinate', 'Blockly.utils.object', 'Blockly.utils.xml'], {});
 goog.addDependency('../../core/block_svg.js', ['Blockly.BlockSvg'], ['Blockly.ASTNode', 'Blockly.Block', 'Blockly.ContextMenu', 'Blockly.ContextMenuRegistry', 'Blockly.Events', 'Blockly.Events.BlockMove', 'Blockly.Events.Ui', 'Blockly.Msg', 'Blockly.RenderedConnection', 'Blockly.TabNavigateCursor', 'Blockly.Tooltip', 'Blockly.Touch', 'Blockly.blockAnimations', 'Blockly.blockRendering.IPathObject', 'Blockly.navigation', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Rect', 'Blockly.utils.deprecation', 'Blockly.utils.dom', 'Blockly.utils.object'], {});
 goog.addDependency('../../core/blockly.js', ['Blockly'], ['Blockly.Events', 'Blockly.Events.Ui', 'Blockly.Procedures', 'Blockly.Tooltip', 'Blockly.Touch', 'Blockly.Variables', 'Blockly.WidgetDiv', 'Blockly.WorkspaceSvg', 'Blockly.Xml', 'Blockly.constants', 'Blockly.inject', 'Blockly.navigation', 'Blockly.utils', 'Blockly.utils.Size', 'Blockly.utils.colour'], {});
 goog.addDependency('../../core/blocks.js', ['Blockly.Blocks'], [], {});
@@ -45,7 +45,7 @@ goog.addDependency('../../core/contextmenu_items.js', ['Blockly.ContextMenuItems
 goog.addDependency('../../core/contextmenu_registry.js', ['Blockly.ContextMenuRegistry'], ['Blockly.ContextMenuItems'], {'lang': 'es5'});
 goog.addDependency('../../core/css.js', ['Blockly.Css'], [], {'lang': 'es5'});
 goog.addDependency('../../core/dropdowndiv.js', ['Blockly.DropDownDiv'], ['Blockly.utils.dom', 'Blockly.utils.math', 'Blockly.utils.style'], {});
-goog.addDependency('../../core/events.js', ['Blockly.Events'], ['Blockly.utils'], {});
+goog.addDependency('../../core/events.js', ['Blockly.Events'], ['Blockly.registry', 'Blockly.utils'], {});
 goog.addDependency('../../core/events_abstract.js', ['Blockly.Events.Abstract'], ['Blockly.Events'], {});
 goog.addDependency('../../core/extensions.js', ['Blockly.Extensions'], ['Blockly.utils'], {});
 goog.addDependency('../../core/field.js', ['Blockly.Field'], ['Blockly.Events', 'Blockly.Events.BlockChange', 'Blockly.Gesture', 'Blockly.utils', 'Blockly.utils.Rect', 'Blockly.utils.Size', 'Blockly.utils.dom', 'Blockly.utils.style', 'Blockly.utils.userAgent'], {'lang': 'es5'});
@@ -158,7 +158,7 @@ goog.addDependency('../../core/tooltip.js', ['Blockly.Tooltip'], ['Blockly.utils
 goog.addDependency('../../core/touch.js', ['Blockly.Touch'], ['Blockly.utils', 'Blockly.utils.global', 'Blockly.utils.string'], {});
 goog.addDependency('../../core/touch_gesture.js', ['Blockly.TouchGesture'], ['Blockly.Gesture', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.object'], {});
 goog.addDependency('../../core/trashcan.js', ['Blockly.Trashcan'], ['Blockly.Scrollbar', 'Blockly.Xml', 'Blockly.utils.Rect', 'Blockly.utils.dom'], {});
-goog.addDependency('../../core/ui_events.js', ['Blockly.Events.Ui'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.utils.object'], {});
+goog.addDependency('../../core/ui_events.js', ['Blockly.Events.Ui'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.registry', 'Blockly.utils.object'], {});
 goog.addDependency('../../core/utils.js', ['Blockly.utils'], ['Blockly.Msg', 'Blockly.constants', 'Blockly.utils.Coordinate', 'Blockly.utils.Rect', 'Blockly.utils.colour', 'Blockly.utils.global', 'Blockly.utils.string', 'Blockly.utils.style', 'Blockly.utils.userAgent'], {});
 goog.addDependency('../../core/utils/aria.js', ['Blockly.utils.aria'], [], {});
 goog.addDependency('../../core/utils/colour.js', ['Blockly.utils.colour'], [], {});
@@ -179,7 +179,7 @@ goog.addDependency('../../core/utils/svg_paths.js', ['Blockly.utils.svgPaths'],
 goog.addDependency('../../core/utils/toolbox.js', ['Blockly.utils.toolbox'], [], {});
 goog.addDependency('../../core/utils/useragent.js', ['Blockly.utils.userAgent'], ['Blockly.utils.global'], {});
 goog.addDependency('../../core/utils/xml.js', ['Blockly.utils.xml'], [], {});
-goog.addDependency('../../core/variable_events.js', ['Blockly.Events.VarBase', 'Blockly.Events.VarCreate', 'Blockly.Events.VarDelete', 'Blockly.Events.VarRename'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.utils.object'], {});
+goog.addDependency('../../core/variable_events.js', ['Blockly.Events.VarBase', 'Blockly.Events.VarCreate', 'Blockly.Events.VarDelete', 'Blockly.Events.VarRename'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.registry', 'Blockly.utils.object'], {});
 goog.addDependency('../../core/variable_map.js', ['Blockly.VariableMap'], ['Blockly.Events', 'Blockly.Events.VarDelete', 'Blockly.Events.VarRename', 'Blockly.Msg', 'Blockly.utils', 'Blockly.utils.object'], {});
 goog.addDependency('../../core/variable_model.js', ['Blockly.VariableModel'], ['Blockly.Events', 'Blockly.Events.VarCreate', 'Blockly.utils'], {});
 goog.addDependency('../../core/variables.js', ['Blockly.Variables'], ['Blockly.Blocks', 'Blockly.Msg', 'Blockly.VariableModel', 'Blockly.Xml', 'Blockly.utils', 'Blockly.utils.xml'], {});
@@ -193,9 +193,9 @@ goog.addDependency('../../core/workspace_comment_render_svg.js', ['Blockly.Works
 goog.addDependency('../../core/workspace_comment_svg.js', ['Blockly.WorkspaceCommentSvg'], ['Blockly.Css', 'Blockly.Events', 'Blockly.Events.CommentCreate', 'Blockly.Events.CommentDelete', 'Blockly.Events.CommentMove', 'Blockly.Events.Ui', 'Blockly.WorkspaceComment', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Rect', 'Blockly.utils.dom', 'Blockly.utils.object'], {});
 goog.addDependency('../../core/workspace_drag_surface_svg.js', ['Blockly.WorkspaceDragSurfaceSvg'], ['Blockly.utils', 'Blockly.utils.dom'], {});
 goog.addDependency('../../core/workspace_dragger.js', ['Blockly.WorkspaceDragger'], ['Blockly.utils.Coordinate'], {});
-goog.addDependency('../../core/workspace_events.js', ['Blockly.Events.FinishedLoading'], ['Blockly.Events', 'Blockly.Events.Ui', 'Blockly.utils.object'], {'lang': 'es5'});
+goog.addDependency('../../core/workspace_events.js', ['Blockly.Events.FinishedLoading'], ['Blockly.Events', 'Blockly.Events.Ui', 'Blockly.registry', 'Blockly.utils.object'], {'lang': 'es5'});
 goog.addDependency('../../core/workspace_svg.js', ['Blockly.WorkspaceSvg'], ['Blockly.BlockSvg', 'Blockly.ConnectionDB', 'Blockly.ContextMenuRegistry', 'Blockly.Events', 'Blockly.Events.BlockCreate', 'Blockly.Gesture', 'Blockly.Grid', 'Blockly.MarkerManager', 'Blockly.Msg', 'Blockly.Options', 'Blockly.ThemeManager', 'Blockly.Themes.Classic', 'Blockly.TouchGesture', 'Blockly.Workspace', 'Blockly.WorkspaceAudio', 'Blockly.WorkspaceDragSurfaceSvg', 'Blockly.Xml', 'Blockly.blockRendering', 'Blockly.constants', 'Blockly.navigation', 'Blockly.registry', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Metrics', 'Blockly.utils.Rect', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.toolbox'], {});
-goog.addDependency('../../core/ws_comment_events.js', ['Blockly.Events.CommentBase', 'Blockly.Events.CommentChange', 'Blockly.Events.CommentCreate', 'Blockly.Events.CommentDelete', 'Blockly.Events.CommentMove'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.utils.Coordinate', 'Blockly.utils.object', 'Blockly.utils.xml'], {});
+goog.addDependency('../../core/ws_comment_events.js', ['Blockly.Events.CommentBase', 'Blockly.Events.CommentChange', 'Blockly.Events.CommentCreate', 'Blockly.Events.CommentDelete', 'Blockly.Events.CommentMove'], ['Blockly.Events', 'Blockly.Events.Abstract', 'Blockly.registry', 'Blockly.utils.Coordinate', 'Blockly.utils.object', 'Blockly.utils.xml'], {});
 goog.addDependency('../../core/xml.js', ['Blockly.Xml'], ['Blockly.Events', 'Blockly.Events.BlockCreate', 'Blockly.Events.FinishedLoading', 'Blockly.Events.VarCreate', 'Blockly.utils', 'Blockly.utils.dom', 'Blockly.utils.global', 'Blockly.utils.xml'], {});
 goog.addDependency('../../core/zoom_controls.js', ['Blockly.ZoomControls'], ['Blockly.Css', 'Blockly.Scrollbar', 'Blockly.Touch', 'Blockly.utils.dom'], {'lang': 'es5'});
 goog.addDependency("base.js", [], []);
diff --git a/core/block_events.js b/core/block_events.js
index aa6d2754a..06bd0fc5f 100644
--- a/core/block_events.js
+++ b/core/block_events.js
@@ -22,6 +22,7 @@ goog.provide('Blockly.Events.Move');  // Deprecated.
 
 goog.require('Blockly.Events');
 goog.require('Blockly.Events.Abstract');
+goog.require('Blockly.registry');
 goog.require('Blockly.utils.Coordinate');
 goog.require('Blockly.utils.object');
 goog.require('Blockly.utils.xml');
@@ -31,19 +32,26 @@ goog.require('Blockly.utils.xml');
 
 /**
  * Abstract class for a block event.
- * @param {Blockly.Block} block The block this event corresponds to.
+ * @param {!Blockly.Block=} opt_block The block this event corresponds to.
+ *     Undefined for a blank event.
  * @extends {Blockly.Events.Abstract}
  * @constructor
  */
-Blockly.Events.BlockBase = function(block) {
+Blockly.Events.BlockBase = function(opt_block) {
   Blockly.Events.BlockBase.superClass_.constructor.call(this);
+  this.isBlank = typeof opt_block == 'undefined';
 
   /**
    * The block id for the block this event pertains to
    * @type {string}
    */
-  this.blockId = block.id;
-  this.workspaceId = block.workspace.id;
+  this.blockId = this.isBlank ? '' : opt_block.id;
+
+  /**
+   * The workspace identifier for this event.
+   * @type {string}
+   */
+  this.workspaceId = this.isBlank ? '' : opt_block.workspace.id;
 };
 Blockly.utils.object.inherits(Blockly.Events.BlockBase,
     Blockly.Events.Abstract);
@@ -69,33 +77,36 @@ Blockly.Events.BlockBase.prototype.fromJson = function(json) {
 
 /**
  * Class for a block change event.
- * @param {Blockly.Block} block The changed block.  Null for a blank event.
- * @param {string} element One of 'field', 'comment', 'disabled', etc.
- * @param {?string} name Name of input or field affected, or null.
- * @param {*} oldValue Previous value of element.
- * @param {*} newValue New value of element.
+ * @param {!Blockly.Block=} opt_block The changed block.  Undefined for a blank
+ *     event.
+ * @param {string=} opt_element One of 'field', 'comment', 'disabled', etc.
+ * @param {?string=} opt_name Name of input or field affected, or null.
+ * @param {*=} opt_oldValue Previous value of element.
+ * @param {*=} opt_newValue New value of element.
  * @extends {Blockly.Events.BlockBase}
  * @constructor
  */
-Blockly.Events.Change = function(block, element, name, oldValue, newValue) {
-  if (!block) {
+Blockly.Events.Change = function(opt_block, opt_element, opt_name, opt_oldValue,
+    opt_newValue) {
+  Blockly.Events.Change.superClass_.constructor.call(this, opt_block);
+  if (!opt_block) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.Change.superClass_.constructor.call(this, block);
-  this.element = element;
-  this.name = name;
-  this.oldValue = oldValue;
-  this.newValue = newValue;
+  this.element = typeof opt_element == 'undefined' ? '' : opt_element;
+  this.name = typeof opt_name == 'undefined' ? '' : opt_name;
+  this.oldValue = typeof opt_oldValue == 'undefined' ? '' : opt_oldValue;
+  this.newValue = typeof opt_newValue == 'undefined' ? '' : opt_newValue;
 };
 Blockly.utils.object.inherits(Blockly.Events.Change, Blockly.Events.BlockBase);
 
 /**
  * Class for a block change event.
- * @param {Blockly.Block} block The changed block.  Null for a blank event.
- * @param {string} element One of 'field', 'comment', 'disabled', etc.
- * @param {?string} name Name of input or field affected, or null.
- * @param {*} oldValue Previous value of element.
- * @param {*} newValue New value of element.
+ * @param {!Blockly.Block=} opt_block The changed block.  Undefined for a blank
+ *     event.
+ * @param {string=} opt_element One of 'field', 'comment', 'disabled', etc.
+ * @param {?string=} opt_name Name of input or field affected, or null.
+ * @param {*=} opt_oldValue Previous value of element.
+ * @param {*=} opt_newValue New value of element.
  * @extends {Blockly.Events.BlockBase}
  * @constructor
  */
@@ -197,28 +208,30 @@ Blockly.Events.Change.prototype.run = function(forward) {
 
 /**
  * Class for a block creation event.
- * @param {Blockly.Block} block The created block.  Null for a blank event.
+ * @param {!Blockly.Block=} opt_block The created block.  Undefined for a blank
+ *     event.
  * @extends {Blockly.Events.BlockBase}
  * @constructor
  */
-Blockly.Events.Create = function(block) {
-  if (!block) {
+Blockly.Events.Create = function(opt_block) {
+  Blockly.Events.Create.superClass_.constructor.call(this, opt_block);
+  if (!opt_block) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.Create.superClass_.constructor.call(this, block);
 
-  if (block.workspace.rendered) {
-    this.xml = Blockly.Xml.blockToDomWithXY(block);
+  if (opt_block.workspace.rendered) {
+    this.xml = Blockly.Xml.blockToDomWithXY(opt_block);
   } else {
-    this.xml = Blockly.Xml.blockToDom(block);
+    this.xml = Blockly.Xml.blockToDom(opt_block);
   }
-  this.ids = Blockly.Events.getDescendantIds(block);
+  this.ids = Blockly.Events.getDescendantIds(opt_block);
 };
 Blockly.utils.object.inherits(Blockly.Events.Create, Blockly.Events.BlockBase);
 
 /**
  * Class for a block creation event.
- * @param {Blockly.Block} block The created block. Null for a blank event.
+ * @param {!Blockly.Block=} block The created block. Undefined for a blank
+ *     event.
  * @extends {Blockly.Events.BlockBase}
  * @constructor
  */
@@ -276,25 +289,26 @@ Blockly.Events.Create.prototype.run = function(forward) {
 
 /**
  * Class for a block deletion event.
- * @param {Blockly.Block} block The deleted block.  Null for a blank event.
+ * @param {!Blockly.Block=} opt_block The deleted block.  Undefined for a blank
+ *     event.
  * @extends {Blockly.Events.BlockBase}
  * @constructor
  */
-Blockly.Events.Delete = function(block) {
-  if (!block) {
+Blockly.Events.Delete = function(opt_block) {
+  Blockly.Events.Delete.superClass_.constructor.call(this, opt_block);
+  if (!opt_block) {
     return;  // Blank event to be populated by fromJson.
   }
-  if (block.getParent()) {
+  if (opt_block.getParent()) {
     throw Error('Connected blocks cannot be deleted.');
   }
-  Blockly.Events.Delete.superClass_.constructor.call(this, block);
 
-  if (block.workspace.rendered) {
-    this.oldXml = Blockly.Xml.blockToDomWithXY(block);
+  if (opt_block.workspace.rendered) {
+    this.oldXml = Blockly.Xml.blockToDomWithXY(opt_block);
   } else {
-    this.oldXml = Blockly.Xml.blockToDom(block);
+    this.oldXml = Blockly.Xml.blockToDom(opt_block);
   }
-  this.ids = Blockly.Events.getDescendantIds(block);
+  this.ids = Blockly.Events.getDescendantIds(opt_block);
 };
 Blockly.utils.object.inherits(Blockly.Events.Delete, Blockly.Events.BlockBase);
 
@@ -356,15 +370,17 @@ Blockly.Events.Delete.prototype.run = function(forward) {
 
 /**
  * Class for a block move event.  Created before the move.
- * @param {Blockly.Block} block The moved block.  Null for a blank event.
+ * @param {!Blockly.Block=} opt_block The moved block.  Undefined for a blank
+ *     event.
  * @extends {Blockly.Events.BlockBase}
  * @constructor
  */
-Blockly.Events.Move = function(block) {
-  if (!block) {
+Blockly.Events.Move = function(opt_block) {
+  Blockly.Events.Move.superClass_.constructor.call(this, opt_block);
+  if (!opt_block) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.Move.superClass_.constructor.call(this, block);
+
   var location = this.currentLocation_();
   this.oldParentId = location.parentId;
   this.oldInputName = location.inputName;
@@ -509,3 +525,12 @@ Blockly.Events.Move.prototype.run = function(forward) {
     }
   }
 };
+
+Blockly.registry.register(Blockly.registry.Type.EVENT, Blockly.Events.CREATE,
+    Blockly.Events.Create);
+Blockly.registry.register(Blockly.registry.Type.EVENT, Blockly.Events.DELETE,
+    Blockly.Events.Delete);
+Blockly.registry.register(Blockly.registry.Type.EVENT, Blockly.Events.CHANGE,
+    Blockly.Events.Change);
+Blockly.registry.register(Blockly.registry.Type.EVENT, Blockly.Events.MOVE,
+    Blockly.Events.Move);
diff --git a/core/events.js b/core/events.js
index 3f567b132..e803af0ab 100644
--- a/core/events.js
+++ b/core/events.js
@@ -16,6 +16,7 @@
  */
 goog.provide('Blockly.Events');
 
+goog.require('Blockly.registry');
 goog.require('Blockly.utils');
 
 
@@ -338,53 +339,15 @@ Blockly.Events.getDescendantIds = function(block) {
  * @param {!Object} json JSON representation.
  * @param {!Blockly.Workspace} workspace Target workspace for event.
  * @return {!Blockly.Events.Abstract} The event represented by the JSON.
+ * @throws {Error} if an event type is not found in the registry.
  */
 Blockly.Events.fromJson = function(json, workspace) {
-  // TODO: Should I have a way to register a new event into here?
-  var event;
-  switch (json.type) {
-    case Blockly.Events.CREATE:
-      event = new Blockly.Events.Create(null);
-      break;
-    case Blockly.Events.DELETE:
-      event = new Blockly.Events.Delete(null);
-      break;
-    case Blockly.Events.CHANGE:
-      event = new Blockly.Events.Change(null, '', '', '', '');
-      break;
-    case Blockly.Events.MOVE:
-      event = new Blockly.Events.Move(null);
-      break;
-    case Blockly.Events.VAR_CREATE:
-      event = new Blockly.Events.VarCreate(null);
-      break;
-    case Blockly.Events.VAR_DELETE:
-      event = new Blockly.Events.VarDelete(null);
-      break;
-    case Blockly.Events.VAR_RENAME:
-      event = new Blockly.Events.VarRename(null, '');
-      break;
-    case Blockly.Events.UI:
-      event = new Blockly.Events.Ui(null, '', '', '');
-      break;
-    case Blockly.Events.COMMENT_CREATE:
-      event = new Blockly.Events.CommentCreate(null);
-      break;
-    case Blockly.Events.COMMENT_CHANGE:
-      event = new Blockly.Events.CommentChange(null, '', '');
-      break;
-    case Blockly.Events.COMMENT_MOVE:
-      event = new Blockly.Events.CommentMove(null);
-      break;
-    case Blockly.Events.COMMENT_DELETE:
-      event = new Blockly.Events.CommentDelete(null);
-      break;
-    case Blockly.Events.FINISHED_LOADING:
-      event = new Blockly.Events.FinishedLoading(workspace);
-      break;
-    default:
-      throw Error('Unknown event type.');
+  var eventClass = Blockly.registry.getClass(Blockly.registry.Type.EVENT,
+      json.type);
+  if (!eventClass) {
+    throw Error('Unknown event type.');
   }
+  var event = new eventClass();
   event.fromJson(json);
   event.workspaceId = workspace.id;
   return event;
diff --git a/core/events_abstract.js b/core/events_abstract.js
index cbe048b9d..948649f9a 100644
--- a/core/events_abstract.js
+++ b/core/events_abstract.js
@@ -21,6 +21,13 @@ goog.require('Blockly.Events');
  * @constructor
  */
 Blockly.Events.Abstract = function() {
+
+  /**
+   * Whether or not the event is blank (to be populated by fromJson).
+   * @type {?boolean}
+   */
+  this.isBlank = null;
+
   /**
    * The workspace identifier for this event.
    * @type {string|undefined}
@@ -61,6 +68,7 @@ Blockly.Events.Abstract.prototype.toJson = function() {
  * @param {!Object} json JSON representation.
  */
 Blockly.Events.Abstract.prototype.fromJson = function(json) {
+  this.isBlank = false;
   this.group = json['group'];
 };
 
diff --git a/core/registry.js b/core/registry.js
index b0e808ac4..05cb95160 100644
--- a/core/registry.js
+++ b/core/registry.js
@@ -14,6 +14,7 @@
 goog.provide('Blockly.registry');
 
 goog.requireType('Blockly.blockRendering.Renderer');
+goog.requireType('Blockly.Events.Abstract');
 goog.requireType('Blockly.Field');
 goog.requireType('Blockly.IConnectionChecker');
 goog.requireType('Blockly.IFlyout');
@@ -60,21 +61,25 @@ Blockly.registry.Type.prototype.toString = function() {
   return this.name_;
 };
 
-/** @type {!Blockly.registry.Type<Blockly.blockRendering.Renderer>} */
-Blockly.registry.Type.RENDERER = new Blockly.registry.Type('renderer');
+/** @type {!Blockly.registry.Type<Blockly.IConnectionChecker>} */
+Blockly.registry.Type.CONNECTION_CHECKER =
+    new Blockly.registry.Type('connectionChecker');
+
+/** @type {!Blockly.registry.Type<Blockly.Events.Abstract>} */
+Blockly.registry.Type.EVENT = new Blockly.registry.Type('event');
 
 /** @type {!Blockly.registry.Type<Blockly.Field>} */
 Blockly.registry.Type.FIELD = new Blockly.registry.Type('field');
 
+/** @type {!Blockly.registry.Type<Blockly.blockRendering.Renderer>} */
+Blockly.registry.Type.RENDERER = new Blockly.registry.Type('renderer');
+
 /** @type {!Blockly.registry.Type<Blockly.IToolbox>} */
 Blockly.registry.Type.TOOLBOX = new Blockly.registry.Type('toolbox');
 
 /** @type {!Blockly.registry.Type<Blockly.Theme>} */
 Blockly.registry.Type.THEME = new Blockly.registry.Type('theme');
 
-/** @type {!Blockly.registry.Type<Blockly.IConnectionChecker>} */
-Blockly.registry.Type.CONNECTION_CHECKER =
-    new Blockly.registry.Type('connectionChecker');
 
 /** @type {!Blockly.registry.Type<Blockly.IFlyout>} */
 Blockly.registry.Type.FLYOUTS_VERTICAL_TOOLBOX =
diff --git a/core/ui_events.js b/core/ui_events.js
index 693b7443d..7991f6b86 100644
--- a/core/ui_events.js
+++ b/core/ui_events.js
@@ -14,6 +14,7 @@ goog.provide('Blockly.Events.Ui');
 
 goog.require('Blockly.Events');
 goog.require('Blockly.Events.Abstract');
+goog.require('Blockly.registry');
 goog.require('Blockly.utils.object');
 
 
@@ -23,20 +24,25 @@ goog.require('Blockly.utils.object');
  * editing to work (e.g. scrolling the workspace, zooming, opening toolbox
  * categories).
  * UI events do not undo or redo.
- * @param {Blockly.Block} block The affected block.
- * @param {string} element One of 'selected', 'comment', 'mutatorOpen', etc.
- * @param {*} oldValue Previous value of element.
- * @param {*} newValue New value of element.
+ * @param {?Blockly.Block=} opt_block The affected block.  Null for UI events
+ *     that do not have an associated block.  Undefined for a blank event.
+ * @param {string=} opt_element One of 'selected', 'comment', 'mutatorOpen',
+ *     etc.
+ * @param {*=} opt_oldValue Previous value of element.
+ * @param {*=} opt_newValue New value of element.
  * @extends {Blockly.Events.Abstract}
  * @constructor
  */
-Blockly.Events.Ui = function(block, element, oldValue, newValue) {
+Blockly.Events.Ui = function(opt_block, opt_element, opt_oldValue,
+    opt_newValue) {
   Blockly.Events.Ui.superClass_.constructor.call(this);
-  this.blockId = block ? block.id : null;
-  this.workspaceId = block ? block.workspace.id : undefined;
-  this.element = element;
-  this.oldValue = oldValue;
-  this.newValue = newValue;
+  this.isBlank = typeof opt_block == 'undefined';
+
+  this.blockId = opt_block ? opt_block.id : null;
+  this.workspaceId = opt_block ? opt_block.workspace.id : undefined;
+  this.element = typeof opt_element == 'undefined' ? '' : opt_element;
+  this.oldValue = typeof opt_oldValue == 'undefined' ? '' : opt_oldValue;
+  this.newValue = typeof opt_newValue == 'undefined' ? '' : opt_newValue;
   // UI events do not undo or redo.
   this.recordUndo = false;
 };
@@ -74,3 +80,6 @@ Blockly.Events.Ui.prototype.fromJson = function(json) {
   this.newValue = json['newValue'];
   this.blockId = json['blockId'];
 };
+
+Blockly.registry.register(Blockly.registry.Type.EVENT, Blockly.Events.UI,
+    Blockly.Events.Ui);
diff --git a/core/variable_events.js b/core/variable_events.js
index 78c6d65c4..c908ff26a 100644
--- a/core/variable_events.js
+++ b/core/variable_events.js
@@ -17,25 +17,32 @@ goog.provide('Blockly.Events.VarRename');
 
 goog.require('Blockly.Events');
 goog.require('Blockly.Events.Abstract');
+goog.require('Blockly.registry');
 goog.require('Blockly.utils.object');
 
 
 /**
  * Abstract class for a variable event.
- * @param {Blockly.VariableModel} variable The variable this event corresponds
- *     to.
+ * @param {!Blockly.VariableModel=} opt_variable The variable this event
+ *     corresponds to.  Undefined for a blank event.
  * @extends {Blockly.Events.Abstract}
  * @constructor
  */
-Blockly.Events.VarBase = function(variable) {
+Blockly.Events.VarBase = function(opt_variable) {
   Blockly.Events.VarBase.superClass_.constructor.call(this);
+  this.isBlank = typeof opt_variable == 'undefined';
 
   /**
    * The variable id for the variable this event pertains to.
    * @type {string}
    */
-  this.varId = variable.getId();
-  this.workspaceId = variable.workspace.id;
+  this.varId = this.isBlank ? '' : opt_variable.getId();
+
+  /**
+   * The workspace identifier for this event.
+   * @type {string}
+   */
+  this.workspaceId = this.isBlank ? '' : opt_variable.workspace.id;
 };
 Blockly.utils.object.inherits(Blockly.Events.VarBase, Blockly.Events.Abstract);
 
@@ -60,18 +67,19 @@ Blockly.Events.VarBase.prototype.fromJson = function(json) {
 
 /**
  * Class for a variable creation event.
- * @param {Blockly.VariableModel} variable The created variable.
- *     Null for a blank event.
+ * @param {!Blockly.VariableModel=} opt_variable The created variable. Undefined
+ *     for a blank event.
  * @extends {Blockly.Events.VarBase}
  * @constructor
  */
-Blockly.Events.VarCreate = function(variable) {
-  if (!variable) {
+Blockly.Events.VarCreate = function(opt_variable) {
+  Blockly.Events.VarCreate.superClass_.constructor.call(this, opt_variable);
+  if (!opt_variable) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.VarCreate.superClass_.constructor.call(this, variable);
-  this.varType = variable.type;
-  this.varName = variable.name;
+
+  this.varType = opt_variable.type;
+  this.varName = opt_variable.name;
 };
 Blockly.utils.object.inherits(Blockly.Events.VarCreate, Blockly.Events.VarBase);
 
@@ -117,18 +125,19 @@ Blockly.Events.VarCreate.prototype.run = function(forward) {
 
 /**
  * Class for a variable deletion event.
- * @param {Blockly.VariableModel} variable The deleted variable.
- *     Null for a blank event.
+ * @param {!Blockly.VariableModel=} opt_variable The deleted variable. Undefined
+ *     for a blank event.
  * @extends {Blockly.Events.VarBase}
  * @constructor
  */
-Blockly.Events.VarDelete = function(variable) {
-  if (!variable) {
+Blockly.Events.VarDelete = function(opt_variable) {
+  Blockly.Events.VarDelete.superClass_.constructor.call(this, opt_variable);
+  if (!opt_variable) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.VarDelete.superClass_.constructor.call(this, variable);
-  this.varType = variable.type;
-  this.varName = variable.name;
+
+  this.varType = opt_variable.type;
+  this.varName = opt_variable.name;
 };
 Blockly.utils.object.inherits(Blockly.Events.VarDelete, Blockly.Events.VarBase);
 
@@ -174,19 +183,20 @@ Blockly.Events.VarDelete.prototype.run = function(forward) {
 
 /**
  * Class for a variable rename event.
- * @param {Blockly.VariableModel} variable The renamed variable.
- *     Null for a blank event.
- * @param {string} newName The new name the variable will be changed to.
+ * @param {!Blockly.VariableModel=} opt_variable The renamed variable. Undefined
+ *     for a blank event.
+ * @param {string=} newName The new name the variable will be changed to.
  * @extends {Blockly.Events.VarBase}
  * @constructor
  */
-Blockly.Events.VarRename = function(variable, newName) {
-  if (!variable) {
+Blockly.Events.VarRename = function(opt_variable, newName) {
+  Blockly.Events.VarRename.superClass_.constructor.call(this, opt_variable);
+  if (!opt_variable) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.VarRename.superClass_.constructor.call(this, variable);
-  this.oldName = variable.name;
-  this.newName = newName;
+
+  this.oldName = opt_variable.name;
+  this.newName = typeof newName == 'undefined' ? '' : newName;
 };
 Blockly.utils.object.inherits(Blockly.Events.VarRename, Blockly.Events.VarBase);
 
@@ -229,3 +239,10 @@ Blockly.Events.VarRename.prototype.run = function(forward) {
     workspace.renameVariableById(this.varId, this.oldName);
   }
 };
+
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.VAR_CREATE, Blockly.Events.VarCreate);
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.VAR_DELETE, Blockly.Events.VarDelete);
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.VAR_RENAME, Blockly.Events.VarRename);
diff --git a/core/workspace_events.js b/core/workspace_events.js
index de98c5896..f150eb454 100644
--- a/core/workspace_events.js
+++ b/core/workspace_events.js
@@ -14,6 +14,7 @@ goog.provide('Blockly.Events.FinishedLoading');
 
 goog.require('Blockly.Events');
 goog.require('Blockly.Events.Ui');
+goog.require('Blockly.registry');
 goog.require('Blockly.utils.object');
 
 
@@ -22,17 +23,24 @@ goog.require('Blockly.utils.object');
  * Used to notify the developer when the workspace has finished loading (i.e
  * domToWorkspace).
  * Finished loading events do not record undo or redo.
- * @param {!Blockly.Workspace} workspace The workspace that has finished
- *    loading.
- * @extends {Blockly.Events.Abstract}
+ * @param {!Blockly.Workspace=} opt_workspace The workspace that has finished
+ *    loading.  Undefined for a blank event.
+ * @extends {Blockly.Events.Ui}
  * @constructor
  */
-Blockly.Events.FinishedLoading = function(workspace) {
+Blockly.Events.FinishedLoading = function(opt_workspace) {
+
+  /**
+   * Whether or not the event is blank (to be populated by fromJson).
+   * @type {boolean}
+   */
+  this.isBlank = typeof opt_workspace == 'undefined';
+
   /**
    * The workspace identifier for this event.
    * @type {string}
    */
-  this.workspaceId = workspace.id;
+  this.workspaceId = opt_workspace ? opt_workspace.id : '';
 
   /**
    * The event group ID for the group this event belongs to. Groups define
@@ -76,6 +84,10 @@ Blockly.Events.FinishedLoading.prototype.toJson = function() {
  * @param {!Object} json JSON representation.
  */
 Blockly.Events.FinishedLoading.prototype.fromJson = function(json) {
+  this.isBlank = false;
   this.workspaceId = json['workspaceId'];
   this.group = json['group'];
 };
+
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.FINISHED_LOADING, Blockly.Events.FinishedLoading);
diff --git a/core/ws_comment_events.js b/core/ws_comment_events.js
index 6a88af290..82369fd37 100644
--- a/core/ws_comment_events.js
+++ b/core/ws_comment_events.js
@@ -18,6 +18,7 @@ goog.provide('Blockly.Events.CommentMove');
 
 goog.require('Blockly.Events');
 goog.require('Blockly.Events.Abstract');
+goog.require('Blockly.registry');
 goog.require('Blockly.utils.Coordinate');
 goog.require('Blockly.utils.object');
 goog.require('Blockly.utils.xml');
@@ -27,23 +28,30 @@ goog.require('Blockly.utils.xml');
 
 /**
  * Abstract class for a comment event.
- * @param {Blockly.WorkspaceComment} comment The comment this event corresponds
- *     to.
+ * @param {!Blockly.WorkspaceComment=} opt_comment The comment this event
+ *     corresponds to.  Undefined for a blank event.
  * @extends {Blockly.Events.Abstract}
  * @constructor
  */
-Blockly.Events.CommentBase = function(comment) {
+Blockly.Events.CommentBase = function(opt_comment) {
+
+  /**
+   * Whether or not an event is blank.
+   * @type {boolean}
+   */
+  this.isBlank = typeof opt_comment == 'undefined';
+
   /**
    * The ID of the comment this event pertains to.
    * @type {string}
    */
-  this.commentId = comment.id;
+  this.commentId = this.isBlank ? '' : opt_comment.id;
 
   /**
    * The workspace identifier for this event.
    * @type {string}
    */
-  this.workspaceId = comment.workspace.id;
+  this.workspaceId = this.isBlank ? '' : opt_comment.workspace.id;
 
   /**
    * The event group id for the group this event belongs to. Groups define
@@ -85,20 +93,24 @@ Blockly.Events.CommentBase.prototype.fromJson = function(json) {
 
 /**
  * Class for a comment change event.
- * @param {Blockly.WorkspaceComment} comment The comment that is being changed.
- *     Null for a blank event.
- * @param {string} oldContents Previous contents of the comment.
- * @param {string} newContents New contents of the comment.
+ * @param {!Blockly.WorkspaceComment=} opt_comment The comment that is being
+ *     changed.  Undefined for a blank event.
+ * @param {string=} opt_oldContents Previous contents of the comment.
+ * @param {string=} opt_newContents New contents of the comment.
  * @extends {Blockly.Events.CommentBase}
  * @constructor
  */
-Blockly.Events.CommentChange = function(comment, oldContents, newContents) {
-  if (!comment) {
+Blockly.Events.CommentChange = function(opt_comment, opt_oldContents,
+    opt_newContents) {
+  Blockly.Events.CommentChange.superClass_.constructor.call(this, opt_comment);
+  if (!opt_comment) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.CommentChange.superClass_.constructor.call(this, comment);
-  this.oldContents_ = oldContents;
-  this.newContents_ = newContents;
+
+  this.oldContents_ = typeof opt_oldContents == 'undefined' ? '' :
+      opt_oldContents;
+  this.newContents_ = typeof opt_newContents == 'undefined' ? '' :
+      opt_newContents;
 };
 Blockly.utils.object.inherits(Blockly.Events.CommentChange,
     Blockly.Events.CommentBase);
@@ -154,18 +166,18 @@ Blockly.Events.CommentChange.prototype.run = function(forward) {
 
 /**
  * Class for a comment creation event.
- * @param {Blockly.WorkspaceComment} comment The created comment.
- *     Null for a blank event.
+ * @param {!Blockly.WorkspaceComment=} opt_comment The created comment.
+ *     Undefined for a blank event.
  * @extends {Blockly.Events.CommentBase}
  * @constructor
  */
-Blockly.Events.CommentCreate = function(comment) {
-  if (!comment) {
+Blockly.Events.CommentCreate = function(opt_comment) {
+  Blockly.Events.CommentCreate.superClass_.constructor.call(this, opt_comment);
+  if (!opt_comment) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.CommentCreate.superClass_.constructor.call(this, comment);
 
-  this.xml = comment.toXmlWithXY();
+  this.xml = opt_comment.toXmlWithXY();
 };
 Blockly.utils.object.inherits(Blockly.Events.CommentCreate,
     Blockly.Events.CommentBase);
@@ -228,18 +240,18 @@ Blockly.Events.CommentCreateDeleteHelper = function(event, create) {
 };
 /**
  * Class for a comment deletion event.
- * @param {Blockly.WorkspaceComment} comment The deleted comment.
- *     Null for a blank event.
+ * @param {!Blockly.WorkspaceComment=} opt_comment The deleted comment.
+ *     Undefined for a blank event.
  * @extends {Blockly.Events.CommentBase}
  * @constructor
  */
-Blockly.Events.CommentDelete = function(comment) {
-  if (!comment) {
+Blockly.Events.CommentDelete = function(opt_comment) {
+  Blockly.Events.CommentDelete.superClass_.constructor.call(this, opt_comment);
+  if (!opt_comment) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.CommentDelete.superClass_.constructor.call(this, comment);
 
-  this.xml = comment.toXmlWithXY();
+  this.xml = opt_comment.toXmlWithXY();
 };
 Blockly.utils.object.inherits(Blockly.Events.CommentDelete,
     Blockly.Events.CommentBase);
@@ -278,29 +290,29 @@ Blockly.Events.CommentDelete.prototype.run = function(forward) {
 
 /**
  * Class for a comment move event.  Created before the move.
- * @param {Blockly.WorkspaceComment} comment The comment that is being moved.
- *     Null for a blank event.
+ * @param {!Blockly.WorkspaceComment=} opt_comment The comment that is being
+ *     moved.  Undefined for a blank event.
  * @extends {Blockly.Events.CommentBase}
  * @constructor
  */
-Blockly.Events.CommentMove = function(comment) {
-  if (!comment) {
+Blockly.Events.CommentMove = function(opt_comment) {
+  Blockly.Events.CommentMove.superClass_.constructor.call(this, opt_comment);
+  if (!opt_comment) {
     return;  // Blank event to be populated by fromJson.
   }
-  Blockly.Events.CommentMove.superClass_.constructor.call(this, comment);
 
   /**
    * The comment that is being moved.  Will be cleared after recording the new
    * location.
    * @type {Blockly.WorkspaceComment}
    */
-  this.comment_ = comment;
+  this.comment_ = opt_comment;
 
   /**
    * The location before the move, in workspace coordinates.
    * @type {!Blockly.utils.Coordinate}
    */
-  this.oldCoordinate_ = comment.getXY();
+  this.oldCoordinate_ = opt_comment.getXY();
 
   /**
    * The location after the move, in workspace coordinates.
@@ -394,3 +406,12 @@ Blockly.Events.CommentMove.prototype.run = function(forward) {
   var current = comment.getXY();
   comment.moveBy(target.x - current.x, target.y - current.y);
 };
+
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.COMMENT_CREATE, Blockly.Events.CommentCreate);
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.COMMENT_CHANGE, Blockly.Events.CommentChange);
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.COMMENT_MOVE, Blockly.Events.CommentMove);
+Blockly.registry.register(Blockly.registry.Type.EVENT,
+    Blockly.Events.COMMENT_DELETE, Blockly.Events.CommentDelete);
diff --git a/tests/mocha/event_test.js b/tests/mocha/event_test.js
index c7d0bbcd5..d5fa8576d 100644
--- a/tests/mocha/event_test.js
+++ b/tests/mocha/event_test.js
@@ -344,7 +344,7 @@ suite('Events', function() {
     suite('fromJson', function() {
       test('Var create', function() {
         var event = new Blockly.Events.VarCreate(this.variable);
-        var event2 = new Blockly.Events.VarCreate(null);
+        var event2 = new Blockly.Events.VarCreate();
         var json = event.toJson();
         event2.fromJson(json);
 
@@ -352,7 +352,7 @@ suite('Events', function() {
       });
       test('Var delete', function() {
         var event = new Blockly.Events.VarDelete(this.variable);
-        var event2 = new Blockly.Events.VarDelete(null);
+        var event2 = new Blockly.Events.VarDelete();
         var json = event.toJson();
         event2.fromJson(json);
 
@@ -360,7 +360,7 @@ suite('Events', function() {
       });
       test('Var rename', function() {
         var event = new Blockly.Events.VarRename(this.variable, '');
-        var event2 = new Blockly.Events.VarRename(null);
+        var event2 = new Blockly.Events.VarRename();
         var json = event.toJson();
         event2.fromJson(json);
 
diff --git a/tests/mocha/trashcan_test.js b/tests/mocha/trashcan_test.js
index 315ebc66e..2256afc7a 100644
--- a/tests/mocha/trashcan_test.js
+++ b/tests/mocha/trashcan_test.js
@@ -10,7 +10,7 @@ suite("Trashcan", function() {
         '<xml xmlns="https://developers.google.com/blockly/xml">' +
         xmlString + '</xml>');
     xml = xml.children[0];
-    var event = new Blockly.Events.Delete(null);
+    var event = new Blockly.Events.Delete();
     event.oldXml = xml;
     event.workspaceId = workspace.id;
     Blockly.Events.fire(event);