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<h2>
XPCOM Code FAQ</h2>
Suresh Duddi &lt;<a href="mailto:dp@netscape.com">dp@netscape.com</a>>
<br>Last Modified: April 2 1999
<br>
<hr WIDTH="100%">
<br>I am documenting things randomly as I am replying to people's questions.
So this looks more like an FAQ.
<h3>
Table of Contents</h3>
<ol>
<li>
<a href="#What are the Global Objects that XPCOM maintains">What are the
Global Objects that XPCOM maintains</a></li>
<li>
<a href="#What are the static classes that XPCOM maintains">What are the
static classes that XPCOM maintains</a></li>
<li>
<a href="#Is there any restriction on which static class I should call fi">Is
there any restriction on which static class I should call first</a></li>
<li>
<a href="#What is the order of creation of the ServiceManager, ComponentM">What
is the order of creation of the ServiceManager, ComponentManager and Registry</a></li>
<li>
<a href="#Is there a global Registry being maintained">Is there a global
Registry being maintained</a></li>
<li>
<font color="#000000"><a href="#ComponentManager Vs ServiceManager">ComponentManager
Vs ServiceManager</a></font></li>
<li>
<a href="#ProgID Vs CLSID">ProgID Vs CLSID</a></li>
<li>
<a href="#How to debug components ?">How to debug components ?</a></li>
</ol>
<hr WIDTH="100%">
<h4>
<a NAME="What are the Global Objects that XPCOM maintains"></a>What are
the Global Objects that XPCOM maintains</h4>
<ul>mGlobalServiceManager
<br>mGlobalComponentManager</ul>
<h4>
<a NAME="What are the static classes that XPCOM maintains"></a>What are
the static classes that XPCOM maintains</h4>
<blockquote>nsComponentManager
<br>nsServiceManager</blockquote>
<h4>
<a NAME="Is there any restriction on which static class I should call fi"></a>Is
there any restriction on which static class I should call first</h4>
<blockquote>No restrictions. You can call any function from the static
classes nsComponentManager and nsServiceManager. XPCOM will do the right
thing to initialize itself at both places.
<p>Autoregistration() can happen only after Init_XPCOM() is called since
the registy might be required by SelfRegister() functions of the dlls and
it is only in Init_XPCOM() do we create register the RegistryFactory()
with the ComponentManager.</blockquote>
<h4>
<a NAME="What is the order of creation of the ServiceManager, ComponentM"></a>What
is the order of creation of the ServiceManager, ComponentManager and Registry</h4>
<blockquote>Init_XPCOM()
<blockquote>
<li>
create the global service manager</li>
<li>
create the global component manager and register as service with the global
service manager</li>
<li>
RegisterFactory(...RegistryFactory...)&nbsp; Register the RegistryFactory
with the component manager so that new registry objects can be created.</li>
</blockquote>
Now the hard problem is when to trigger Init_XPCOM() There are two static
objects nsComponentManager and nsServiceManager. Any function in either
of them can be called first. Today nsServiceManager::GetService() is the
first one that gets called. All the members of the static nsServiceManager
use the NS_GetGlobalServiceManager() to get to the global service manager.
All members of the static nsComponentManager use the NS_GetGlobalComponentManager()
to get to the global component manager. Hence if we trigger Init_XPCOM()
from both NS_GetGlobalComponentManager() and NS_GetGlobalServiceManager()
we will be safe.</blockquote>
<h4>
<a NAME="Is there a global Registry being maintained"></a>Is there a global
Registry being maintained</h4>
<blockquote>No. The nsIRegistry is designed to be lightweight access to
the registry. Consumers who need to access the registry should use the
component manager to create the their own registry access object. This
is required because the open() call is supported by the nsIRegistry() and
if we maintain a global registry arbitrating which registry file is opened
is going to be a major headach.
<p>The ProgID for the registry will be <font color="#990000">component://netscape/registry</font>
<br>&nbsp;</blockquote>
<h4>
<a NAME="ComponentManager Vs ServiceManager"></a><font color="#000000">ComponentManager
Vs ServiceManager</font></h4>
<blockquote><font color="#000000">ComponentManager is the only way for
component creation. ComponentManager always uses the component's factory
to create the component instance. Clients (code that calls CreateInstance()
to create and use a component) call the ComponentManager to create instances.
Components (the code that implemented NSRegisterSelf()) calls the ComponentManager
to register itself and gets called when a Client wants to instantiate a
component.</font>
<p><font color="#000000">ServiceManager is a convinience for getting singleton
components, components for which only one instance stays alive for the
entire application e.g Netlib. It enforces only one of a kind of a component
to exist. Hence the notion of getting a service not creating one. (as opposed
to the notion of Creating instances with the componentManager). ServiceManager
is a convenience because components can technically force singletonism
by making their factory return the same instance if one was created already.
The other big use of ServiceManager is the (still unimplemented) notion
of Shutting down a service.</font>
<p><b><i><font color="#000000">Client</font></i></b>
<ul>
<li>
<i><font color="#000000">When does a client use the service manager vs
component manager</font></i></li>
<br><font color="#000000">When a client knows that the component that they
are trying to instantiate is a singleton, they need to call service manager
instead of component manager. Clients dont have to worry about calling
the ComponentManager at all in this case. The ServiceManager will take
care of creating the instance if the first one doesn't exist already.</font>
<br>&nbsp;
<li>
<i><font color="#000000">When does a client use the Component Manager as
opposed to Service Manager</font></i></li>
<br><font color="#000000">When a client wants a private instance of a component,
they call the Component Manager. From the Clients point of view, a new
xpcom object creation happens everytime they call CreateInstance() Anything
else is an implementation detail that the Client need not worry about.</font>
<br>&nbsp;
<li>
<i><font color="#000000">How does a Client know that they have to instantiate
a singleton</font></i></li>
<br><font color="#000000">For now, the Client just has to know. There is
no way of telling which component is a Service and which isn't. In fact,
in todays xpcom (Mar 1999) any component can be accessed as a Service.
Use your judgement until there is a proper method or service manager is
eliminated. There is nothing even in the code that detects Services from
Instances.</font>
<p><b><font color="#CC0000">Need a solution for this. Email suggestion
to <a href="mailto:warren@netscape.com,dp@netscape.com">warren@netscape.com,
dp@netscape.com</a></font></b>
<br>&nbsp;</ul>
<b><i><font color="#000000">Component</font></i></b>
<ul>
<li>
<i><font color="#000000">Can a component enforce use only as a Service</font></i></li>
<br><font color="#000000">No. The notion of the ServiceManager is available
only to Clients.</font>
<p><font color="#000000">Note that at some points when a component wants
another component, it actually behaves as a client and hence follows the
rules of the Client above to either CreateInstance() or GetService() the
needed component.</font>
<p><b><tt><font color="#990000">Workaround:</font></tt></b><font color="#000000">
If however a component wants only one of its instances to exist and cannot
ensure that Clients understand well enough only to use the Service Manager
to get it, it can implement singletonism in its factory. Basically the
factory on first instance creation should hang on to the instance. On subsequence
instance creations, addref the instance it is holding to and return that
instead creating a new one.</font>
<ul>&nbsp;
<br><font color="#000000">E.g preferences does this.</font> Code sample
at
<a href="http://lxr.mozilla.org/seamonkey/source/modules/libpref/src/nsPref.cpp#621">nsPref.cpp
nsPrefFactory::CreateInstance()</a> and&nbsp; <a href="http://lxr.mozilla.org/seamonkey/source/modules/libpref/src/nsPref.cpp#227">nsPref.cpp
nsPref::GetInstance()</a> With this implementation, whether Clients get
to it by calling nsIServiceManager::GetService() or nsIComponentManager::CreateInstance(),
the same object will be returned hence guaranteeing singletonism.</ul>
<li>
<i><font color="#000000">Should a component do anything at creation to
become a Service</font></i></li>
<br><font color="#000000">No. Again, the notion of a ServiceManager is
available only to Clients.</font>
<br>&nbsp;
<li>
<i><font color="#000000">Can a component advertise that it is a service
so clients can use it as one</font></i></li>
<br>No. There isn't a way other than a comment in the interface of the
header file.</ul>
</blockquote>
<h4>
<a NAME="ProgID Vs CLSID"></a>ProgID Vs CLSID</h4>
<blockquote>ClassID or CLSID is the unique indentification of a component.
It is a structure of huge numbers generated by using uuidgen on a windows
box. It is represented as a string in documentation as {108d75a0-bab5-11d2-96c4-0060b0fb9956}
<p>ProgID is the string identification of an implementation of a component
the client is looking for. The representation takes a URI syntax. Eg. <i>component://netscape/network/protocol?name=http;description=Http%20Protocol%20Handler</i>
<br>Some simplify this to, ProgID is a more readable string form of a CLSID.
That is acceptable on the periphery. The ProgID is a Client thing. Components
register with component manager to claim that they are the implementation
for a ProgID. A component can register to be the implementation for multiple
ProgIDs (not implemented yet).
<p><b><i>Client</i></b>
<ul>
<li>
<i>Should CreateInstance() calls use ProgID or CLSID</i></li>
<br>ProgID is what Clients should use to CreateInstances. Clients should
not even know about the CLSID unless they are hell bent on creating a particular
implementation of a component.
<br>&nbsp;</ul>
<b><i>Component</i></b>
<ul>
<li>
<i>Should Components register with both a CID and ProgID</i></li>
<br>Absolutely.</ul>
</blockquote>
<h4>
<a NAME="How to debug components ?"></a>How to debug components ?</h4>
<blockquote>XPCOM provides log output. To enable the logging:
<blockquote><tt>setenv NSPR_LOG_MODULES nsComponentManager:5</tt>
<br><tt>setenv NSPR_LOG_FILE xpcom.log</tt></blockquote>
Start your application after setting the above environment variables. Debug
log from xpcom would be in the file <tt>xpcom.log</tt>
<p>Since components are dynamically loaded only on demand, debugging them
could be a hard. Here are some tips to debugging components.
<p><b><i>Windows: VC5.0 VC6.0</i></b>
<ul>Include your component library in the Project->Settings, Additional
Dll. drop down. After that breakpoints can be enabled.</ul>
<b><i>Unix: gdb</i></b>
<blockquote>Let the program run until you are sure that your component
is loaded. Type Control-C. Now all symbols from your component will be
available in gdb. Put your breakpoints and restart the app. Gdb will complain
that it cannot set the breakpoint, and that it is temporarily disabling
it, but when the *.so is loaded, the breakpoint is enabled automatically.
- &lt;<a href="mailto:erik@netscape.com">Eric Van Der Poel</a>>
<p>I think typing "dir components" (assuming you're in dist/bin) will also
allow you to see the symbols in your stack the first time. - &lt;<a href="mailto:alecf@netscape.com">Alec
Flett</a>></blockquote>
<b><i>Mac: Codewarrior</i></b>
<blockquote>Just open the appropriate .xSYM file in the debugger; the debugger
will target the library when the application is run. - &lt;<a href="mailto:sfraser@netscape.com">Simon
Fraser</a>></blockquote>
</blockquote>
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