XPCOMUtils.jsm

The XPCOMUtils.jsm JavaScript code module offers utility routines for JavaScript components loaded by the JavaScript component loader.

To use this, you first need to import the code module into your JavaScript scope:

Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");

Using XPCOMUtils

Exposing a JavaScript class as a component using these utility methods requires four key steps:

1. Import XPCOMUtils.jsm, as explained previously.
2. Declare the class (or multiple classes) implementing the component(s).
3. Create an array of component constructors.
4. Define the NSGetFactory() or NSGetModule() entry point.

Pseudocode

This section provides some pseudocode that demonstrates how to put together a JavaScript class based on the steps listed above.

Constructor

The constructor is a simple method that handles any required initialization tasks.

function MyComponent() {
  // initialize the component here
} 

Class declaration

Declare the class prototype, using a form similar to this.

MyComponent.prototype = {
  // properties required for XPCOM registration:
  classDescription: "unique text description",
  classID:          Components.ID("{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}"),
  contractID:       "@example.com/xxx;1",

  // [optional] custom factory (an object implementing nsIFactory). If not
  // provided, the default factory is used, which returns
  // |(new MyComponent()).QueryInterface(iid)| in its createInstance().
  _xpcom_factory: { ... },

  // [optional] an array of categories to register this component in.
  _xpcom_categories: [{
    // Each object in the array specifies the parameters to pass to
    // nsICategoryManager.addCategoryEntry(). 'true' is passed for
    // both aPersist and aReplace params.
    category: "some-category",
    // optional, defaults to the object's classDescription
    entry: "entry name",
    // optional, defaults to the object's contractID (unless
    // 'service' is specified)
    value: "...",
    // optional, defaults to false. When set to true, and only if 'value'
    // is not specified, the concatenation of the string "service," and the
    // object's contractID is passed as aValue parameter of addCategoryEntry.
    service: true,
    // optional array of applications' IDs in the form:
    // [ "{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}", ... ]
    // If this is defined, the component is registered in this
    // category only on the specified applications.
    apps: [ ... ]
  }],

  // QueryInterface implementation, e.g. using the generateQI helper
  QueryInterface: XPCOMUtils.generateQI(
    [Components.interfaces.nsIObserver,
     Components.interfaces.nsIMyInterface,
     "nsIFoo",
     "nsIBar" ]),

  // [optional] classInfo implementation, e.g. using the generateCI helper.
  // Will be automatically returned from QueryInterface if that was
  // generated with the generateQI helper.
  classInfo: XPCOMUtils.generateCI(
    {classID: Components.ID("{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}"),
     contractID: "@example.com/xxx;1",
     classDescription: "unique text description",
     interfaces: [Components.interfaces.nsIObserver,
                  Components.interfaces.nsIMyInterface,
                  "nsIFoo",
                  "nsIBar"],
     flags: Ci.nsIClassInfo.SINGLETON}),

  // ...component implementation...
};

Notice that the QueryInterface() method implemented by the component simply calls the generateQI() method provided by the XPCOMUtils code module.

Create an array of component constructors

You need to create an array that lists the constructors for each component. This array can of course have just one entry:

var components = [MyComponent];

Here, we're calling the array components.

Create the NSGetFactory() or NSGetModule() entry point

Finally, you need to implement the NSGetModule() entry point so Gecko can start up your component:

// "components" is the array created in the previous section
if ("generateNSGetFactory" in XPCOMUtils)
  var NSGetFactory = XPCOMUtils.generateNSGetFactory(components);  // Gecko 2.0+
else
  var NSGetModule = XPCOMUtils.generateNSGetModule(components);    // Gecko 1.9.x

Method overview

Attributes

Methods

Getter functions in JavaScript give you a way to define a property of an object, but not calculate the property's value until it is accessed. A getter defers the cost of calculating the value until the value is needed, and if it is never needed, you never pay the cost.

A "lazy getter" provides an additional optimization: the value is calculated the first time the getter is called, and is then cached (or memoized), so subsequent accesses return the cached value without recalculating it.

This means that you shouldn't use a lazy getter for a property whose value you expect to change, because the getter will not recalculate the value.

defineLazyGetter takes three arguments:

• the object to define the property on
• the name of the property defined
• the getter function itself, which returns the value and which will be called just once, the first time code tries to access the property.

Example for this is seen at bottom of this page here.

function defineLazyGetter(
  aObject,
  aName,
  aLambda
); 

Parameters

aObject

The object into which to add the new lazy getter function.

aName

The name of the getter function to create.

aLambda

A function that returns the value the getter should return. This function is called exactly once.  this will reference aObject during execution of the function.

Defines a getter on a specified object for a module. The module will not be imported until first use.

function defineLazyModuleGetter(
  aObject,
  aName,
  aResource,
  aSymbol
); 

Parameters

aObject

The object to define the lazy getter on.

aName

The name of the getter to define on aObject for the module.

aResource

The URL used to obtain the module.

aSymbol

The name of the symbol exported by the module. This parameter is optional and defaults to aName.

Defines a function on a specified object which acts as a getter for a service. The service isn't obtained until the first time it's used.

function defineLazyServiceGetter(
  aObject,
  aName,
  aContract,
  aInterfaceName
);

Parameters

aObject

The object into which to add the new lazy service getter function.

aName

The name of the getter function to create.

aContract

The contract to use to obtain the service.

aInterfaceName

The name of the interface to query the service to.

Generates a module implementation.

nsIModule generateModule(
  componentsArray,
  postRegister,
  preUnregister
); 

Parameters

componentsArray

An array of component constructors.

postRegister

An optional function that is called after the module is registered. See Post-registration callback.

preUnregister

An optional function that's called before the module is unregistered. See Pre-unregistration callback.

Return value

An nsIModule that implements the module.

Generates the NSGetFactory() function along with the factory definition.

Function generateNSGetFactory(
  componentsArray
); 

Parameters

componentsArray

An array of component constructors.

Return value

A function that will return the factory for the components and can be assigned to NSGetFactory global variable.

Generates the NSGetModule() function along with the module definition. You can use this instead of generateModule() to create both the module and the NSGetModule() function at the same time.

nsIModule generateNSGetModule(
  componentsArray,
  postRegister,
  preUnregister
); 

Parameters

componentsArray

An array of component constructors.

postRegister

An optional function that is called after the module is registered. See Post-registration callback.

preUnregister

An optional function that's called before the module is unregistered. See Pre-unregistration callback.

Return value

An nsIModule that implements the module.

Generates an nsIClassInfo implementation for a component. The returned object should be assigned to the classInfo property of a JS object, the QueryInterface() function generated by generateQI will return it automatically then.

function generateCI(
  classInfo
); 

Parameters

classInfo

An object containing the optional properties interfaces, contractID, classDescription, classID, flags. This parameter should not be the component itself because that would cause a memory leak.

Return value

An nsIClassInfo implementation returning the values of the properties from the classInfo parameter in its various properties.

Exceptions thrown

This method throws an exception with the message "In generateCI, don't use a component for generating classInfo" if classInfo parameter is an XPCOM component.

generateQI()

Generates a QueryInterface() function implementation. You need to assign the returned function to the QueryInterface property of a JavaScript object.

When the generated method is invoked on that object, it checks to see if the specified IID is listed in the array specified by the interfaces parameter; if it is, this (that is, the object itself) is returned. Otherwise, null is returned.

function generateQI(
  interfaces
); 

Parameters

interfaces

An array of interfaces implemented by the component.

Return value

A QueryInterface() function implementation.

Remarks

When you implement an interface that inherits from another one, you should generally list all the base interfaces explicitly, except for nsISupports. For example, if your component implements nsIStreamConverter:

MyComponent.prototype = {
  QueryInterface: XPCOMUtils.generateQI([
    Components.interfaces.nsIRequestObserver,
    Components.interfaces.nsIStreamListener,
    Components.interfaces.nsIStreamConverter,
  ]),

  // ...methods...
}

Imports a JavaScript code module given the calling JavaScript code module's global object (you should specify this) and a path relative to that module. This lets modules bundled in a package load one another without having to hard-code full paths.

void importRelative(
  that,
  path,
  scope
); 

Parameters

that

The JavaScript Object of the calling JavaScript code module's global scope. You should simply pass this.

path

The relative path of the JavaScript code module to import.

scope

An optional object to import into; if omitted, the object passed in for the that parameter is used.

Remarks

This lets an extension bundle its own JavaScript code modules within itself and have them load one another. For example, if an extension named "MyExtension" bundles foo.jsm and bar.jsm, and foo.jsm needs to load bar.jsm, it can do so like this:

Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
XPCOMUtils.importRelative(this, "bar.jsm"); 

In other words: importRelative will only work from other code modules (such as JSM files). It will NOT work from overlay scripts or bootstrap.js or etc. Details can be found here: bug 628669

Wraps an nsISimpleEnumerator instance into a JavaScript generator that can be easily iterated over.

generator IterSimpleEnumerator(
  enumerator,
  interface
); 

Parameters

enumerator

The nsISimpleEnumerator instance to iterate over.

interface

The expected interface for each element.

Return value

A generator yielding enumerated objects.

Example

const nsIFile = Components.interfaces.nsIFile;
for (var file in XPCOMUtils.IterSimpleEnumerator(dir.directoryEntries, nsIFile))
  console.log(file.path);

Wraps an nsIStringEnumerator or nsIUTF8StringEnumerator instance into a JavaScript generator that can be easily iterated over.

generator IterStringEnumerator(
  enumerator
); 

Parameters

enumerator

The nsIStringEnumerator or nsIUTF8StringEnumerator instance to iterate over.

Return value

A generator yielding enumerated strings.

Example

for (var section in XPCOMUtils.IterStringEnumerator(iniParser.getSections()))
  console.log(section);

Post-registration callback

The post-registration callback called by generateModule() should have the following signature:

postRegister(
  nsIComponentManager compMgr,
  nsIFile fileSpec,
  componentsArray
);

Parameters

compMgr

An nsIComponentManager instance to use for managing the component.

fileSpec

An nsIFile instance for... what?

componentsArray

An array of the components, as passed to generateModule().

The function doesn't need to return a value.

Pre-unregistration callback

The pre-unregistration callback passed to generateModule() should have the following signature:

preUnregister(
  nsIComponentManager compMgr,
  nsIFile fileSpec,
  componentsArray
); 

Parameters

compMgr

The nsIComponentManager instance to use for managing the component.

fileSpec

An nsIFile instance for... what?

componentsArray

The array of components passed to generateModule().

This function doesn't need to return a value.

Examples

defineLazyGetter

var myServices = {};
Cu.import('resource://gre/modules/XPCOMUtils.jsm');

//set it up
XPCOMUtils.defineLazyGetter(myServices, 'as', function () { return Cc['@mozilla.org/alerts-service;1'].getService(Ci.nsIAlertsService) });

//when you need to use it
myServices.as.showAlertNotification('chrome://branding/content/icon64.png', 'this was lazyloaded', 'this is a notification from myServices.as', null, null);

See also

• Using JavaScript code modules
• JavaScript code modules
• XPCOMUtils.jsm source on DXR