gistfile1.rb

require 'rubygems'
require 'spec'

class Mine
  attr_reader :foo, :bar
  def initialize
    @foo = 'unchanged'
    @bar = 'unchanged'
  end

  def atomic(*ivars, &block)
    ivars = instance_variables if ivars.empty?
    before_vals = ivars.inject({}) {|h,i| h[i]=instance_variable_get("@#{i}"); h}
    begin
      instance_eval &block
    rescue => e
      before_vals.each {|k,v| instance_variable_set("@#{k}", v) }
      raise e
    end
  end
end

describe "atomic changes" do
  it "should reraise an error raised" do
    lambda {
      Mine.new.atomic(:foo, :bar) do
        @foo = 'ignore'
        @bar = 'ignore'
        raise 'an error'
      end
    }.should raise_error(/an error/)
  end

  it "should reset the ivars if there's an error" do
    o = Mine.new
    o.atomic(:foo, :bar) {
      @foo = 'changed'
      @bar = 'changed'
      raise 'an error'
    } rescue nil

    o.foo.should == 'unchanged'
    o.bar.should == 'unchanged'
  end

  it "should make all ivars atomic if none specified" do
    o = Mine.new
    o.atomic {
      @foo = 'changed'
      @bar = 'changed'
      raise 'an error'
    } rescue nil

    o.foo.should == 'unchanged'
    o.bar.should == 'unchanged'
  end
end