path: root/test/scanners/yaml/faq.in.yml

---
- "What is a...":
  - "container?": >-
      A _container_ is collection of service points and other containers. It
      is used to organize services. Each container has access to all of the
      service points in its ancestor containers.

  - "registry?": >-
      A _registry_ is a special kind of container that has no parent container.
      It also defines a few services (such as the LoggingInterceptor, and
      the various service models and pipeline elements), so that they are
      available by default to all of the services it contains.

  - "service point?": >-
      A _service point_ is the definition of a service. Just as a class is the
      definition of an object, and you instantiate an object from a class, so
      do you instantiate services from service points.

  - "service?": >-
      A _service_ is the instantiation of a service point.

  - "parameterized service?": >-
      A _parameterized_ service is a service that allows contextual parameters
      to be passed to the service when it is created. Such services are
      typically used in conjunction with the @multiton@ service model, but
      the only real requirement is that they _not_ be used with a service model
      that does not support multiple parameters (like @singleton@ or
      @threaded@).

  - "service model?": >-
      A _service model_ is a description of the lifecycle of a service. By
      default, all services are _singletons_, meaning that every time you ask
      a container for a particular service, you'll get the same object
      instance back.


      There are other service models available, though, including "prototype"
      (which returns a new instance for each request of a service) and
      "deferred" (which returns a proxy, deferring the instatiation of the
      service itself until a method is invoked on the service). 

  - "interceptor?": >-
      An _interceptor_ is an object that may be placed between the client and
      a service. Every request to the service is thus _intercepted_ by that
      object, which can do operations on the request (such as logging) and may
      even reroute or ignore the request altogether. This provides a kind of
      "poor man's AOP", since you can do "before", "after", and "around" advice
      on the methods of a service.


      Needle comes with one standard interceptor, the LoggingInterceptor. It
      will log a message on method entry and exit, and also when an exception
      is raised.

  - "pipeline?": >-
      In Needle, the _instantiation pipeline_ is used to control how and when
      services are instantiated. The _service models_ are implemented as
      pipelines.


      Just as the _interceptors_ are for hooking into method invocations, the
      pipelines are for hooking into service instantiations. Every time a
      service is requested, it's instantiation pipeline is executed. By
      choosing the appropriate kinds of pipeline elements, all of the available
      service models can be implemented (prototype, prototype_deferred,
      singleton, singleton_deferred, etc.).

- "How do I...":
  - "create a new registry?": >-
      There are several ways to create a new registry. The simplist is just to
      invoke Registry#new.


      <pre>
        reg = Needle::Registry.new
      </pre>


      This will create a new Registry instance. You can also send a block to
      #new, in which case the new registry will be yielded to it:


      <pre>
        reg = Needle::Registry.new do |r|
          ...
        end
      </pre>


      There are two other factory methods you can use for creating a Registry
      instance. Both require a block.


      <pre>
        r1 = Needle::Registry.define do |builder|
          ...
        end

        r2 = Needle::Registry.define! do
          ...
        end
      </pre>


      Registry#define creates a "builder" object that you can use define
      services more conveniently. Register#define! (with a bang) does the same
      thing, but evaluates the block within the context of the builder.

  - "register a service?": >-
      The first way to register a service is by calling #register on the
      registry (or a namespace):


      <pre>
        reg.register( :foo ) { Foo.new }
      </pre>


      The (first) parameter to #register is the name of the service, and the
      block should return the implementation of the service. If needed, the
      block can accept two parameters--the container that the service is being
      registered with, and an object that represents the service being defined
      (called a "service point"):


      <pre>
        reg.register( :foo ) do |container,point|
          Foo.new( container[:bar], point.fullname )
        end
      </pre>


      You can also use Container#define and Container#define! to register
      services.  These approaches are friendlier if you are needing to register
      several services at once.


      <pre>
        reg.define do |builder|
          builder.foo { Foo.new }
          builder.bar { |c,p| Bar.new( c[:foo], p.name ) }
        end

        reg.define! do
          baz { |c,p| Baz.new( c[:bar], p.name ) }
          zoom { Buggy.new }
        end
      </pre>


      Container#define yields a new "builder" object to the block. Messages
      sent to the builder are interpreted as service names, and if a block is
      sent with the message, a new service is registered under that name.

      
      Container#define! does likewise, except it evaluates the block within the
      context of the builder object.


      If you do not pass a block to #define, it will return the builder object,
      so you could do something like the following if you only need to define
      one or two services:


      <pre>
        reg.define.foo { ... }
      </pre>


      Lastly, you can get the builder directly and add services using it:


      <pre>
        builder = reg.builder
        builder.baz { ... }
        builder.bar { ... }
      </pre>


      (This last is the same as calling #define without arguments, but is more
      readable if you intend to use the builder object multiple times.)
      
  - "reference a service?": >-
      Referencing a service can be done in either of two ways. The first is to
      treat the container (i.e., registry) as a hash, passing the name of the
      service as an argument to Container#[]:


      <pre>
        svc = registry[:foo]
        svc.do_something_interesting
      </pre>


      A more convenient (but slightly more peril-fraught) approach is to send
      the name of the method to the registry as a message:


      <pre>
        svc = registry.foo
      </pre>


      Be aware that this latter approach will only work when the service name
      does not conflict with the name of an existing method on the container.
      For example, if you were to do:


      <pre>
        registry.register( :hash ) { "hello, world" }
        p registry.hash
      </pre>


      You would get the hash value of the registry object, instead of the value
      value of the service (which would be "hello, world").

  - "select a service model for a service (i.e., change the default model of lifecycle management)?": >-
      By default, a service will be managed as a singleton, i.e., every request
      of that service will return the same object instance. This is the
      _singleton_ service model.


      To select a different service model, pass it as an option when you
      register the service:


      <pre>
        registry.register( :foo, :model => :prototype ) {...}
        registry.define.bar( :model => :threaded ) {...}
        registry.define! do
          baz( :model => :singleton_deferred ) {...}
          ...
        end
        ...
      </pre>

  - "create a namespace?": >-
      Namespaces allow you to organize your services into hierarchical
      packages. You can create namespaces in a few ways. The first (and
      simplest) is to just call Container#namespace:


      <pre>
        registry.namespace( :stuff )
      </pre>


      This will create a namespace in the registry, called stuff. If you send a
      block as well, the block will be invoked (with the new namespace yielded
      to it) the first time the namespace is requested:


      <pre>
        registry.namespace( :stuff ) do |ns|
          ns.register( :foo ) {...}
          ns.define.bar {...}
          ns.define! do
            baz {...}
            buf {...}
          end
        end
      </pre>

      Because it is so common to immediately define services on the new
      namespace, there are some convenience methods to make this more...
      convenient.


      <pre>
        registry.namespace_define!( :stuff ) do
          foo {...}
          bar {...}
          baz {...}
        end

        registry.namespace_define( :more_stuff ) do |b|
          b.blah {...}
          b.argh {...}
          b.hack {...}
        end
      </pre>


      The first one, above, creates the namespace and calls Container#define!.
      The second creates the namespace and calls Container#define. In both
      cases, _the namespace is created immediately_, unlike Container#namespace
      which only creates the namespace when it is first requested.


      Lastly, note that namespace's are just special services. Thus, you can
      pass options to the namespace methods just as you can with
      Container#register and friends.

  - "write log messages?": >-
      You can obtain a new logger instance from the @:logs@ and @:log_for@
      services. Once you have a logger instance, you can invoke the #debug,
      #info, #warn, #error, and #fatal methods on the instance to log messages
      of the corresponding severity.


      <pre>
        logger = registry.logs.get( "a name for my logger" )
        logger.debug "This is a debug message"
        logger.info "This is an informational message"
        ...
        logger2 = registry.log_for( "another logger name" )
        ...
      </pre>


      The two approaches shown above are identical--the second approach (using
      the @log_for@ service) is just a convenience for @logs.get@.


      Log messages are written, by default, to a file called "needle.log", in
      the same directory that the application was invoked from.


      You can also use a logging interceptor to automatically log all external
      method invocations on a service. This includes method entry and exit, as
      well as any exceptions that are raised inside the method.


      <pre>
        registry.register( :foo ) { ... }
        registry.intercept( :foo ).with { |r| r.logging_interceptor }

        foo.something
        foo.another_method( 1, 2, 3 )
      </pre>


      See the chapter in the "User's Manual":http://needle.rubyforge.org about
      logging for more information on how to use and configure loggers.

  - "exclude methods from being intercepted?": >-
      Only interceptors that explicitly support exclusion of methods can help
      you here.  Fortunately, the LoggingInterceptor is one of them. (If you
      write your own interceptor and would like similar functionality, see the
      IncludeExclude module.)


      In the case of the LoggingInterceptor, just pass an array of patterns
      (matching method names and/or arities) as the "exclude" option, when
      declaring the interceptor:


      <pre>
        registry.register( :foo ) { ... }
        registry.intercept( :foo ).
          with { |r| r.logging_interceptor }.
          with_options :exclude => [ 'foo', 'bar(>4)', '*(<2)' ]
      </pre>


      The above will exclude from interception any method named 'foo', or any
      invocation of 'bar' with more than 4 arguments, or any method invocation
      with fewer than two arguments.


      You can also give an array of patterns to _include_. These cause methods
      to be explicitly intercepted even if they match an exclude pattern:


      <pre>
        registry.register( :foo ) { ... }
        registry.intercept( :foo ).
          with { |r| r.logging_interceptor }.
          with_options :exclude => [ 'foo', 'bar(>4)', '*(<2)' ],
                       :include => [ 'baz' ]

        foo = registry.foo
        foo.baz
      </pre>


      This would result in the call to #baz being intercepted, even though it
      matches an exclude pattern (@*(<2)@).

  - "include services defined in another library?": >-
      This requires that the other library be implemented in such a way that it
      expects to be "included" by other libraries/applications. For example,
      Needle encourages the use of a method called @register_services@, which
      accepts a container as a parameter:


      <pre>
        module A
          module B
            def register_services( container )
              ...
            end
            module_function :register_services
          end
        end
      </pre>


      If the library has been implemented in this way, you can simply do a
      require of the library and then invoke the @register_services@ method.


      There is a convenience method in Container for doing this. Just call
      Container#require, passing the file to require and a string (or symbol)
      identifying the name of the module that contains the registration method.
      You can also pass a symbol as the third parameter naming the registration
      method, but it defaults to @:register_services@.


      <pre>
        require 'a/b'
        A::B.register_services( container )

        # or

        container.require( 'a/b', "A::B" )
      </pre>


      The definition context (i.e., the "builder" object) also supports the
      require method, so you can do:


      <pre>
        container.define do |b|
          b.require "a/b", "A::B"
          b.foo { ... }
          ...
        end
      </pre>

- "When should I...":
  - "use a different service model?":
    - "Like, :prototype?": >-
        The prototype service model is appropriate when the service:


        * has internal state

        * will be used multiple times for different situations


        For example, if you have a GUI library, a "button" service could be a
        prototype, because you will likely have many buttons in an application,
        with each button being an independent instance.
      
    - "Like, :singleton?": >-
        The singleton service model is the default, so you should rarely need
        to explicitly specify it as a model. It is appropriate for services
        that:


        * guard some specific functionality

        * represent state that is global across an application

    - "Like, :threaded?": >-
        Threaded is similar to singleton, but it allows one unique instance of
        the service _per thread_. Thus, it is appropriate to the same
        situations as singleton, but specific to a thread, instead of an
        application. This is useful for web applications that are run in a
        single virtual machine, and which share a single registry.

    - "Like, deferred?": >-
        Deferred models use a proxy to enforce lazy initialization of the
        service. A service using a deferred service model (ie,
        @:prototype_deferred@, @:multiton_deferred@, @:singleton_deferred@, or
        @:threaded_deferred@) will not be instantiated until the first time a
        method is invoked on the service.


        This makes a deferred model appropriate when a service is expensive to
        instantiate, since you can wait to do the expensive initialization
        until it is really needed. Applications will start up faster when their
        dependences use deferred instantiation.

    - "Like, initialize?": >-
        This is useful when you have a method that you want to be invoked
        automatically after a service has been instantiated. Consider the case
        where a service is initialized primarily using setters, but requires
        some logic to be executed to complete the initialization phase. In this
        case, you could always explicitly invoke the initialization method(s)
        in the constructor block, but if many services use the same
        initialization method, it can be more convenient to use an "initialize"
        service model.

    - "Like, multiton?": >-
        Multitons are useful for factories, where you have a class that
        differentiates its instances based on some construction parameters that
        need to be determined at runtime. Thus, multitons are always used with
        parameterized services.