Editor module structure

This document explains the structure of the editor module and overview of classes.


This module implements the builtin editors of editable elements or documents, and this does not implement the interface with DOM API and visual feedback of the editing UI. In other words, this module implements DOM tree editors.



Previously, this directory contained “Composer” UI related code. However, currently, this directory contains nsEditingSession and ComposerCommandsUpdater.


This is the main directory which contains “core” implementation of editors.


Despite of the directory name, implementation of the spellchecker is not here. This directory contains only a bridge between editor classes and the spellchecker and serialized text of editable content for spellchecking.


This directory contains transaction items and transaction classes. They were designed for generic use cases, e.g., managing undo/redo of bookmarks/history of browser, etc, but they are used only by the editor.

Main classes


EditorBase class is an abstract class of editors. This inherits nsIEditor XPCOM interface, implement common features which work with instance of classes, and exposed by mozilla/EditorBase.h.


TextEditor class is the implementation of plaintext editor which works with <input> and <textarea>. Its exposed root is the host HTML elements, however, the editable root is an anonymous <div> created in a native anonymous subtree under the exposed root elements. This creates a Text node as the first child of the anonymous <div> and modify its data. If the text data ends with a line-break, i.e., the last line is empty, append a <br> element for making the empty last line visible.

This also implements password editor. It works almost same as normal text editor, but each character may be masked by masked character such as “●” or “*” by the layout module for the privacy. Therefore, this manages masked/unmasked range of password and maybe making typed character automatically after a while for mobile devices.

This is exposed with mozilla/TextEditor.h.

Selection in TextEditor

Independent Selection and nsFrameSelection per <input> or <textarea>.

Lifetime of TextEditor

Created when an editable <textarea> is created or a text-editable <input> element gets focus. Note that the initialization may run asynchronously if it’s requested when it’s not safe to run script. Destroyed when the element becomes invisible. Note that TextEditor is recreated when every reframe of the host element. This means that when the size of <input> or <textarea> is changed for example, TextEditor is recreated and forget undo/redo transactions, but takes over the value, selection ranges and composition of IME from the previous instance.


HTMLEditor class is the implementation of rich text editor which works with contenteditable, Document.designMode and XUL <editor>. Its instance is created per document even if the document has multiple elements having contenteditable attribute. Therefore, undo/redo transactions are shared in all editable regions.

This is exposed with mozilla/HTMLEditor.h.

Selection in HTMLEditor

The instance for the Document and Window. When an editable element gets focus, HTMLEditor sets the ancestor limit of Selection to the focused element or the <body> of the Document. Then, Selection cannot cross boundary of the limiter element.

Lifetime of HTMLEditor

Created when first editable region is created in the Document. Destroyed when last editable region becomes non-editable.

Currently, even while HTMLEditor is handling an edit command/operation (called edit action in editor classes), each DOM mutation can be tracked with legacy DOM mutation events synchronously. Thus, after changing the DOM tree from HTMLEditor, any state could occur, e.g., the editor itself may have been destroyed, the DOM tree have been modified, the Selection have been modified, etc. This issue is tracked in bug 1710784.


This class has only static utility methods which are used by EditorBase or TextEditor and may be used by HTMLEditor too. I.e., the utility methods which are used not only by HTMLEditor should be implemented in this class.

Typically, sateless methods should be implemented as static methods of utility classes because editor classes have too many methods and fields.

This class is not exposed.


This class has only static utility methods which are used only by HTMLEditor.

This class is not exposed.


This class is a stack only class and intended to copy of normal selection ranges. In the new code, Selection shouldn’t be referred directly, instead, methods should take reference to this instance and modify it. Finally, root caller should apply the ranges to Selection. Then, HTMLEditor does not need to take care of unexpected Selection updates by legacy DOM mutation event listeners.

This class is not exposed.

EditorDOMPoint, EditorRawDOMPoint, EditorDOMPointInText, EditorRawDOMPointInText

It represents a point in a DOM tree with one of the following:

  • Container node and offset in it

  • Container node and child node in it

  • Container node and both offset and child node in it

In most cases, instances are initialized with a container and only offset or child node. Then, when Offset() or GetChild() is called, the last one is “fixed”. After inserting new child node before the offset and/or the child node, IsSetAndValid() will return false since the child node is not the child at the offset.

If you want to keep using after modifying the DOM tree, you can make the instance forget offset or child node with AutoEditorDOMPointChildInvalidator and AutoEditorDOMRangeChildrenInvalidator. The reason why the forgetting methods are not simply exposed is, Offset() and GetChild() are available even after the DOM tree is modified to get the cached offset and child node, additionally, which method may modify the DOM tree may be not clear for developers. Therefore, creating a block only for these helper classes makes the updating point clearer.

These classes are exposed with mozilla/EditorDOMPoint.h.

EditorDOMRange, EditorRawDOMRange, EditorDOMRangeInTexts, EditorRawDOMRangeInTexts

It represents 2 points in a DOM tree with 2 Editor*DOMPoint(InText). Different from nsRange, the instances do not track the DOM tree changes. Therefore, the initialization is much faster than nsRange and can be in the stack.

These classes are exposed with mozilla/EditorDOMPoint.h.

AutoTrackDOMPoint, AutoTrackDOMRange

These methods updates Editor*DOMPoint(InText) or Editor*DOMRange(InTexts) at destruction with applying the changes caused by the editor instance. In other words, they don’t track the DOM tree changes by the web apps like changes from legacy DOM mutation event listeners.

These classes are currently exposed with mozilla/SelectionState.h, but we should stop exposing them.


A helper class of HTMLEditor. This class scans previous or (inclusive) next visible thing from a DOM point or a DOM node. This is typically useful for considering whether a <br> is visible or invisible due to near a block element boundary, finding nearest editable character from caret position, etc. However, the running cost is not cheap, thus if you find another way to consider it simpler, use it instead, and also this does not check the actual style of the nodes (visible vs. invisible, block vs. inline), thus you’d get unexpected result in tricky cases.

This class is not exposed.


A helper class of HTMLEditor to handle collapsible white-spaces as what user expected. This class currently handles white-space normalization (e.g., when user inputs multiple collapsible white-spaces, this replaces some of them to NBSPs), but the behavior is different from the other browsers. We should re-implement this with emulating the other browsers’ behavior as far as possible, but currently it’s put off due to not affecting UX (tracked in bug 1658699.

This class is not exposed.


*Transaction classes represents a small transaction of updating the DOM tree and implements “do”, “undo” and “redo” of the update.

Note that each class instance is created too many (one edit action may cause multiple transactions). Therefore, each instance must be smaller as far as possible, and if you have an idea to collapse multiple instances to one instance, you should fix it. Then, users can run Firefox with smaller memory devices especially if the transaction is used in TextEditor.