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<html>
<head>
<title>Needle Manual :: Chapter 9: Customizing Needle</title>
<link type="text/css" rel="stylesheet" href="manual.css" />
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<div class="title">
<span class="product">Needle—</span><br />
<span class="tagline">to the point --></span>
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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">
<li>
<a href="chapter-1.html">
Introduction
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<ol type="1">
<li><a href="chapter-1.html#s1">What is Needle?</a></li>
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Registry
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Dependency Injection
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Interceptors
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Service Libraries
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<a href="chapter-9.html">
Customizing Needle
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<ol type="1">
<li><a href="chapter-9.html#s1">Namespaces</a></li>
<li><a href="chapter-9.html#s2">Interceptors</a></li>
<li><a href="chapter-9.html#s3">Contexts</a></li>
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<li><a href="http://needle.rubyforge.org/api/index.html">Needle API</a></li>
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This manual is licensed under a <a href="http://creativecommons.org/licenses/by-sa/2.0/">Creative Commons License</a>.
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</td><td valign='top' width="100%">
<div id="content">
<h1>9. Customizing Needle</h1>
<h2>
<a name="s1"></a>
9.1. Namespaces
</h2>
<div class="section">
<p>By default, when you create a namespace in Needle, the namespace is registered as a service. The type of the service is determined by the <code>:namespace_impl_factory</code> service, which (by default) returns the <code>Needle::Container</code> class.</p>
<p>You can specify your own custom implementation for namespaces by registering your own <code>:namespace_impl_factory</code> service. In fact, each namespace can have its own implementation of subnamespaces—just register a <code>:namespace_impl_factory</code> in each one that you want to be specialized.</p>
<p>Here’s a contrived example. Suppose you want each namespace to keep track of the precise time that it was created.</p>
<pre>
class TimeTrackerNamespace < Needle::Container
attr_reader :birth_date
def initialize( *args )
super
@birth_date = Time.now
end
end
reg = Needle::Registry.new
reg.register( :namespace_impl_factory ) { TimeTrackerNamespace }
reg.namespace :hello
p reg.hello.birth_date
</pre>
<p>In general, you’ll be better off having your custom implementation extend <code>Needle::Container</code>, although the only <em>real</em> requirement is that your implementation publish the same interface as the default namespace implementation.</p>
</div>
<h2>
<a name="s2"></a>
9.2. Interceptors
</h2>
<div class="section">
<p>When you attach an interceptor to a service, that new interceptor is wrapped in a definition object that includes various metadata about the interceptor, including its implementation, its priority, its name, and so forth. The implementation of this interceptor definition is determined by the value of the <code>:interceptor_impl_factory</code> service, which by default returns <code>Needle::Interceptor</code>.</p>
<p>It is this wrapper object that allows interceptor definitions to be done using method chaining:</p>
<pre>
reg.intercept( :foo ).with { ... }.with_options(...)
</pre>
<p>If you wish to add custom, domain-specific functionality to the interceptor wrapper, you can register your own implementation of the <code>:interceptor_impl_factory</code>. Consider the following contrived example, where an “only_if” clause is given to determine when the interceptor should be invoked.</p>
<pre>
class OnlyIfInterceptor < Needle::Interceptor
def only_if( &block )
@only_if = block
self
end
def action
action_proc = super
lambda do |chain,ctx|
if @only_if.call( chain, ctx )
action_proc.call( chain, ctx )
else
chain.process_next( ctx )
end
end
end
end
reg = Needle::Registry.new
reg.register( :interceptor_impl_factory ) { OnlyIfInterceptor }
reg.register( :foo ) { Bar.new }
reg.intercept( :foo ).
with { |c| c.logging_interceptor }.
only_if { |ch,ctx| something_is_true( ch, ctx ) }.
with_options(...)
</pre>
</div>
<h2>
<a name="s3"></a>
9.3. Contexts
</h2>
<div class="section">
<p>A <em>definition context</em> is used when registering services using any of the <code>#define</code> interfaces. For example, <code>Container#define</code> yields an instance of a definition context to the given block, and <code>Container#define!</code> uses the block in an <code>instance_eval</code> on a definition context.</p>
<p>The default implementation used for definition contexts is defined by the <code>:definition_context_factory</code> service. By default, this service returns <code>Needle::DefinitionContext</code>, but you can specify your own definition context implementations by overriding this service. In fact, each namespace could have its own definition context implementation, if needed.</p>
<p>Consider the following contrived example, where you want to provide a convenient way to register services of type Hash.</p>
<pre>
class MyDefinitionContext < Needle::DefinitionContext
def register_hash( name, opts={} )
this_container.register( name, opts ) { Hash.new }
end
end
reg = Needle::Registry.new
reg.register( :definition_context_factory ) { MyDefinitionContext }
reg.define do |b|
b.register_hash( :test1 )
b.register_hash( :test2 )
end
reg.test1[:key] = "value"
reg.test2[:foo] = "bar"
</pre>
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