Ruby Blocks as Dynamic Callbacks

November 27, 2011

Callbacks are a great technique for achieving simplicity and flexibility. Simply put, a callback is a block of code passed as an argument to a method. In Ruby, code blocks are everywhere and Ruby makes it trivial to pass a block of code to methods. For example:

def foo(bar, &block)
  callback = block
  callback.call(bar)
end

foo(5) {|x| x * x} # => 25

But what do we do when a method needs two blocks of code or more? Consider the classic case where we want a method to execute a block of code if an action succeeds or call different code if an action fails.

In this article, I will demonstrate how we can pass multiple blocks to a method and with some metaprogramming, we can achieve a dynamic callback mechanism with just a few lines of code.

Let’s add a method called callback to the Proc class:

class Proc
  def callback(callable, *args)
    self === Class.new do
      method_name = callable.to_sym
      define_method(method_name) { |&block| block.nil? ? true : block.call(*args) }
      define_method("#{method_name}?") { true }
      def method_missing(method_name, *args, &block) false; end
    end.new
  end
end

That’s it! The above Proc#callback method simply yields an anonymous class with methods defined to handle our callbacks. This allows for the capability of creating and storing dynamic callbacks, which can later be looked up and executed as needed.

Notice anything unusual? We’re using the === operand to invoke the block. Proc#=== is an alias for Proc.call. Anything on the right side of === acts as the proc’s parameter. Normally, this is to allow a proc object to be a target of a when clause in case statements, but we’re using it as a super simple way of invoking our anonymous class.

Let’s try it with something useful. Let’s say we’re writing something which needs to happen in an all-or-nothing, atomic fashion. Either the whole thing works, or none of it does. A simple case is tweeting:

def tweet(message, &block)
  Twitter.update(message)
  block.callback :success
rescue => e
  block.callback :failure, e.message
end

The tweet method accepts a message string and &block parameters. We call callback on the block and give it a name. Any name will work :success, :error, :fail!, whatever. In addition, we can pass arguments to the blocks (more on that later). Now we can provide a status if the tweet was successful or not:

tweet "Ruby methods with multiple blocks. #lolruby" do |on|
  on.success do
    puts "Tweet successful!"
  end
  on.failure do |status|
    puts "Error: #{status}"
  end
end

The advantage here is that we define our own mini DSL. We don’t need to worry about passing too many or unexpected blocks. We could have easily said where.success or on.error or update.fail!. Also note the on.failure block includes a status parameter - this contains the exception message captured in the tweet method above. So if Twitter was down for whatever reason, the on.failure block would be invoked and printed ‘Error: Twitter is down or being upgraded’.

Bonus: In addition to wrapping code in blocks, our Proc#callback method defines boolean style methods. So we could have call the tweet method like this if we wanted to:

tweet "Ruby methods with multiple blocks. #lolruby" do |update|
  puts "Tweet successful!" if update.success?
  puts "Sorry, something went wrong." if update.failure?
end

Put the Proc#callback method in a utility library and your code will look neat and tidy.

As always, I welcome your thoughts and feedback. Let me know what you think of the techniques shown here, or share your own favorite code block tricks.