Using XPCOM without chrome

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We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

Don't use these techniques to develop new add-ons. Use WebExtensions instead.

If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.


Using sdk XPCOM

With the low-level module sdk/platform/xpcom , it's possible to exclude chrome and XPCOMUtils in some cases.
Below is an example for the XPCOM bookmark observer.


Bookmarks Observer

Normally, a bookmark observer would require Chrome Components and XPCOMUtils as described in the following links:

However, with the sdk module , we can remove this need.
Below is an example, where we extend the xpcom module's Unknown class with an nsINavBookmarkObserverinterface and one of its optional interface methods (onItemchanged). We can then create an object from it and add it as an observer to the bookmarks service.

// This removes the need to import Ci and the XPCOMUtils
const { Class }       = require("sdk/core/heritage");
const { Unknown }     = require('sdk/platform/xpcom');
const { PlacesUtils } = require("resource://gre/modules/PlacesUtils.jsm");

let bmListener = Class({
 extends: Unknown,
 interfaces: [ "nsINavBookmarkObserver" ],
 //This event most often handles all events
 onItemChanged: function(bId, prop, an, nV, lM, type, parentId, aGUID, aParentGUID) {
  console.log("onItemChanged", "bId: "+bId, "property: "+prop, "isAnno: "+an, "new value: "+nV, "lastMod: "+lM, "type: "+type, "parentId:"+parentId, "aGUID:"+aGUID);0

  // Code to handle the event here


//We just have a class, but need an object. Notice the small l
var bmlistener = bmListener();
PlacesUtils.bookmarks.addObserver(bmlistener, false);


Document Tags and Contributors

 Contributors to this page: wbamberg, PushpitaPikuDey, e-motiv
 Last updated by: wbamberg,