In Files
- forwardable.rb
- forwardable/impl.rb
Class/Module Index
![[+]](./images/find.png "show/hide quicksearch")
Forwardable
The Forwardable module provides delegation of specified methods to a designated object, using the methods def_delegator and def_delegators.
For example, say you have a class RecordCollection which contains an array @records. You could provide the lookup method record_number(), which simply calls [] on the @records array, like this:
require 'forwardable' class RecordCollection attr_accessor :records extend Forwardable def_delegator :@records, :[], :record_number end
We can use the lookup method like so:
r = RecordCollection.new r.records = [4,5,6] r.record_number(0) # => 4
Further, if you wish to provide the methods size, <<, and map, all of which delegate to @records, this is how you can do it:
class RecordCollection # re-open RecordCollection class def_delegators :@records, :size, :<<, :map end r = RecordCollection.new r.records = [1,2,3] r.record_number(0) # => 1 r.size # => 3 r << 4 # => [1, 2, 3, 4] r.map { |x| x * 2 } # => [2, 4, 6, 8]
You can even extend regular objects with Forwardable.
my_hash = Hash.new my_hash.extend Forwardable # prepare object for delegation my_hash.def_delegator "STDOUT", "puts" # add delegation for STDOUT.puts() my_hash.puts "Howdy!"
Another example¶ ↑
We want to rely on what has come before obviously, but with delegation we can take just the methods we need and even rename them as appropriate. In many cases this is preferable to inheritance, which gives us the entire old interface, even if much of it isn't needed.
class Queue extend Forwardable def initialize @q = [ ] # prepare delegate object end # setup preferred interface, enq() and deq()... def_delegator :@q, :push, :enq def_delegator :@q, :shift, :deq # support some general Array methods that fit Queues well def_delegators :@q, :clear, :first, :push, :shift, :size end q = Queue.new q.enq 1, 2, 3, 4, 5 q.push 6 q.shift # => 1 while q.size > 0 puts q.deq end q.enq "Ruby", "Perl", "Python" puts q.first q.clear puts q.first
This should output:
2 3 4 5 6 Ruby nil
Notes¶ ↑
Be advised, RDoc will not detect delegated methods.
forwardable.rb provides single-method delegation via the def_delegator and def_delegators methods. For full-class delegation via DelegateClass, see delegate.rb.
Constants
- FORWARDABLE_VERSION
Version of
forwardable.rb
Public Instance Methods
Define method as delegator instance method with an optional alias name ali. Method calls to ali will be delegated to accessor.method.
class MyQueue extend Forwardable attr_reader :queue def initialize @queue = [] end def_delegator :@queue, :push, :mypush end q = MyQueue.new q.mypush 42 q.queue #=> [42] q.push 23 #=> NoMethodError
# File forwardable.rb, line 180
def def_instance_delegator(accessor, method, ali = method)
gen = Forwardable._delegator_method(self, accessor, method, ali)
# If it's not a class or module, it's an instance
(Module === self ? self : singleton_class).module_eval(&gen)
end
Shortcut for defining multiple delegator methods, but with no provision for using a different name. The following two code samples have the same effect:
def_delegators :@records, :size, :<<, :map def_delegator :@records, :size def_delegator :@records, :<< def_delegator :@records, :map
# File forwardable.rb, line 153
def def_instance_delegators(accessor, *methods)
methods.delete("__send__")
methods.delete("__id__")
for method in methods
def_instance_delegator(accessor, method)
end
end
Takes a hash as its argument. The key is a symbol or an array of symbols. These symbols correspond to method names. The value is the accessor to which the methods will be delegated.
# File forwardable.rb, line 132
def instance_delegate(hash)
hash.each do |methods, accessor|
unless defined?(methods.each)
def_instance_delegator(accessor, methods)
else
methods.each {|method| def_instance_delegator(accessor, method)}
end
end
end