Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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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.
+
+.. _build system: https://searchfox.org/mozilla-central/source/xpcom/idl-parser/xpidl
+.. _Rust: https://searchfox.org/mozilla-central/source/__GENERATED__/dist/xpcrs/rt
+.. _C++: https://searchfox.org/mozilla-central/source/__GENERATED__/dist/include
+
+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.
+
+.. _Browser Console: https://developer.mozilla.org/en-US/docs/Tools/Browser_Console
+
+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
+     * 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, :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
+     * 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 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
+     * 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 :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:
+            # 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 :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
+    # 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.
+
+.. code:: javascript
+
+    console.log(Services.componentName.isAlive);
+    > true