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tag: v4.0.1.rc3
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class Module
# Provides a +delegate+ class method to easily expose contained objects'
# public methods as your own.
#
# The macro receives one or more method names (specified as symbols or
# strings) and the name of the target object via the <tt>:to</tt> option
# (also a symbol or string).
#
# Delegation is particularly useful with Active Record associations:
#
# class Greeter < ActiveRecord::Base
# def hello
# 'hello'
# end
#
# def goodbye
# 'goodbye'
# end
# end
#
# class Foo < ActiveRecord::Base
# belongs_to :greeter
# delegate :hello, to: :greeter
# end
#
# Foo.new.hello # => "hello"
# Foo.new.goodbye # => NoMethodError: undefined method `goodbye' for #<Foo:0x1af30c>
#
# Multiple delegates to the same target are allowed:
#
# class Foo < ActiveRecord::Base
# belongs_to :greeter
# delegate :hello, :goodbye, to: :greeter
# end
#
# Foo.new.goodbye # => "goodbye"
#
# Methods can be delegated to instance variables, class variables, or constants
# by providing them as a symbols:
#
# class Foo
# CONSTANT_ARRAY = [0,1,2,3]
# @@class_array = [4,5,6,7]
#
# def initialize
# @instance_array = [8,9,10,11]
# end
# delegate :sum, to: :CONSTANT_ARRAY
# delegate :min, to: :@@class_array
# delegate :max, to: :@instance_array
# end
#
# Foo.new.sum # => 6
# Foo.new.min # => 4
# Foo.new.max # => 11
#
# It's also possible to delegate a method to the class by using +:class+:
#
# class Foo
# def self.hello
# "world"
# end
#
# delegate :hello, to: :class
# end
#
# Foo.new.hello # => "world"
#
# Delegates can optionally be prefixed using the <tt>:prefix</tt> option. If the value
# is <tt>true</tt>, the delegate methods are prefixed with the name of the object being
# delegated to.
#
# Person = Struct.new(:name, :address)
#
# class Invoice < Struct.new(:client)
# delegate :name, :address, to: :client, prefix: true
# end
#
# john_doe = Person.new('John Doe', 'Vimmersvej 13')
# invoice = Invoice.new(john_doe)
# invoice.client_name # => "John Doe"
# invoice.client_address # => "Vimmersvej 13"
#
# It is also possible to supply a custom prefix.
#
# class Invoice < Struct.new(:client)
# delegate :name, :address, to: :client, prefix: :customer
# end
#
# invoice = Invoice.new(john_doe)
# invoice.customer_name # => 'John Doe'
# invoice.customer_address # => 'Vimmersvej 13'
#
# If the target is +nil+ and does not respond to the delegated method a
# +NoMethodError+ is raised, as with any other value. Sometimes, however, it
# makes sense to be robust to that situation and that is the purpose of the
# <tt>:allow_nil</tt> option: If the target is not +nil+, or it is and
# responds to the method, everything works as usual. But if it is +nil+ and
# does not respond to the delegated method, +nil+ is returned.
#
# class User < ActiveRecord::Base
# has_one :profile
# delegate :age, to: :profile
# end
#
# User.new.age # raises NoMethodError: undefined method `age'
#
# But if not having a profile yet is fine and should not be an error
# condition:
#
# class User < ActiveRecord::Base
# has_one :profile
# delegate :age, to: :profile, allow_nil: true
# end
#
# User.new.age # nil
#
# Note that if the target is not +nil+ then the call is attempted regardless of the
# <tt>:allow_nil</tt> option, and thus an exception is still raised if said object
# does not respond to the method:
#
# class Foo
# def initialize(bar)
# @bar = bar
# end
#
# delegate :name, to: :@bar, allow_nil: true
# end
#
# Foo.new("Bar").name # raises NoMethodError: undefined method `name'
#
def delegate(*methods)
options = methods.pop
unless options.is_a?(Hash) && to = options[:to]
raise ArgumentError, 'Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, to: :greeter).'
end
prefix, allow_nil = options.values_at(:prefix, :allow_nil)
if prefix == true && to =~ /^[^a-z_]/
raise ArgumentError, 'Can only automatically set the delegation prefix when delegating to a method.'
end
method_prefix = \
if prefix
"#{prefix == true ? to : prefix}_"
else
''
end
file, line = caller.first.split(':', 2)
line = line.to_i
to = to.to_s
to = 'self.class' if to == 'class'
methods.each do |method|
# Attribute writer methods only accept one argument. Makes sure []=
# methods still accept two arguments.
definition = (method =~ /[^\]]=$/) ? 'arg' : '*args, &block'
# The following generated methods call the target exactly once, storing
# the returned value in a dummy variable.
#
# Reason is twofold: On one hand doing less calls is in general better.
# On the other hand it could be that the target has side-effects,
# whereas conceptualy, from the user point of view, the delegator should
# be doing one call.
if allow_nil
module_eval(<<-EOS, file, line - 3)
def #{method_prefix}#{method}(#{definition}) # def customer_name(*args, &block)
_ = #{to} # _ = client
if !_.nil? || nil.respond_to?(:#{method}) # if !_.nil? || nil.respond_to?(:name)
_.#{method}(#{definition}) # _.name(*args, &block)
end # end
end # end
EOS
else
exception = %(raise "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
module_eval(<<-EOS, file, line - 2)
def #{method_prefix}#{method}(#{definition}) # def customer_name(*args, &block)
_ = #{to} # _ = client
_.#{method}(#{definition}) # _.name(*args, &block)
rescue NoMethodError => e # rescue NoMethodError => e
location = "%s:%d:in `%s'" % [__FILE__, __LINE__ - 2, '#{method_prefix}#{method}'] # location = "%s:%d:in `%s'" % [__FILE__, __LINE__ - 2, 'customer_name']
if _.nil? && e.backtrace.first == location # if _.nil? && e.backtrace.first == location
#{exception} # # add helpful message to the exception
else # else
raise # raise
end # end
end # end
EOS
end
end
end
end
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