зеркало из https://github.com/mozilla/gecko-dev.git
Bug 1758012 - Add a tutorial for writing an XPCOM Interface; r=xpcom-reviewers,kmag
I wrote this tutorial while documenting my process of adding a component. I attempted to figure out the process rather than just cargo culting from examples. The idea is that you can follow along with this to get a new component going, and then reference the more detailed documentation for implementation details. Differential Revision: https://phabricator.services.mozilla.com/D140262
This commit is contained in:
Greg Tatum
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9884018277
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@ -4,6 +4,11 @@
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Defining XPCOM C++-implemented Components
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=========================================
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This document explains how to write a :code:`components.conf` file. For
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documentation on the idl format see :ref:`XPIDL`. For a tutorial on writing
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a new XPCOM interface, see
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:ref:`writing_xpcom_interface`.
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Native XPCOM components are registered at build time, and compiled into static
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data structures which allow them to be accessed with little runtime overhead.
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Each module which wishes to register components must provide a manifest
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@ -13,6 +13,7 @@ These pages contain documentation for Mozilla's Cross-Platform Component Object
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huntingleaks
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collections
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xpidl
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writing-xpcom-interface
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dmd
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hashtables
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hashtables_detailed
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@ -0,0 +1,284 @@
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.. _writing_xpcom_interface:
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Tutorial for Writing a New XPCOM Interface
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==========================================
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High Level Overview
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-------------------
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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.
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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.
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.. _build system: https://searchfox.org/mozilla-central/source/xpcom/idl-parser/xpidl
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.. _Rust: https://searchfox.org/mozilla-central/source/__GENERATED__/dist/xpcrs/rt
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.. _C++: https://searchfox.org/mozilla-central/source/__GENERATED__/dist/include
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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.
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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.
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.. _Browser Console: https://developer.mozilla.org/en-US/docs/Tools/Browser_Console
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From C++, components can be accessed via :code:`mozilla::components::ComponentName::Create()` using the :code:`name` option in the :code:`components.conf`.
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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.
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.. code:: javascript
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let component = Cc["@mozilla.org/component-name;1"].createInstance(
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Ci.nsIComponentName
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);
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.. code:: c++
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nsCOMPtr<nsIComponentName> component = do_CreateInstance(
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"@mozilla.org/component-name;1");
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Writing an XPIDL
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----------------
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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.
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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`.
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To start, create an :ref:`XPIDL` file:
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.. code:: bash
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touch path/to/nsIComponentName.idl
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And hook it up to the :code:`path/to/moz.build`
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.. code:: python
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XPIDL_SOURCES += [
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"nsIComponentName.idl",
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]
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Next write the initial :code:`.idl` file: :code:`path/to/nsIComponentName.idl`
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.. code:: c++
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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// This is the base include which defines nsISupports. This class defines
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// the QueryInterface method.
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#include "nsISupports.idl"
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// `scriptable` designates that this object will be used with JavaScript
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// `uuid` The example below uses a UUID with all Xs. Replace the Xs with
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// your own UUID generated here:
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// http://mozilla.pettay.fi/cgi-bin/mozuuid.pl
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/**
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* Make sure to document your interface.
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*/
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[scriptable, uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)]
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interface nsIComponentName : nsISupports {
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// Fill out your definition here. This example attribute only returns a bool.
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/**
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* Make sure to document your attributes.
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*/
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readonly attribute bool isAlive;
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};
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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.
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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:
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:code:`{obj-directory}/dist/include/nsIComponentName.h`
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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/>`_.
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Writing the C++ implementation
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------------------------------
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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.
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Add the C++ sources to :code:`path/to/moz.build`
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.. code:: python
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EXPORTS.mozilla += [
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"ComponentName.h",
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]
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UNIFIED_SOURCES += [
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"ComponentName.cpp",
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]
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Now write the header: :code:`path/to/ComponentName.h`
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.. code:: c++
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_nsComponentName_h__
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#define mozilla_nsComponentName_h__
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// This will pull in the header auto-generated by the .idl file:
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// {obj-directory}/dist/include/nsIComponentName.h
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#include "nsIComponentName.h"
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// The implementation can be namespaced, while the XPCOM interface is globally namespaced.
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namespace mozilla {
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// Notice how the class name does not need to be prefixed, as it is defined in the
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// `mozilla` namespace.
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class ComponentName final : public nsIComponentName {
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// This first macro includes the necessary information to use the base nsISupports.
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// This includes the QueryInterface method.
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NS_DECL_ISUPPORTS
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// This second macro includes the declarations for the attributes. There is
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// no need to duplicate these declarations.
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//
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// In our case it includes a declaration for the isAlive attribue:
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// GetIsAlive(bool *aIsAlive)
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NS_DECL_NSICOMPONENTNAME
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public:
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ComponentName() = default;
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private:
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// A private destructor must be declared.
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~ComponentName() = default;
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};
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} // namespace mozilla
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#endif
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Now write the definitions: :code:`path/to/ComponentName.cpp`
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.. code:: c++
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#include "ComponentName.h"
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namespace mozilla {
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// Use the macro to inject all of the definitions for nsISupports.
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NS_IMPL_ISUPPORTS(ComponentName, nsIComponentName)
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// This is the actual implementation of the `isAlive` attribute. Note that the
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// method name is somewhat different than the attribute. We specified "read-only"
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// in the attribute, so only a getter, not a setter was defined for us. Here
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// the name was adjusted to be `GetIsAlive`.
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//
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// Another common detail of implementing an XPIDL interface is that the return values
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// are passed as out parameters. The methods are treated as fallible, and the return
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// value is an `nsresult`. See the XPIDL documentation for the full nitty gritty
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// details.
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//
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// A common way to know the exact function signature for a method implementation is
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// to copy and paste from existing examples, or inspecting the generated file
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// directly: {obj-directory}/dist/include/nsIComponentName.h
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NS_IMETHODIMP
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ComponentName::GetIsAlive(bool* aIsAlive) {
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*aIsAlive = true;
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return NS_OK;
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}
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} // namespace: mozilla
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Registering the component
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-------------------------
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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`.
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.. code:: bash
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touch path/to/components.conf
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Now update the :code:`moz.build` to point to it.
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.. code:: python
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XPCOM_MANIFESTS += [
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"components.conf",
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]
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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.
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.. code:: python
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Classes = [
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{
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# This CID is the ID for component entries, and needs a separate UUID from
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# the .idl file. Replace the Xs with a uuid from:
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# http://mozilla.pettay.fi/cgi-bin/mozuuid.pl
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'cid': '{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}',
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'interfaces': ['nsIComponentName'],
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# A contract ID is a human-readable identifier for an _implementation_ of
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# an XPCOM interface.
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#
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# "@mozilla.org/process/environment;1"
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# ^^^^^^^^^^^^ ^^^^^^^ ^^^^^^^^^^^ ^
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# | | | |
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# | | | The version number, usually just 1.
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# | | Component name
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# | Module
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# Domain
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#
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# This design goes back to a time when XPCOM was intended to be a generalized
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# solution for the Gecko Runtime Environment (GRE). At this point most (if
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# not all) of mozilla-central has an @mozilla domain.
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'contract_ids': ['@mozilla.org/component-name;1'],
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# This is the name of the C++ type that implements the interface.
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'type': 'mozilla::ComponentName',
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# The header file to pull in for the implementation of the interface.
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'headers': ['path/to/ComponentName.h'],
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# In order to hook up this interface to the `Services` object, we can
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# provide the "js_name" parameter. This is an ergonomic way to access
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# the component.
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'js_name': 'componentName',
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},
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]
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At this point the full :code:`moz.build` file should look like:
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.. code:: python
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# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
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# vim: set filetype=python:
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# This Source Code Form is subject to the terms of the Mozilla Public
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# License, v. 2.0. If a copy of the MPL was not distributed with this
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# file, You can obtain one at http://mozilla.org/MPL/2.0/.
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XPIDL_SOURCES += [
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"nsIComponentName.idl",
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]
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XPCOM_MANIFESTS += [
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"components.conf",
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]
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EXPORTS.mozilla += [
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"ComponentName.h",
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]
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UNIFIED_SOURCES += [
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"ComponentName.cpp",
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]
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This completes the implementation of a basic XPCOM Interface using C++. The component should be available via the `Browser Console`_ or other chrome contexts.
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.. code:: javascript
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console.log(Services.componentName.isAlive);
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> true
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