adapter.md

Adapter Pattern

Problem

We want an object talk to some other object but their interfaces don't match.

Solution

We simply wrap the adaptee with our new adapter class. This class implements an interface that the invoker understands, although all the work is performed by the adapted object.

Example

Let's think of a class that takes two open files (a reader and a writer) and encrypts a file.

class Encrypter
  def initialize(key)
    @key = key
  end

  def encrypt(reader, writer)
    key_index = 0
    while not reader.eof?
      clear_char = reader.getc
      encrypted_char = clear_char ^ @key[key_index]
      writer.putc(encrypted_char)
      key_index = (key_index + 1) % @key.size
    end
  end
end

But what happens if the data we want to secure happens to be in a string, rather than in a file? We need an object that looks like an open file, that is, supports the same interface as the Ruby IO object. We can create an StringIOAdapter to achieve so:

class StringIOAdapter
  def initialize(string)
    @string = string
    @position = 0
  end

  def getc
    if @position >= @string.length
      raise EOFError
    end
    ch = @string[@position]
    @position += 1
    return ch
  end

  def eof?
    return @position >= @string.length
  end
end

Now we can use a String as if it were an open file (it only implements a small part of the IO interface, essentally what we need).

encrypter = Encrypter.new('XYZZY')
reader= StringIOAdapter.new('We attack at dawn')
writer=File.open('out.txt', 'w')
encrypter.encrypt(reader, writer)