gecko-dev/devtools/server/actors/stylesheets.js

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

const { Ci } = require("chrome");
const defer = require("devtools/shared/defer");
const protocol = require("devtools/shared/protocol");
const { LongStringActor } = require("devtools/server/actors/string");
const { fetch } = require("devtools/shared/DevToolsUtils");
const {
  mediaRuleSpec,
  styleSheetSpec,
  styleSheetsSpec,
} = require("devtools/shared/specs/stylesheets");
const InspectorUtils = require("InspectorUtils");

loader.lazyRequireGetter(
  this,
  "CssLogic",
  "devtools/shared/inspector/css-logic"
);
loader.lazyRequireGetter(
  this,
  "addPseudoClassLock",
  "devtools/server/actors/highlighters/utils/markup",
  true
);
loader.lazyRequireGetter(
  this,
  "removePseudoClassLock",
  "devtools/server/actors/highlighters/utils/markup",
  true
);
loader.lazyRequireGetter(
  this,
  "loadSheet",
  "devtools/shared/layout/utils",
  true
);
loader.lazyRequireGetter(
  this,
  "ReplayDebugger",
  "devtools/server/actors/replay/debugger"
);
loader.lazyRequireGetter(
  this,
  "ReplayInspector",
  "devtools/server/actors/replay/inspector"
);

var TRANSITION_PSEUDO_CLASS = ":-moz-styleeditor-transitioning";
var TRANSITION_DURATION_MS = 500;
var TRANSITION_BUFFER_MS = 1000;
var TRANSITION_RULE_SELECTOR = `:root${TRANSITION_PSEUDO_CLASS}, :root${TRANSITION_PSEUDO_CLASS} *`;

var TRANSITION_SHEET =
  "data:text/css;charset=utf-8," +
  encodeURIComponent(`
  ${TRANSITION_RULE_SELECTOR} {
    transition-duration: ${TRANSITION_DURATION_MS}ms !important;
    transition-delay: 0ms !important;
    transition-timing-function: ease-out !important;
    transition-property: all !important;
  }
`);

// The possible kinds of style-applied events.
// UPDATE_PRESERVING_RULES means that the update is guaranteed to
// preserve the number and order of rules on the style sheet.
// UPDATE_GENERAL covers any other kind of change to the style sheet.
const UPDATE_PRESERVING_RULES = 0;
exports.UPDATE_PRESERVING_RULES = UPDATE_PRESERVING_RULES;
const UPDATE_GENERAL = 1;
exports.UPDATE_GENERAL = UPDATE_GENERAL;

// If the user edits a style sheet, we stash a copy of the edited text
// here, keyed by the style sheet.  This way, if the tools are closed
// and then reopened, the edited text will be available.  A weak map
// is used so that navigation by the user will eventually cause the
// edited text to be collected.
const modifiedStyleSheets = new WeakMap();

/**
 * A MediaRuleActor lives on the server and provides access to properties
 * of a DOM @media rule and emits events when it changes.
 */
var MediaRuleActor = protocol.ActorClassWithSpec(mediaRuleSpec, {
  get window() {
    return this.parentActor.window;
  },

  get document() {
    return this.window.document;
  },

  get matches() {
    return this.mql ? this.mql.matches : null;
  },

  initialize: function(mediaRule, parentActor) {
    protocol.Actor.prototype.initialize.call(this, null);

    this.rawRule = mediaRule;
    this.parentActor = parentActor;
    this.conn = this.parentActor.conn;

    this._matchesChange = this._matchesChange.bind(this);

    this.line = InspectorUtils.getRuleLine(mediaRule);
    this.column = InspectorUtils.getRuleColumn(mediaRule);

    try {
      this.mql = this.window.matchMedia(mediaRule.media.mediaText);
    } catch (e) {
      // Ignored
    }

    if (this.mql) {
      this.mql.addListener(this._matchesChange);
    }
  },

  destroy: function() {
    if (this.mql) {
      this.mql.removeListener(this._matchesChange);
    }

    protocol.Actor.prototype.destroy.call(this);
  },

  form: function() {
    const form = {
      actor: this.actorID, // actorID is set when this is added to a pool
      mediaText: this.rawRule.media.mediaText,
      conditionText: this.rawRule.conditionText,
      matches: this.matches,
      line: this.line,
      column: this.column,
      parentStyleSheet: this.parentActor.actorID,
    };

    return form;
  },

  _matchesChange: function() {
    this.emit("matches-change", this.matches);
  },
});

function getSheetText(sheet, consoleActor) {
  const cssText = modifiedStyleSheets.get(sheet);
  if (cssText !== undefined) {
    return Promise.resolve(cssText);
  }

  if (!sheet.href) {
    // this is an inline <style> sheet
    const content = sheet.ownerNode.textContent;
    return Promise.resolve(content);
  }

  return fetchStylesheet(sheet, consoleActor).then(({ content }) => content);
}

exports.getSheetText = getSheetText;

/**
 * Get the charset of the stylesheet.
 */
function getCSSCharset(sheet) {
  if (sheet) {
    // charset attribute of <link> or <style> element, if it exists
    if (sheet.ownerNode && sheet.ownerNode.getAttribute) {
      const linkCharset = sheet.ownerNode.getAttribute("charset");
      if (linkCharset != null) {
        return linkCharset;
      }
    }

    // charset of referring document.
    if (sheet.ownerNode && sheet.ownerNode.ownerDocument.characterSet) {
      return sheet.ownerNode.ownerDocument.characterSet;
    }
  }

  return "UTF-8";
}

/**
 * Fetch a stylesheet at the provided URL. Returns a promise that will resolve the
 * result of the fetch command.
 *
 * @return {Promise} a promise that resolves with an object with the following members
 *         on success:
 *           - content: the document at that URL, as a string,
 *           - contentType: the content type of the document
 *         If an error occurs, the promise is rejected with that error.
 */
async function fetchStylesheet(sheet, consoleActor) {
  const href = sheet.href;

  let result;
  if (consoleActor) {
    result = await consoleActor.getRequestContentForURL(href);
    if (result) {
      return result;
    }
  }

  // When replaying, fetch the stylesheets from the replaying process, so that
  // we get the same sheets which were used when recording.
  if (isReplaying) {
    const dbg = new ReplayDebugger();
    return dbg.replayingContent(href);
  }

  const options = {
    loadFromCache: true,
    policy: Ci.nsIContentPolicy.TYPE_INTERNAL_STYLESHEET,
    charset: getCSSCharset(sheet),
  };

  // Bug 1282660 - We use the system principal to load the default internal
  // stylesheets instead of the content principal since such stylesheets
  // require system principal to load. At meanwhile, we strip the loadGroup
  // for preventing the assertion of the userContextId mismatching.

  // chrome|file|resource|moz-extension protocols rely on the system principal.
  const excludedProtocolsRe = /^(chrome|file|resource|moz-extension):\/\//;
  if (!excludedProtocolsRe.test(href)) {
    // Stylesheets using other protocols should use the content principal.
    if (sheet.ownerNode) {
      // eslint-disable-next-line mozilla/use-ownerGlobal
      options.window = sheet.ownerNode.ownerDocument.defaultView;
      options.principal = sheet.ownerNode.ownerDocument.nodePrincipal;
    }
  }

  try {
    result = await fetch(href, options);
  } catch (e) {
    // The list of excluded protocols can be missing some protocols, try to use the
    // system principal if the first fetch failed.
    console.error(
      `stylesheets actor: fetch failed for ${href},` +
        ` using system principal instead.`
    );
    options.window = undefined;
    options.principal = undefined;
    result = await fetch(href, options);
  }

  return result;
}

/**
 * A StyleSheetActor represents a stylesheet on the server.
 */
var StyleSheetActor = protocol.ActorClassWithSpec(styleSheetSpec, {
  toString: function() {
    return "[StyleSheetActor " + this.actorID + "]";
  },

  /**
   * Window of target
   */
  get window() {
    return isReplaying ? ReplayInspector.window : this.parentActor.window;
  },

  /**
   * Document of target.
   */
  get document() {
    return this.window.document;
  },

  /**
   * StyleSheet's window.
   */
  get ownerWindow() {
    // eslint-disable-next-line mozilla/use-ownerGlobal
    return this.ownerDocument.defaultView;
  },

  get ownerNode() {
    return this.rawSheet.ownerNode;
  },

  /**
   * URL of underlying stylesheet.
   */
  get href() {
    return this.rawSheet.href;
  },

  /**
   * Returns the stylesheet href or the document href if the sheet is inline.
   */
  get safeHref() {
    let href = this.href;
    if (!href) {
      if (this.ownerNode.nodeType == this.ownerNode.DOCUMENT_NODE) {
        href = this.ownerNode.location.href;
      } else if (
        this.ownerNode.ownerDocument &&
        this.ownerNode.ownerDocument.location
      ) {
        href = this.ownerNode.ownerDocument.location.href;
      }
    }
    return href;
  },

  /**
   * Retrieve the index (order) of stylesheet in the document.
   *
   * @return number
   */
  get styleSheetIndex() {
    if (this._styleSheetIndex == -1) {
      const styleSheets = InspectorUtils.getAllStyleSheets(this.document, true);
      for (let i = 0; i < styleSheets.length; i++) {
        if (styleSheets[i] == this.rawSheet) {
          this._styleSheetIndex = i;
          break;
        }
      }
    }
    return this._styleSheetIndex;
  },

  destroy: function() {
    if (this._transitionTimeout && this.window) {
      this.window.clearTimeout(this._transitionTimeout);
      removePseudoClassLock(
        this.document.documentElement,
        TRANSITION_PSEUDO_CLASS
      );
    }
  },

  initialize: function(styleSheet, parentActor) {
    protocol.Actor.prototype.initialize.call(this, null);

    this.rawSheet = styleSheet;
    this.parentActor = parentActor;
    this.conn = this.parentActor.conn;

    // text and index are unknown until source load
    this.text = null;
    this._styleSheetIndex = -1;

    // When the style is imported, `styleSheet.ownerNode` is null,
    // so retrieve the topmost parent style sheet which has an ownerNode
    let parentStyleSheet = styleSheet;
    while (parentStyleSheet.parentStyleSheet) {
      parentStyleSheet = parentStyleSheet.parentStyleSheet;
    }
    // When the style is injected via nsIDOMWindowUtils.loadSheet, even
    // the parent style sheet has no owner, so default back to target actor
    // document
    if (parentStyleSheet.ownerNode) {
      this.ownerDocument = parentStyleSheet.ownerNode.ownerDocument;
    } else {
      this.ownerDocument = parentActor.window;
    }
  },

  /**
   * Test whether this sheet has been modified by CSSOM.
   * @return {Boolean} true if changed by CSSOM.
   */
  hasRulesModifiedByCSSOM: function() {
    return InspectorUtils.hasRulesModifiedByCSSOM(this.rawSheet);
  },

  /**
   * Get the raw stylesheet's cssRules once the sheet has been loaded.
   *
   * @return {Promise}
   *         Promise that resolves with a CSSRuleList
   */
  getCSSRules: function() {
    let rules;
    try {
      rules = this.rawSheet.cssRules;
    } catch (e) {
      // sheet isn't loaded yet
    }

    if (rules) {
      return Promise.resolve(rules);
    }

    if (!this.ownerNode) {
      return Promise.resolve([]);
    }

    if (this._cssRules) {
      return this._cssRules;
    }

    const deferred = defer();

    const onSheetLoaded = event => {
      this.ownerNode.removeEventListener("load", onSheetLoaded);

      deferred.resolve(this.rawSheet.cssRules);
    };

    this.ownerNode.addEventListener("load", onSheetLoaded);

    // cache so we don't add many listeners if this is called multiple times.
    this._cssRules = deferred.promise;

    return this._cssRules;
  },

  /**
   * Get the current state of the actor
   *
   * @return {object}
   *         With properties of the underlying stylesheet, plus 'text',
   *        'styleSheetIndex' and 'parentActor' if it's @imported
   */
  form: function() {
    let docHref;
    if (this.ownerNode) {
      if (this.ownerNode.nodeType == this.ownerNode.DOCUMENT_NODE) {
        docHref = this.ownerNode.location.href;
      } else if (
        this.ownerNode.ownerDocument &&
        this.ownerNode.ownerDocument.location
      ) {
        docHref = this.ownerNode.ownerDocument.location.href;
      }
    }

    const form = {
      actor: this.actorID, // actorID is set when this actor is added to a pool
      href: this.href,
      nodeHref: docHref,
      disabled: this.rawSheet.disabled,
      title: this.rawSheet.title,
      system: !CssLogic.isAuthorStylesheet(this.rawSheet),
      styleSheetIndex: this.styleSheetIndex,
      sourceMapURL: this.rawSheet.sourceMapURL,
    };

    try {
      form.ruleCount = this.rawSheet.cssRules.length;
    } catch (e) {
      // stylesheet had an @import rule that wasn't loaded yet
      this.getCSSRules().then(() => {
        this._notifyPropertyChanged("ruleCount");
      });
    }
    return form;
  },

  /**
   * Toggle the disabled property of the style sheet
   *
   * @return {object}
   *         'disabled' - the disabled state after toggling.
   */
  toggleDisabled: function() {
    this.rawSheet.disabled = !this.rawSheet.disabled;
    this._notifyPropertyChanged("disabled");

    return this.rawSheet.disabled;
  },

  /**
   * Send an event notifying that a property of the stylesheet
   * has changed.
   *
   * @param  {string} property
   *         Name of the changed property
   */
  _notifyPropertyChanged: function(property) {
    this.emit("property-change", property, this.form()[property]);
  },

  /**
   * Protocol method to get the text of this stylesheet.
   */
  getText: function() {
    return this._getText().then(text => {
      return new LongStringActor(this.conn, text || "");
    });
  },

  /**
   * Fetch the text for this stylesheet from the cache or network. Return
   * cached text if it's already been fetched.
   *
   * @return {Promise}
   *         Promise that resolves with a string text of the stylesheet.
   */
  _getText: function() {
    if (typeof this.text === "string") {
      return Promise.resolve(this.text);
    }

    return getSheetText(this.rawSheet, this._consoleActor).then(text => {
      this.text = text;
      return text;
    });
  },

  /**
   * Try to locate the console actor if it exists via our parent actor (the tab).
   *
   * Keep this in sync with the BrowsingContextTargetActor version.
   */
  get _consoleActor() {
    if (this.parentActor.exited) {
      return null;
    }
    const form = this.parentActor.form();
    return this.conn._getOrCreateActor(form.consoleActor);
  },

  /**
   * Protocol method to get the media rules for the stylesheet.
   */
  getMediaRules: function() {
    return this._getMediaRules();
  },

  /**
   * Get all the @media rules in this stylesheet.
   *
   * @return {promise}
   *         A promise that resolves with an array of MediaRuleActors.
   */
  _getMediaRules: function() {
    return this.getCSSRules().then(rules => {
      const mediaRules = [];
      for (let i = 0; i < rules.length; i++) {
        const rule = rules[i];
        if (rule.type != CSSRule.MEDIA_RULE) {
          continue;
        }
        const actor = new MediaRuleActor(rule, this);
        this.manage(actor);

        mediaRules.push(actor);
      }
      return mediaRules;
    });
  },

  /**
   * Update the style sheet in place with new text.
   *
   * @param  {object} request
   *         'text' - new text
   *         'transition' - whether to do CSS transition for change.
   *         'kind' - either UPDATE_PRESERVING_RULES or UPDATE_GENERAL
   */
  update: function(text, transition, kind = UPDATE_GENERAL) {
    InspectorUtils.parseStyleSheet(this.rawSheet, text);

    modifiedStyleSheets.set(this.rawSheet, text);

    this.text = text;

    if (kind != UPDATE_PRESERVING_RULES) {
      this._notifyPropertyChanged("ruleCount");
    }

    if (transition) {
      this._startTransition(kind);
    } else {
      this.emit("style-applied", kind, this);
    }

    this._getMediaRules().then(rules => {
      this.emit("media-rules-changed", rules);
    });
  },

  /**
   * Insert a catch-all transition sheet into the document. Set a timeout
   * to remove the transition after a certain time.
   */
  _startTransition: function(kind) {
    if (!this._transitionSheetLoaded) {
      this._transitionSheetLoaded = true;
      // We don't remove this sheet. It uses an internal selector that
      // we only apply via locks, so there's no need to load and unload
      // it all the time.
      loadSheet(this.window, TRANSITION_SHEET);
    }

    addPseudoClassLock(this.document.documentElement, TRANSITION_PSEUDO_CLASS);

    // Set up clean up and commit after transition duration (+buffer)
    // @see _onTransitionEnd
    this.window.clearTimeout(this._transitionTimeout);
    this._transitionTimeout = this.window.setTimeout(
      this._onTransitionEnd.bind(this, kind),
      TRANSITION_DURATION_MS + TRANSITION_BUFFER_MS
    );
  },

  /**
   * This cleans up class and rule added for transition effect and then
   * notifies that the style has been applied.
   */
  _onTransitionEnd: function(kind) {
    this._transitionTimeout = null;
    removePseudoClassLock(
      this.document.documentElement,
      TRANSITION_PSEUDO_CLASS
    );
    this.emit("style-applied", kind, this);
  },
});

exports.StyleSheetActor = StyleSheetActor;

/**
 * Creates a StyleSheetsActor. StyleSheetsActor provides remote access to the
 * stylesheets of a document.
 */
var StyleSheetsActor = protocol.ActorClassWithSpec(styleSheetsSpec, {
  /**
   * The window we work with, taken from the parent actor.
   */
  get window() {
    return this.parentActor.window;
  },

  /**
   * The current content document of the window we work with.
   */
  get document() {
    return this.window.document;
  },

  form: function() {
    return { actor: this.actorID };
  },

  initialize: function(conn, targetActor) {
    protocol.Actor.prototype.initialize.call(this, null);

    this.parentActor = targetActor;

    this._onNewStyleSheetActor = this._onNewStyleSheetActor.bind(this);
    this._onSheetAdded = this._onSheetAdded.bind(this);
    this._onWindowReady = this._onWindowReady.bind(this);
    this._transitionSheetLoaded = false;

    this.parentActor.on("stylesheet-added", this._onNewStyleSheetActor);
    this.parentActor.on("window-ready", this._onWindowReady);

    // We listen for StyleSheetApplicableStateChanged rather than
    // StyleSheetAdded, because the latter will be sent before the
    // rules are ready.  Using the former (with a check to ensure that
    // the sheet is enabled) ensures that the sheet is ready before we
    // try to make an actor for it.
    this.parentActor.chromeEventHandler.addEventListener(
      "StyleSheetApplicableStateChanged",
      this._onSheetAdded,
      true
    );

    // This is used when creating a new style sheet, so that we can
    // pass the correct flag when emitting our stylesheet-added event.
    // See addStyleSheet and _onNewStyleSheetActor for more details.
    this._nextStyleSheetIsNew = false;
  },

  destroy: function() {
    for (const win of this.parentActor.windows) {
      // This flag only exists for devtools, so we are free to clear
      // it when we're done.
      win.document.styleSheetChangeEventsEnabled = false;
    }

    this.parentActor.off("stylesheet-added", this._onNewStyleSheetActor);
    this.parentActor.off("window-ready", this._onWindowReady);

    this.parentActor.chromeEventHandler.removeEventListener(
      "StyleSheetApplicableStateChanged",
      this._onSheetAdded,
      true
    );

    protocol.Actor.prototype.destroy.call(this);
  },

  /**
   * Event handler that is called when a the target actor emits window-ready.
   *
   * @param {Event} evt
   *        The triggering event.
   */
  _onWindowReady: function(evt) {
    this._addStyleSheets(evt.window);
  },

  /**
   * Event handler that is called when a the target actor emits stylesheet-added.
   *
   * @param {StyleSheetActor} actor
   *        The new style sheet actor.
   */
  _onNewStyleSheetActor: function(actor) {
    // Forward it to the client side.
    this.emit("stylesheet-added", actor, this._nextStyleSheetIsNew);
    this._nextStyleSheetIsNew = false;
  },

  /**
   * Protocol method for getting a list of StyleSheetActors representing
   * all the style sheets in this document.
   */
  async getStyleSheets() {
    let actors = [];

    const windows = isReplaying
      ? [ReplayInspector.window]
      : this.parentActor.windows;
    for (const win of windows) {
      const sheets = await this._addStyleSheets(win);
      actors = actors.concat(sheets);
    }
    return actors;
  },

  /**
   * Check if we should be showing this stylesheet.
   *
   * @param {DOMCSSStyleSheet} sheet
   *        Stylesheet we're interested in
   *
   * @return boolean
   *         Whether the stylesheet should be listed.
   */
  _shouldListSheet: function(sheet) {
    // Special case about:PreferenceStyleSheet, as it is generated on the
    // fly and the URI is not registered with the about: handler.
    // https://bugzilla.mozilla.org/show_bug.cgi?id=935803#c37
    if (
      sheet.href &&
      sheet.href.toLowerCase() == "about:preferencestylesheet"
    ) {
      return false;
    }

    return true;
  },

  /**
   * Event handler that is called when a new style sheet is added to
   * a document.  In particular,  StyleSheetApplicableStateChanged is
   * listened for, because StyleSheetAdded is sent too early, before
   * the rules are ready.
   *
   * @param {Event} evt
   *        The triggering event.
   */
  _onSheetAdded: function(evt) {
    const sheet = evt.stylesheet;
    if (this._shouldListSheet(sheet) && !this._haveAncestorWithSameURL(sheet)) {
      this.parentActor.createStyleSheetActor(sheet);
    }
  },

  /**
   * Add all the stylesheets for the document in this window to the map and
   * create an actor for each one if not already created.
   *
   * @param {Window} win
   *        Window for which to add stylesheets
   *
   * @return {Promise}
   *         Promise that resolves to an array of StyleSheetActors
   */
  _addStyleSheets: function(win) {
    return async function() {
      const doc = win.document;
      // We have to set this flag in order to get the
      // StyleSheetApplicableStateChanged events.  See Document.webidl.
      doc.styleSheetChangeEventsEnabled = true;

      const documentOnly = !doc.nodePrincipal.isSystemPrincipal;
      const styleSheets = InspectorUtils.getAllStyleSheets(doc, documentOnly);

      let actors = [];
      for (let i = 0; i < styleSheets.length; i++) {
        const sheet = styleSheets[i];
        if (!this._shouldListSheet(sheet)) {
          continue;
        }

        const actor = this.parentActor.createStyleSheetActor(sheet);
        actors.push(actor);

        // Get all sheets, including imported ones
        const imports = await this._getImported(doc, actor);
        actors = actors.concat(imports);
      }
      return actors;
    }.bind(this)();
  },

  /**
   * Get all the stylesheets @imported from a stylesheet.
   *
   * @param  {Document} doc
   *         The document including the stylesheet
   * @param  {DOMStyleSheet} styleSheet
   *         Style sheet to search
   * @return {Promise}
   *         A promise that resolves with an array of StyleSheetActors
   */
  _getImported: function(doc, styleSheet) {
    return async function() {
      const rules = await styleSheet.getCSSRules();
      let imported = [];

      for (let i = 0; i < rules.length; i++) {
        const rule = rules[i];
        if (rule.type == CSSRule.IMPORT_RULE) {
          // With the Gecko style system, the associated styleSheet may be null
          // if it has already been seen because an import cycle for the same
          // URL.  With Stylo, the styleSheet will exist (which is correct per
          // the latest CSSOM spec), so we also need to check ancestors for the
          // same URL to avoid cycles.
          const sheet = rule.styleSheet;
          if (
            !sheet ||
            this._haveAncestorWithSameURL(sheet) ||
            !this._shouldListSheet(sheet)
          ) {
            continue;
          }
          const actor = this.parentActor.createStyleSheetActor(rule.styleSheet);
          imported.push(actor);

          // recurse imports in this stylesheet as well
          const children = await this._getImported(doc, actor);
          imported = imported.concat(children);
        } else if (rule.type != CSSRule.CHARSET_RULE) {
          // @import rules must precede all others except @charset
          break;
        }
      }

      return imported;
    }.bind(this)();
  },

  /**
   * Check all ancestors to see if this sheet's URL matches theirs as a way to
   * detect an import cycle.
   *
   * @param {DOMStyleSheet} sheet
   */
  _haveAncestorWithSameURL(sheet) {
    const sheetHref = sheet.href;
    while (sheet.parentStyleSheet) {
      if (sheet.parentStyleSheet.href == sheetHref) {
        return true;
      }
      sheet = sheet.parentStyleSheet;
    }
    return false;
  },

  /**
   * Create a new style sheet in the document with the given text.
   * Return an actor for it.
   *
   * @param  {object} request
   *         Debugging protocol request object, with 'text property'
   * @return {object}
   *         Object with 'styelSheet' property for form on new actor.
   */
  addStyleSheet: function(text) {
    // This is a bit convoluted.  The style sheet actor may be created
    // by a notification from platform.  In this case, we can't easily
    // pass the "new" flag through to createStyleSheetActor, so we set
    // a flag locally and check it before sending an event to the
    // client.  See |_onNewStyleSheetActor|.
    this._nextStyleSheetIsNew = true;

    const parent = this.document.documentElement;
    const style = this.document.createElementNS(
      "http://www.w3.org/1999/xhtml",
      "style"
    );
    style.setAttribute("type", "text/css");

    if (text) {
      style.appendChild(this.document.createTextNode(text));
    }
    parent.appendChild(style);

    const actor = this.parentActor.createStyleSheetActor(style.sheet);
    return actor;
  },
});

exports.StyleSheetsActor = StyleSheetsActor;