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+.. _defining_xpcom_components:
+
+=========================================
+Defining XPCOM C++-implemented Components
+=========================================
+
+This document explains how to write a :code:`components.conf` file. For
+documentation on the idl format see :ref:`XPIDL`. For a tutorial on writing
+a new XPCOM interface, see
+:ref:`writing_xpcom_interface`.
+
+Native XPCOM components are registered at build time, and compiled into static
+data structures which allow them to be accessed with little runtime overhead.
+Each module which wishes to register components must provide a manifest
+describing each component it implements, its type, and how it should be
+constructed.
+
+Manifest files are Python data files registered in ``moz.build`` files in a
+``XPCOM_MANIFESTS`` file list:
+
+.. code-block:: python
+
+    XPCOM_MANIFESTS += [
+      'components.conf',
+    ]
+
+The files may define any of the following special variables:
+
+.. code-block:: python
+
+    # Optional: A function to be called once, the first time any component
+    # listed in this manifest is instantiated.
+    InitFunc = 'nsInitFooModule'
+    # Optional: A function to be called at shutdown if any component listed in
+    # this manifest has been instantiated.
+    UnloadFunc = 'nsUnloadFooModule'
+
+    # Optional: A processing priority, to determine how early or late the
+    # manifest is processed. Defaults to 50. In practice, this mainly affects
+    # the order in which unload functions are called at shutdown, with higher
+    # priority numbers being called later.
+    Priority = 10
+
+    # Optional: A list of header files to include before calling init or
+    # unload functions, or any legacy constructor functions.
+    #
+    # Any header path beginning with a `/` is loaded relative to the root of
+    # the source tree, and must not rely on any local includes.
+    #
+    # Any relative header path must be exported.
+    Headers = [
+        '/foo/nsFooModule.h',
+        'nsFoo.h',
+    ]
+
+    # A list of component classes provided by this module.
+    Classes = [
+        {
+            # ...
+        },
+        # ...
+    ]
+
+    # A list of category registrations
+    Categories = {
+        'category': {
+            'name': 'value',
+            'other-name': ('value', ProcessSelector.MAIN_PROCESS_ONLY),
+            # ...
+        },
+        # ...
+    }
+
+Class definitions may have the following properties:
+
+``name`` (optional)
+  If present, this component will generate an entry with the given name in the
+  ``mozilla::components`` namespace in ``mozilla/Components.h``, which gives
+  easy access to its CID, service, and instance constructors as (e.g.,)
+  ``components::Foo::CID()``, ``components::Foo::Service()``, and
+  ``components::Foo::Create()``, respectively.
+
+``cid``
+  A UUID string containing this component's CID, in the form
+  ``'{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}'``.
+
+``contract_ids`` (optional)
+  A list of contract IDs to register for this class.
+
+``categories`` (optional)
+  A dict of category entries to register for this component's contract ID.
+  Each key in the dict is the name of the category. Each value is either a
+  string containing a single entry, or a list of entries.  Each entry is either
+  a string name, or a dictionary of the form ``{'name': 'value', 'backgroundtasks':
+  BackgroundTasksSelector.ALL_TASKS}``.  By default, category entries are registered
+  for **no background tasks**: they have
+  ``'backgroundtasks': BackgroundTasksSelector.NO_TASKS``.
+
+``type`` (optional, default=``nsISupports``)
+  The fully-qualified type of the class implementing this component. Defaults
+  to ``nsISupports``, but **must** be provided if the ``init_method`` property
+  is specified, or if neither the ``constructor`` nor ``legacy_constructor``
+  properties are provided.
+
+``headers`` (optional)
+  A list of headers to include in order to call this component's constructor,
+  in the same format as the global ``Headers`` property.
+
+``init_method`` (optional)
+  The name of a method to call on newly-created instances of this class before
+  returning them. The method must take no arguments, and must return a
+  ``nsresult``. If it returns failure, that failure is propagated to the
+  ``getService`` or ``createInstance`` caller.
+
+``constructor`` (optional)
+  The fully-qualified name of a constructor function to call in order to
+  create instances of this class. This function must be declared in one of the
+  headers listed in the ``headers`` property, must take no arguments, and must
+  return ``already_AddRefed<iface>`` where ``iface`` is the interface provided
+  in the ``type`` property.
+
+  This property is incompatible with ``legacy_constructor``.
+
+``jsm`` (optional)
+  If provided, must be the URL of a JavaScript module which contains a
+  JavaScript implementation of the component. The ``constructor`` property
+  must contain the name of an exported function which can be constructed to
+  create a new instance of the component.
+
+``legacy_constructor`` (optional)
+  This property is deprecated, and should not be used in new code.
+
+  The fully-qualified name of a constructor function to call in order to
+  create instances of this class. This function must be declared in one of the
+  headers listed in the ``headers`` property, and must have the signature
+  ``nsresult(const nsID& aIID, void** aResult)``, and behave equivalently to
+  ``nsIFactory::CreateInstance``.
+
+  This property is incompatible with ``constructor``.
+
+``singleton`` (optional, default=``False``)
+  If true, this component's constructor is expected to return the same
+  singleton for every call, and no ``mozilla::components::<name>::Create()``
+  method will be generated for it.
+
+``overridable`` (optional, default=``False``)
+  If true, this component's contract ID is expected to be overridden by some
+  tests, and its ``mozilla::components::<name>::Service()`` getter will
+  therefore look it up by contract ID for every call. This component must,
+  therefore, provide at least one contract ID in its ``contract_ids`` array.
+
+  If false, the ``Service()`` getter will always retrieve the service based on
+  its static data, and it cannot be overridden.
+
+  Note: Enabling this option is expensive, and should not be done when it can
+  be avoided, or when the getter is used by any hot code.
+
+``external`` (optional, default=``False`` if any ``headers`` are provided, ``True`` otherwise)
+  If true, a constructor for this component's ``type`` must be defined in
+  another translation unit, using ``NS_IMPL_COMPONENT_FACTORY(type)``. The
+  constructor must return an ``already_AddRefed<nsISupports>``, and will be
+  used to construct instances of this type.
+
+  This option should only be used in cases where the headers which define the
+  component's concrete type cannot be easily included without local includes.
+
+  Note: External constructors may not specify an ``init_method``, since the
+  generated code will not have the necessary type information required to call
+  it. This option is also incompatible with ``constructor`` and
+  ``legacy_constructor``.
+
+``processes`` (optional, default=``ProcessSelector.ANY_PROCESS``)
+  An optional specifier restricting which types of process this component may
+  be loaded in. This must be a property of ``ProcessSelector`` with the same
+  name as one of the values in the ``Module::ProcessSelector`` enum.
+
+
+Conditional Compilation
+=======================
+
+This manifest may run any appropriate Python code to customize the values of
+the ``Classes`` array based on build configuration. To simplify this process,
+the following globals are available:
+
+``defined``
+  A function which returns true if the given build config setting is defined
+  and true.
+
+``buildconfig``
+  The ``buildconfig`` python module, with a ``substs`` property containing a
+  dict of all available build substitutions.
+
+
+Component Constructors
+======================
+
+There are several ways to define component constructors, which vary mostly
+depending on how old the code that uses them is:
+
+Class Constructors
+------------------
+
+This simplest way to define a component is to include a header defining a
+concrete type, and let the component manager call that class's constructor:
+
+.. code-block:: python
+
+  'type': 'mozilla::foo::Foo',
+  'headers': ['mozilla/Foo.h'],
+
+This is generally the preferred method of defining non-singleton constructors,
+but may not be practicable for classes which rely on local includes for their
+definitions.
+
+Singleton Constructors
+----------------------
+
+Singleton classes are generally expected to provide their own constructor
+function which caches a singleton instance the first time it is called, and
+returns the same instance on subsequent calls. This requires declaring the
+constructor in an included header, and implementing it in a separate source
+file:
+
+.. code-block:: python
+
+  'type': 'mozilla::foo::Foo',
+  'headers': ['mozilla/Foo.h'],
+  'constructor': 'mozilla::Foo::GetSingleton',
+
+``Foo.h``
+
+.. code-block:: c++
+
+    class Foo final : public nsISupports {
+     public:
+      static already_AddRefed<Foo> GetSingleton();
+    };
+
+``Foo.cpp``
+
+.. code-block:: c++
+
+    already_AddRefed<Foo> Foo::GetSingleton() {
+      // ...
+    }
+
+External Constructors
+---------------------
+
+For types whose headers can't easily be included, constructors can be defined
+using a template specialization on an incomplete type:
+
+.. code-block:: python
+
+  'type': 'mozilla::foo::Foo',
+  'external: True,'
+
+``Foo.cpp``
+
+.. code-block:: c++
+
+    NS_IMPL_COMPONENT_FACTORY(Foo) {
+      return do_AddRef(new Foo()).downcast<nsISupports>();
+    }
+
+Legacy Constructors
+-------------------
+
+These should not be used in new code, and are left as an exercise for the
+reader.
+
+
+Registering Categories
+======================
+
+Classes which need define category entries with the same value as their
+contract ID may do so using the following:
+
+.. code-block:: python
+
+    'contract_ids': ['@mozilla.org/foo;1'],
+    'categories': {
+        'content-policy': 'm-foo',
+        'Gecko-Content-Viewers': ['image/jpeg', 'image/png'],
+    },
+
+This will define each of the following category entries:
+
+* ``"content-policy"`` ``"m-foo",`` ``"@mozilla.org/foo;1"``
+* ``"Gecko-Content-Viewers"`` ``"image/jpeg"`` ``"@mozilla.org/foo;1"``
+* ``"Gecko-Content-Viewers"`` ``"image/png"`` ``"@mozilla.org/foo;1"``
+
+Some category entries do not have a contract ID as a value. These entries can
+be specified by adding to a global ``Categories`` dictionary:
+
+.. code-block:: python
+
+    Categories = {
+        'app-startup': {
+            'Mapi Support': 'service,@mozilla.org/mapisupport;1',
+        }
+    }
+
+It is possible to limit these on a per-process basis by using a tuple as the
+value:
+
+.. code-block:: python
+
+    Categories = {
+        'app-startup': {
+            'MainProcessSingleton': ('service,@mozilla.org/main-process-singleton;1', ProcessSelector.MAIN_PROCESS_ONLY),
+        }
+    }