Component Internals

使用C++创建组件

让我们开始研究怎样用c++创建XPCOM组件. 最常见的组件是以C++编写并编译成共享库（如Windows平台的DLL或者Unix平台的DSO）。

The illustration below shows the basic relationship between the shared library containing the component implementation code you write and the XPCOM framework itself. In this diagram, the outer boundary is that of the module, the shared library in which a component is defined.

var el = env.locale; A Component in the XPCOM Framework

As A Component in the XPCOM Framework illustrates, in addition to the NSGetModule entry point, there are nsIModule and nsIFactory interfaces that control the actual creation of the component, and also the string and XPCOM glue parts, which we'll discuss in some detail in the next section (see XPCOM Glue). These latter supply ease-of-development utilities like smart pointers, generic modules support, and simple string implementations. The largest and possibly most complex part of a component is the code specific to the component itself.

XPCOM Initialization

To understand why and when your component library gets called, it is important to understand the XPCOM initalization process. When an application starts up, that application may initialize XPCOM. The sequence of events that kicks off this XPCOM initialization may be triggered by user action or by the application startup itself. A web browser that embeds Gecko, for example, may initialize XPCOM at startup through the embedding APIs. Another application may delay this startup until XPCOM is needed for the first time. In either case, the initialization sequence within XPCOM is the same.

XPCOM starts when the application makes a call to initialize it. Parameters passed to this startup call allow you to configure some aspects of XPCOM, including the customization of location of specific directories. The main purpose of the API at this point is to change which components directory XPCOM searches when it looks for XPCOM components. This is how the API is used, for example, in the Gecko Runtime Environment (GRE).

XPCOM Registry Manifests

XPCOM uses special files called manifests to track and persist information about the components to the local system. There are two types of manifests that XPCOM uses to track components:

Component Manifests

When XPCOM first starts up, it looks for the component manifest, which is a file that lists all registered components, and stores details on exactly what each component can do. XPCOM uses the component manifest to determine which components have been overridden. Starting with Mozilla 1.2, this file is named compreg.dat and exists in the components directory, but there are efforts to move it out of this location to a less application-centric (and thus more user-centric) location. Any Gecko-based application may choose to locate it elsewhere. XPCOM reads this file into an in-memory database.

When the registration process begins, XPCOM broadcasts to all registered observers a notification that says XPCOM has begun the registration of new components. After all components are registered, another notification is fired saying that XPCOM is done with the registration step. The nsIObserver interface that handles this notification is discussed in Starting WebLock.

Once registration is complete and the notifications have fired, XPCOM is ready to be used by the application. If XPCOM registered your component, then it will be available to other parts of the XPCOM system. The XPCOM Initialization section in this chapter describes registration in more detail.

Autoregistration

The term autoregistration is sometimes used synonymously with registration in XPCOM. In the What Is XPCOM Registration? note, we describe the three ways you can register components with XPCOM. Sometimes, applications use the nsIComponentRegistrar interface and create their own code for watching a particular directory and registering new components that are added there, which is what's often referred to as autoregistration. You should always know what the installation and registration requirements are for the applications that will be using your component.

The Shutdown Process

When the application is ready to shutdown XPCOM, it calls NS_ShutdownXPCOM. When that method is called, the following sequence of events occurs:

1. XPCOM fires a shutdown notification to all registered observers.
2. XPCOM closes down the Component Manager, the Service Manager and associated services.
3. XPCOM frees all global services.
4. NS_ShutdownXPCOM returns and the application may exit normally.

One layer above the core object is the factory code. The factory object provides a basic abstraction of the core XPCOM object. An Overview of XPCOM discussed the factory design pattern that's used in a factory object. At this layer of the XPCOM Component Library, the factory objects are factories for the core XPCOM objects of the layer below.

One more layer outward is the module code. The module interface provides yet another abstraction - this time of the factories - and allows for multiple factory objects. From the outside of the component library, there is only the single entry point, NSGetModule(). This point of entry may fan out to any number of factories, and from there, to any number of XPCOM objects.

The factory design pattern in XPCOM is represented by the nsIFactory interface. The module layer is represented by the nsIModule interface. Most component libraries only need these two interfaces, along with the nsISupports interface, to have XPCOM load, recognize, and use their core object code.

In the next section, we'll be writing the code that actually compiles into a component library, and you will see how each layer is implemented and how each interface is used. Following this initial, verbose demonstration of the APIs, we will introduce a faster more generic way of implementing the module and factory code using macros, which can make components much easier to create.

XPCOM Glue

XPCOM contains a lot of stuff. Most XPCOM interfaces are not frozen and are only meant to be used by the Gecko internals and not by clients. XPCOM provides many data structures from linked lists to AVL trees. It's tempting to reuse nsVoidArray or another publicly available class instead of writing your own linked list, but this may prove to be a fatal mistake. The class can change at any time and give you unexpected behavior.

XPCOM makes for a very open environment. At runtime you can acquire any service or component by merely knowing a CID or Contract ID along with an IID. At last count there were over 1300 interfaces defined in XPIDL. Of those 1300 interfaces, less than 100 were frozen, which means that a developer is likely to stumble upon useful interfaces that aren't frozen. Unless an interface is explicitly marked "FROZEN" in the IDL comments, your component may possibly break or crash along with a version change.

The Glue Library

In general, you should avoid any interfaces, symbols in XPCOM, or other part of Gecko libraries that aren't frozen. However, there are some unfrozen tools in XPCOM that are used so often they are practically required parts of component programming.

The smart pointer class, nsCOMPtr, for example, which makes reference counting less tedious and error-prone, is not actually frozen, and neither is nsDebug, a class for aiding in tracking down bugs, nor is nsMemory, a class to ensure that everyone uses the same heap, generic factory, and module. Instead of asking every developer to find and copy these various files into their own application, XPCOM provides a single library of "not-ready-to-freeze-but-really-helpful" classes that you can link into your application, as the following figure demonstrates.

var el = env.locale; XPCOM Glue and Tools

The base string types that XPCOM uses are nsAString and nsACString. These classes are described in the Mozilla String Guide (see Gecko Resources).

The string classes that implement these abstract classes are another set of helpful, unfrozen classes in XPCOM. Most components and embedding applications need to link to some string class or other in order to utilize certain Gecko APIs, but the string code that Mozilla uses is highly complex and even more expensive than the glue code in terms of footprint (~100k). nsEmbedString and nsEmbedCString are available as very lightweight string implementations for component development, especially in small embedded applications. This string implementation does the bare minimum to support the nsAString/nsACString functionality.

In your own component, you can go "slim" and restrict yourself to the nsEmbedString or go "hog wild" and use any of the the other strings. WebLock restricts itself to using the simple nsEmbedString family of classes.

var el = env.locale; String Classes and XPCOM

The glue library provides stub functions for the public functions that XPCOM provides (see xpcom/build/nsXPCOM.h). When the glue library is initialized, it dynamically loads these symbols from the XPCOM library, which allows the component to avoid linking directly with the XPCOM library. You shouldn't have to link to the XPCOM library to create a XPCOM component - in fact, if your component has to, then something is wrong.