Tutorial for Writing a New XPCOM Interface

High Level Overview

In order to write code that works in native code (C++, Rust), and JavaScript contexts, it’s necessary to have a mechanism to do so. For chrome privileged contexts, this is the XPCOM Interface Class.

This mechanism starts with an XPIDL file to define the shape of the interface. In the build system, this file is processed, and Rust and C++ code is automatically generated.

Next, the interface’s methods and attributes must be implemented. This can be done through either a JSM module, or through a C++ interface class. Once these steps are done, the new files must be added to the appropriate moz.build files to ensure the build system knows how to find them and process them.

Often these XPCOM components are wired into the Services JavaScript object to allow for ergonomic access to the interface. For example, open the Browser Console and type Services. to interactively access these components.

From C++, components can be accessed via mozilla::components::ComponentName::Create() using the name option in the components.conf.

While Services and mozilla::components are the preferred means of accessing components, many are accessed through the historical (and somewhat arcane) createInstance mechanism. New usage of these mechanisms should be avoided if possible.

let component = Cc["@mozilla.org/component-name;1"].createInstance(
  Ci.nsIComponentName
);
nsCOMPtr<nsIComponentName> component = do_CreateInstance(
  "@mozilla.org/component-name;1");

Writing an XPIDL

First decide on a name. Conventionally the interfaces are prefixed with nsI (historically Netscape) or mozI as they are defined in the global namespace. While the interface is global, the implementation of an interface can be defined in a namespace with no prefix. Historically many component implementations still use the ns prefixes (notice that the I was dropped), but this convention is no longer needed.

This tutorial assumes the component is located at path/to with the name ComponentName. The interface name will be nsIComponentName, while the implementation will be mozilla::ComponentName.

To start, create an XPIDL file:

touch path/to/nsIComponentName.idl

And hook it up to the path/to/moz.build

XPIDL_SOURCES += [
    "nsIComponentName.idl",
]

Next write the initial .idl file: path/to/nsIComponentName.idl

/* 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/. */

// This is the base include which defines nsISupports. This class defines
// the QueryInterface method.
#include "nsISupports.idl"

// `scriptable` designates that this object will be used with JavaScript
// `uuid`       The example below uses a UUID with all Xs. Replace the Xs with
//              your own UUID generated here:
//              http://mozilla.pettay.fi/cgi-bin/mozuuid.pl

/**
 * Make sure to document your interface.
 */
[scriptable, uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)]
interface nsIComponentName : nsISupports {

  // Fill out your definition here. This example attribute only returns a bool.

  /**
   * Make sure to document your attributes.
   */
  readonly attribute bool isAlive;
};

This definition only includes one attribute, isAlive, which will demonstrate that we’ve done our work correctly at the end. For a more comprehensive guide for this syntax, see the XPIDL docs.

Once ./mach build is run, the XPIDL parser will read this file, and give any warnings if the syntax is wrong. It will then auto-generate the C++ (or Rust) code for us. For this example the generated nsIComponentName class will be located in:

{obj-directory}/dist/include/nsIComponentName.h

It might be useful to check out what was automatically generated here, or see the existing generated C++ header files on SearchFox.

Writing the C++ implementation

Now we have a definition for an interface, but no implementation. The interface could be backed by a JavaScript implementation using a JSM, but for this example we’ll use a C++ implementation.

Add the C++ sources to path/to/moz.build

EXPORTS.mozilla += [
    "ComponentName.h",
]

UNIFIED_SOURCES += [
    "ComponentName.cpp",
]

Now write the header: path/to/ComponentName.h

/* 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/. */
#ifndef mozilla_nsComponentName_h__
#define mozilla_nsComponentName_h__

// This will pull in the header auto-generated by the .idl file:
// {obj-directory}/dist/include/nsIComponentName.h
#include "nsIComponentName.h"

// The implementation can be namespaced, while the XPCOM interface is globally namespaced.
namespace mozilla {

// Notice how the class name does not need to be prefixed, as it is defined in the
// `mozilla` namespace.
class ComponentName final : public nsIComponentName {
  // This first macro includes the necessary information to use the base nsISupports.
  // This includes the QueryInterface method.
  NS_DECL_ISUPPORTS

  // This second macro includes the declarations for the attributes. There is
  // no need to duplicate these declarations.
  //
  // In our case it includes a declaration for the isAlive attribue:
  //   GetIsAlive(bool *aIsAlive)
  NS_DECL_NSICOMPONENTNAME

 public:
  ComponentName() = default;

 private:
  // A private destructor must be declared.
  ~ComponentName() = default;
};

}  // namespace mozilla

#endif

Now write the definitions: path/to/ComponentName.cpp

/* 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/. */

#include "ComponentName.h"

namespace mozilla {

// Use the macro to inject all of the definitions for nsISupports.
NS_IMPL_ISUPPORTS(ComponentName, nsIComponentName)

// This is the actual implementation of the `isAlive` attribute. Note that the
// method name is somewhat different than the attribute. We specified "read-only"
// in the attribute, so only a getter, not a setter was defined for us. Here
// the name was adjusted to be `GetIsAlive`.
//
// Another common detail of implementing an XPIDL interface is that the return values
// are passed as out parameters. The methods are treated as fallible, and the return
// value is an `nsresult`. See the XPIDL documentation for the full nitty gritty
// details.
//
// A common way to know the exact function signature for a method implementation is
// to copy and paste from existing examples, or inspecting the generated file
// directly: {obj-directory}/dist/include/nsIComponentName.h
NS_IMETHODIMP
ComponentName::GetIsAlive(bool* aIsAlive) {
  *aIsAlive = true;
  return NS_OK;
}

} // namespace: mozilla

Registering the component

At this point, the component should be correctly written, but it’s not registered with the component system. In order to this, we’ll need to create or modify the components.conf.

touch path/to/components.conf

Now update the moz.build to point to it.

XPCOM_MANIFESTS += [
    "components.conf",
]

It is probably worth reading over Defining XPCOM C++-implemented Components, but the following config will be sufficient to hook up our component to the Services object.

Classes = [
    {
        # This CID is the ID for component entries, and needs a separate UUID from
        # the .idl file. Replace the Xs with a uuid from:
        # http://mozilla.pettay.fi/cgi-bin/mozuuid.pl
        'cid': '{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}',
        'interfaces': ['nsIComponentName'],

        # A contract ID is a human-readable identifier for an _implementation_ of
        # an XPCOM interface.
        #
        # "@mozilla.org/process/environment;1"
        #  ^^^^^^^^^^^^ ^^^^^^^ ^^^^^^^^^^^ ^
        #  |            |       |           |
        #  |            |       |           The version number, usually just 1.
        #  |            |       Component name
        #  |            Module
        #  Domain
        #
        # This design goes back to a time when XPCOM was intended to be a generalized
        # solution for the Gecko Runtime Environment (GRE). At this point most (if
        # not all) of mozilla-central has an @mozilla domain.
        'contract_ids': ['@mozilla.org/component-name;1'],

        # This is the name of the C++ type that implements the interface.
        'type': 'mozilla::ComponentName',

        # The header file to pull in for the implementation of the interface.
        'headers': ['path/to/ComponentName.h'],

        # In order to hook up this interface to the `Services` object, we can
        # provide the "js_name" parameter. This is an ergonomic way to access
        # the component.
        'js_name': 'componentName',
    },
]

At this point the full moz.build file should look like:

# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
# vim: set filetype=python:
# 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/.

XPIDL_SOURCES += [
    "nsIComponentName.idl",
]

XPCOM_MANIFESTS += [
    "components.conf",
]

EXPORTS.mozilla += [
    "ComponentName.h",
]

UNIFIED_SOURCES += [
    "ComponentName.cpp",
]

This completes the implementation of a basic XPCOM Interface using C++. The component should be available via the Browser Console or other chrome contexts.

console.log(Services.componentName.isAlive);
> true