Bug 1796732 - Write first shot of douments of the editor module r=m_kato DONTBUILD

############################

Editor module specific rules

############################

The editor module has not been maintained aggressively about a decade. Therefore, this module needs

to be treated as a young module or in a transition period to align the behavior to the other

browsers and take modern C++ style.

Undoubtedly, this editor module is under rewritten for modern and optimized for current specs.

Additionally, this module does really complicated things which may cause security issues.

Therefore, there are specific rules:

Treat other browsers behavior as standard if the behavior is reasonable

=======================================================================

The editing behavior is not standardized since as you see too many lines in the editor classes, the

number of cases which need to handle edge cases is crazy and that makes it impossible to standardize.

Additionally, our editor behavior is not so stable. Some behaviors were aligned to Internet Explorer,

some other behaviors were not for making "better" UX for users of email composer and HTML composer

which were in SeaMonkey, and the other browser engines (Blink and WebKit) have same roots but the

behavior is different from both IE and Gecko.

Therefore, there were no reference behavior.

In these days, compatibility between browsers becomes more important, and fortunately, the behavior

of Blink (Chrome/Chromium) which has the biggest market share is more reasonable than ours in a lot

of cases. Therefore, if we get web-compat issue reports, we should align the behavior to Blink in

theory.

However, if Blink's behavior is also odd, this is the worst case. In this case, we should try to

align the behavior to WebKit if and only if WebKit's behavior is different from Blink and

reasonable, or doing something "better" for hiding the issue from web-apps and file an issue to the

Editing Working Group with creating a "tentative" web-platform test.

Don't make methods of editor classes public if they are used only by helper classes

===================================================================================

Although this is a silly rule. Of course, APIs of editor classes need to be public for the other

modules. However, the other methods which are used only by helper classes in the editor module --the

methods may be crashed if called by the other modules because editor classes store and guarantee the

colleagues (e.g., ``Selection``) when it starts to handle an edit action (edit command or

operation)--  does not want to do it for the performance reason. Therefore, such methods are now

declared as protected methods and the caller classes are registered as friends.

For solving this issue, we could split the editor classes one is exported and the other is not

exposed, and make the former to proxies and own the latter.  However, this approach might cause

performance regressions and requires a lot of line changes (at least each method definition and

warning messages at the caller sides).  Tracked in

`bug 1555916 <https://bugzilla.mozilla.org/show_bug.cgi?id=1555916>`__.

Steps to handle one editor command or operation

===============================================

One edit command or operation is called "edit action" in the editor module.  Handling it starts

when an XPCOM method or a public method which is named as ``*AsAction``. Those methods create

``AutoEditActionDataSetter`` in the stack first, then, call one of ``CanHandle()``,

``CanHandleAndMaybeDispatchBeforeInputEvent()`` or ``CanHandleAndFlushPendingNotifications()``.

If ``CanHandleAndMaybeDispatchBeforeInputEvent()`` causes dispatching ``beforeinput`` event and if

the event is consumed by the web app, it returns ``NS_ERROR_EDITOR_ACTION_CANCELED``. In this case,

the method can do anything due to the ``beforeinput`` event definition.

At this time, ``AutoEditActionDataSetter`` stores ``Selection`` etc which are required for handling

the edit action in it and set ``EditorBase::mEditActionData`` to its address. Then all methods of

editor can access the objects via the pointer (typically wrapped in inline methods) and the lifetime

of the objects are guaranteed.

Then, the methods call one or more edit-sub action handlers.  E.g., when user types a character

with a non-collapsed selection range, editor needs to delete the selected content first and insert

the character there. For implementing this behavior, "insert text" edit action handler needs to call

"delete selection" sub-action handler and "insert text" sub-action handler. The sub-edit action

handlers are named as ``*AsSubAction``.

The callers of edit sub-action handlers or the handlers themselves create ``AutoPlaceholderBatch``

in the stack. This creates a placeholder transaction to make all transactions undoable with one

"undo" command.

Then, each edit sub-action handler creates ``AutoEditSubActionNotifier`` in the stack and if it's

the topmost edit sub-action handling, ``OnStartToHandleTopLevelEditSubAction()`` is called at the

creation and ``OnEndHandlingTopLevelEditSubAction()`` is called at the destruction. The latter will

clean up the modified range, e.g., remove unnecessary empty nodes.

Finally, the edit sub-actions does something while ``AutoEditSubActionNotifier`` is alive. Helper

methods of edit sub-action handlers are typically named as ``*WithTransaction`` because they are

done with transaction classes for making everything undoable.

Don't update Selection immediately

==================================

Changing the ranges of ``Selection`` is expensive (due ot validating new range, notifying new

selected or unselected frames, notifying selection listeners, etc), and retrieving current

``Selection`` ranges at staring to handle something makes the code statefull which is harder to

debug when you investigate a bug. Therefore, each method should return new caret position or

update ranges given as in/out parameter of ``AutoRangeArray``.  ``Result<CaretPoint, nsresult>``

is a good result type for the former, and the latter is useful style if the method needs to keep

``Selection`` similar to given ranges, e.g., when paragraphs around selection are changed to

different type of blocks. Finally, edit sub-action handler methods should update ``Selection``

before destroying ``AutoEditSubActionNotifier`` whose post-processing requires ``Selection``.

Don't add new things into OnEndHandlingTopLevelEditSubAction()

==============================================================

When the topmost edit sub-action is handled, ``OnEndHandlingTopLevelEditSubAction`` is called and

it cleans up something in (or around) the modified range. However, this "post-processing" approach

makes it harder to change the behavior for fixing web-compat issues. For example, it deletes empty

nodes in the range, but if only some empty nodes are inserted intentionally, it doesn't have the

details and may unexpectedly delete necessary empty nodes.

Instead, new things should be done immediately at or after modifying the DOM tree, and if it

requires to disable the post-processing, add new ``bool`` flag to

``EditorBase::TopLevelEditSubActionData`` and when it's set, make

``OnEndHandlingTopLevelEditSubAction`` stop doing something.

Don't use NS_WARN_IF for checking NS_FAILED, isErr() and Failed()

=================================================================

The warning messages like ``NS_FAILED(rv)`` does not help except the line number, and in the cases

of that we get web-compat reports, somewhere in the editor modules may get unexpected result. For

saving the investigation time of web-compat issues, each failure should warn which method call

failed, for example:

.. code:: cpp

  nsresult rv = DoSomething();

  if (NS_FAILED(rv)) {

    NS_WARNING("HTMLEditor::DoSomething() failed");

    return rv;

  }

These warnings will let you know the stack of failure in debug build. In other words, when you

investigate a web-compat issue in editor, you should do the steps to reproduce in debug build first.

Then, you'd see failure point stack in the terminal.

Return NS_ERROR_EDITOR_DESTROYED when editor gets destroyed

===========================================================

The most critical error while an editor class method is running is what the editor instance is

destroyed by the web app. This can be checked with a call of ``EditorBase::Destroyed()`` and

if it returns ``true``, methods should return ``NS_ERROR_EDITOR_DESTROYED`` with stopping handling

anything. Then, all callers which handle the error result properly will stop handling too.

Finally, public methods should return ``EditorBase::ToGenericNSResult(rv)`` instead of exposing

an internal error of the editor module.

Note that destroying the editor is intentional thing for the web app. Thus we should not throw

exception for this failure reason. Therefore, the public methods shouldn't return error.

When you make a method return ``NS_ERROR_EDITOR_DESTROYED`` properly, you should mark the method

as ``[[nodiscard]]``. In other words, if you see ``[[nodiscard]]`` in method definition and it

returns ``nsresult`` or ``Result<*, nsresult>``, the method callers do not need to check

``Destroyed()`` by themselves.

Use reference instead of pointer as far as possible

===================================================

When you create or redesign a method, it should take references instead of pointers if they take.

This rule forces that the caller to do null-check and this avoids a maybe unexpected case like:

.. code:: cpp

  inline bool IsBRElement(const nsINode* aNode) {

    return aNode && aNode->IsHTMLElement(nsGkAtoms::br);

  }

  void DoSomethingExceptIfBRElement(const nsINode* aNode) {

    if (IsBRElement(aNode)) {

      return;

    }

    // Do something for non-BR element node.

  }

In this case, ``DoSomethingExceptIfBRElement`` expects that ``aNode`` is never ``nullptr`` but it

could be at least in build time. Using reference fixes this mistake at build time.

Use ``EditorUtils`` or ``HTMLEditUtils`` for stateless methods

==============================================================

When you create a new static method to the editor classes or a new inline method in cpp file which

defines the editor classes, please check if it's a common method which may be used from other

places in the editor module.  If it's possible to be used only in ``HTMLEditor`` or its helper

classes, the method should be in ``HTMLEditUtils``.  If it's possible be used in ``EditorBase`` or

``TextEditor`` or their helper classes, it should be in ``EditorUtils``.

Don't use bool argument

=======================

If you create a new method which take one or more ``bool`` arguments, use ``enum class`` instead

since ``true`` or ``false`` in the caller side is not easy to read. For example, you must not

be able to understand what this example mean:

.. code:: cpp

  if (IsEmpty(aNode, true)) {

For avoiding this issue, you should create new ``enum class`` for each.  E.g.,

.. code:: cpp

  if (IsEmpty(aNode, TreatSingleBR::AsVisible)) {

Basically, both ``enum class`` name and its value names explains what it means fluently. However, if

it's impossible, use ``No`` and ``Yes`` for the value like:

.. code:: cpp

  if (DoSomething(aNode, OnlyIfEmpty::Yes)) {

Don't use out parameters

========================

In most cases, editor methods meet error of low level APIs, thus editor methods usually return error

code. On the other hand, a lot of code need to return computed things, e.g., new caret position,

whether it's handled, ignored or canceled, a target node looked for, etc. We used ``nsresult`` for

the return value type and out parameters for the other results, but it makes callers scattering a

lot of auto variables and reusing them makes the code harder to understand.

Now we can use ``mozilla::Result<Foo, nsresult>`` instead.

#######################

Editor module structure

#######################

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

Introduction

============

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.

Directories

===========

composer

--------

Previously, this directory contained "Composer" UI related code.  However, currently, this

directory contains ``nsEditingSession`` and ``ComposerCommandsUpdater``.

libeditor

---------

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

spellchecker

------------

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.

txmgr

-----

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

----------

``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

----------

``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

----------

``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 <https://bugzilla.mozilla.org/show_bug.cgi?id=1710784>`__.

EditorUtils

-----------

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.

HTMLEditUtils

-------------

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

This class is not exposed.

AutoRangeArray

--------------

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.

WSRunScanner

------------

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.

WhiteSpaceVisibilityKeeper

--------------------------

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 <https://bugzilla.mozilla.org/show_bug.cgi?id=1658699>`__.

This class is not exposed.

\*Transaction

-------------

``*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``.

.. toctree::

   :maxdepth: 1

   EditorModuleStructure

   EditorModuleSpecificRules

   IMEHandlingGuide