In Files
- forwardable.rb
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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:
class RecordCollection extend Forwardable def_delegator :@records, :[], :record_number end
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 # extend Forwardable, but we did that above def_delegators :@records, :size, :<<, :map end f = Foo.new f.printf ... f.gets f.content_at(1)
If the object isn't a Module and Class, You can too extend Forwardable module.
printer = String.new printer.extend Forwardable # prepare object for delegation printer.def_delegator "STDOUT", "puts" # add delegation for STDOUT.puts() printer.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
Attributes
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 167
def def_instance_delegator(accessor, method, ali = method)
line_no = __LINE__; str = %{
def #{ali}(*args, &block)
begin
#{accessor}.__send__(:#{method}, *args, &block)
rescue Exception
$@.delete_if{|s| %r"#{Regexp.quote(__FILE__)}"o =~ s} unless Forwardable::debug
::Kernel::raise
end
end
}
# If it's not a class or module, it's an instance
begin
module_eval(str, __FILE__, line_no)
rescue
instance_eval(str, __FILE__, line_no)
end
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 140
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 120
def instance_delegate(hash)
hash.each{ |methods, accessor|
methods = [methods] unless methods.respond_to?(:each)
methods.each{ |method|
def_instance_delegator(accessor, method)
}
}
end