How do I create a Ruby class that initializes and closes when passed a block?

Question

Ruby has the File class that can be initialized using the normal new() method, or using open() and passing a block. How would I write a class that behaved like this?

File.open("myfile.txt","r") do |f|
...
end

Answer 1

The general outline of File.open is something like this:

def open(foo, bar)
  f = do_opening_stuff(foo, bar)
  begin
    yield f
  ensure
    do_closing_stuff(f)
  end
end

It is yield that invokes the block passed by the caller. Putting do_closing_stuff within an ensure guarantees that the file gets closed even if the block raises an exception.

More nitty-gritty on the various ways of calling blocks here: http://innig.net/software/ruby/closures-in-ruby

Answer 2

This is a simple example of passing a block to new/open method

class Foo
  def initialize(args, &block)
    if block_given?
       p block.call(args) # or do_something
    else
      #do_something else
    end

  end

  def self.open(args, &block)
    if block_given?
      a = new(args, &block)  # or do_something
    else
      #do_something else
    end
  ensure
    a.close
  end

  def close
     puts "closing"
  end


end

Foo.new("foo") { |x|  "This is #{x} in new" } 
# >> "This is foo in new"
Foo.open("foo") { |x|  "This is #{x} in open" } 
# >> "This is foo in open"
# >> closing