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<html>
<head>
<title>Needle Manual :: Chapter 8: Service Libraries</title>
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Needle Version: <strong>1.2.0</strong><br />
Manual Last Updated: <strong>2004-11-18 15:36 GMT</strong>
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<h1>Needle Manual</h1>
<h2>Chapters</h2>
<ol type="I">
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Introduction
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Registry
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Service Locator
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Service Libraries
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<li><a href="chapter-8.html#s1">Overview</a></li>
<li><a href="chapter-8.html#s2">Creating Libraries</a></li>
<li><a href="chapter-8.html#s3">Using Libraries</a></li>
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Customizing Needle
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<h1>8. Service Libraries</h1>
<h2>
<a name="s1"></a>
8.1. Overview
</h2>
<div class="section">
<p>Using Needle as it has been presented so far works fine when you are dealing with a single application, all self-encapsulated. When you start dealing with combining multiple libraries, each potentially written by a different author, into a single registry, things get a little more complicated.</p>
<p>Needle provides a way for service authors to share their services. All it requires is that authors centralize their service configuration.</p>
</div>
<h2>
<a name="s2"></a>
8.2. Creating Libraries
</h2>
<div class="section">
<p>This centralization is implemented by creating a module for each library you want to have services for. That module should then define a module function called (by default) <code>register_services</code>. This function should accept a single parameter—the Needle container that the services will be added to.</p>
<p>For example, if I had a library of cryptographic routines and I wanted to make them accessible as Needle services, I would create a module to contain the service definitions. Typically, this module will be defined in a file called “services.rb”, although you can certainly name it whatever you like.</p>
<pre>
module Crypto
def register_services( container )
container.namespace_define( :crypto ) do |b|
b.prng do
require 'crypto/prng'
PRNG.new
end
b.des do
require 'crypto/des'
DES.new
end
...
end
end
module_function :register_services
end
</pre>
<p>Notice that there are no explicit dependencies on Needle, only on the interfaces Needle publishes. Thus, third-parties can add service configuration modules to their libraries without introducing dependencies on Needle.</p>
<p>Once a service library has been created, it can then be accessed from another library or application that wishes to import those services.</p>
</div>
<h2>
<a name="s3"></a>
8.3. Using Libraries
</h2>
<div class="section">
<p>Using the libraries is as simple as requiring the file that has the service definitions, and then invoking the <code>#register_services</code> module function:</p>
<pre>
require 'needle'
reg = Needle::Registry.new
reg.define do |b|
b.foo { Foo.new }
require 'crypto/services'
Crypto.register_services( reg )
end
prng = reg.crypto.prng
</pre>
<p>To make this easier, the Container class has a convenience method named <code>#require</code>:</p>
<pre>
require 'needle'
reg = Needle::Registry.new
reg.define do |b|
b.foo { Foo.new }
b.require 'crypto/services', "Crypto"
end
prng = reg.crypto.prng
</pre>
<p>The <code>Container#require</code> method will require the file, and then look for a <code>#register_services</code> method of the named module. It will execute that method, passing the container as an argument.</p>
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