callwith

callwith

Synopsis

Object#callwith is like instance_eval, but can still delegate back to the original object if a method is not found. Also unlike instance_eval, all instance variables accessed within the block reference the original self object.

gem install callwith

callwith(obj) do
  # stuff
end

(this method was originally called with when it was part of RubyTreasures, but has been renamed to callwith to avoid conflict with the with gem).

Background

One common problem with using instance_eval as a tool for brevity is that it can easily become unclear which object is self. For example, the Tk library that comes with Ruby makes heavy use of instance_eval:

class MyApplication
  def initialize
    @root = TkRoot.new
    TkFrame.new do
      TkButton.new do
        text "Ok"

        # The action() method will never be called, because this
        # block was called with instance_eval, so `self` is not
        # the MyApplication instance.
        command proc { action() }

        pack
      end
      pack
    end
  end

  def action()
    puts "Hello, world"
  end
end

The callwith method will delegate method calls to the passed object; however, if the passed object does not have such a method, callwith will fall back onto calling on self. For example:

class HelloWorld
  def initialize
    @hello_count = 0
  end

  def hello(file)
    callwith(file) do
      write("Hello world") # writes "Hello world" to file
      increment()          # calls increment on the HelloWorld instance
    end
  end

  def increment
    @hello_count += 1
  end
end

All accessed instance variables are in the self object, so we could have written hello() like this:

def hello(file)
  callwith(file) do
    write("Hello world")
    increment()
  end
end

The callwith method can also iterate over multiple objects, e.g.:

def hello(*files)
  callwith(*files) do
    write("Hello world")
    increment()
  end
end

License

License is the MIT License (http://www.opensource.org/licenses/mit-license.html)