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.. _writing_xpcom_interface:
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 :ref:`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 :code:`moz.build` files to ensure the build system knows how to find them and process them.
Often these XPCOM components are wired into the :code:`Services` JavaScript object to allow for ergonomic access to the interface. For example, open the `Browser Console`_ and type :code:`Services.` to interactively access these components.
From C++, components can be accessed via :code:`mozilla::components::ComponentName::Create()` using the :code:`name` option in the :code:`components.conf`.
While :code:`Services` and :code:`mozilla::components` are the preferred means of accessing components, many are accessed through the historical (and somewhat arcane) :code:`createInstance` mechanism. New usage of these mechanisms should be avoided if possible.
.. code:: javascript
let component = Cc["@mozilla.org/component-name;1"].createInstance(
Ci.nsIComponentName
);
.. code:: c++
nsCOMPtr<nsIComponentName> component = do_CreateInstance(
"@mozilla.org/component-name;1");
Writing an XPIDL
----------------
First decide on a name. Conventionally the interfaces are prefixed with :code:`nsI` (historically Netscape) or :code:`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 :code:`ns` prefixes (notice that the :code:`I` was dropped), but this convention is no longer needed.
This tutorial assumes the component is located at :code:`path/to` with the name :code:`ComponentName`. The interface name will be :code:`nsIComponentName`, while the implementation will be :code:`mozilla::ComponentName`.
To start, create an :ref:`XPIDL` file:
.. code:: bash
touch path/to/nsIComponentName.idl
And hook it up to the :code:`path/to/moz.build`
.. code:: python
XPIDL_SOURCES += [
"nsIComponentName.idl",
]
Next write the initial :code:`.idl` file: :code:`path/to/nsIComponentName.idl`
.. _contract_ids:
.. code:: c++
/* 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
// 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:
/**
* 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, :code:`isAlive`, which will demonstrate that we've done our work correctly at the end. For a more comprehensive guide for this syntax, see the :ref:`XPIDL` docs.
Once :code:`./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 :code:`nsIComponentName` class will be located in:
:code:`{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 <https://searchfox.org/mozilla-central/source/__GENERATED__/dist/>`_.
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 :code:`path/to/moz.build`
.. code:: python
EXPORTS.mozilla += [
"ComponentName.h",
]
UNIFIED_SOURCES += [
"ComponentName.cpp",
]
Now write the header: :code:`path/to/ComponentName.h`
.. code:: c++
/* 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
#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 attribute:
// 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: :code:`path/to/ComponentName.cpp`
.. code:: c++
/* 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
#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 :code:`components.conf`.
.. code:: bash
touch path/to/components.conf
Now update the :code:`moz.build` to point to it.
.. code:: python
XPCOM_MANIFESTS += [
"components.conf",
]
It is probably worth reading over :ref:`defining_xpcom_components`, but the following config will be sufficient to hook up our component to the :code:`Services` object.
Services should also be added to ``tools/lint/eslint/eslint-plugin-mozilla/lib/services.json``.
The easiest way to do that is to copy from ``<objdir>/xpcom/components/services.json``.
.. code:: python
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:
'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 :code:`moz.build` file should look like:
.. code:: python
# -*- 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
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.
.. code:: javascript
console.log(Services.componentName.isAlive);
> true