Hooks

Generic hooks with callbacks for Ruby.

Introduction

Hooks lets you define hooks declaratively in your ruby class. You can add callbacks to your hook, which will be run as soon as you run the hook!

It's almost like ActiveSupport::Callbacks but 76,6% less complex. Instead, it is not more than a few lines of code, one method compilation, no method_missing and no magic.

Also, you may pass additional arguments to your callbacks when invoking a hook.

Example

Let's take... a cat.

require 'hooks'

class Cat
  include Hooks

  define_hooks :before_dinner, :after_dinner

Now you can add callbacks to your hook declaratively in your class.

  before_dinner :wash_paws

  after_dinner do
    puts "Ice cream for #{self}!"
  end

  after_dinner :have_a_desert   # => refers to Cat#have_a_desert

  def have_a_desert
    puts "Hell, yeah!"
  end

This will run the block and #have_a_desert from above.

cat.run_hook :after_dinner
# => Ice cream for #<Cat:0x8df9d84>!
     Hell, yeah!

Callback blocks and methods will be executed with instance context. Note how self in the block refers to the Cat instance.

Inheritance

Hooks are inherited, here's a complete example to put it all together.

class Garfield < Cat

  after_dinner :want_some_more

  def want_some_more
    puts "Is that all?"
  end
end


Garfield.new.run_hook :after_dinner
# => Ice cream for #<Cat:0x8df9d84>!
     Hell, yeah!
     Is that all?

Note how the callbacks are invoked in the order they were inherited.

Options for Callbacks

You're free to pass any number of arguments to #run_callback, those will be passed to the callbacks.

cat.run_hook :before_dinner, cat, Time.now

The callbacks should be ready for receiving parameters.

before_dinner :wash_pawns
before_dinner do |who, when|
  ...
end

def wash_pawns(who, when)

Not sure why a cat should have ice cream for dinner. Beside that, I was tempted naming this gem hooker.

Running And Halting Hooks

Using #run_hook doesn't only run all callbacks for this hook but also returns an array of the results from each callback method or block.

class Garfield
  include Hooks
  define_hook :after_dark

  after_dark { "Chase mice" }
  after_dark { "Enjoy supper" }
end

Garfield.new.run_hook :after_dark
# => ["Chase mice", "Enjoy supper"]

This is handy if you need to collect data from your callbacks without having to access a global (brrr) variable.

With the :halts_on_falsey option you can halt the callback chain when a callback returns nil or false.

class Garfield
  include Hooks
  define_hook :after_dark, halts_on_falsey: true

  after_dark { "Chase mice" }
  after_dark { nil }
  after_dark { "Enjoy supper" }
end

result = Garfield.new.run_hook :after_dark
# => ["Chase mice"]

This will only run the first two callbacks. Note that the result doesn't contain the nil value. You even can check if the chain was halted.

result.halted? #=> true

Execution Scope

Normally, callbacks are executed in object context. You're free to provide your own context object using :scope.

define_hook :before_lunch, scope: lambda { |callback, scope| Logger }

This will evaluate the lambda in Logger class context.

Return nil to execute the lambda in the original context.

define_hook :before_lunch, scope: lambda { |*| nil }
what = "hands"
before_lunch << lambda { puts "wash #{what}" } # executed in original context.

The :scope lambda is executed for every added callback per run, hence the block options.

Instance Hooks

You can also define hooks and/or add callbacks per instance. This is helpful if your class should define a basic set of hooks and callbacks that are then extended by instances.

class Cat
  include Hooks
  include Hooks::InstanceHooks

  define_hook :after_dark

  after_dark { "Chase mice" }
end

Note that you have to include Hooks::InstanceHooks to get this additional functionality.

Callbacks can now be added to a single object.

garfield = Cat.new

garfield.after_dark :sleep
garfield.run_hook(:after_dark) # => invoke "Chase mice" hook and #sleep

What happens is that the garfield object inherits existing hooks with all their callbacks from the Cat class, namely, this is :after_dark. It can then add local callbacks (or even more hooks) to itself without affecting the Cat class.

Naturally, adding new hooks works like-wise.

garfield.define_hook :before_six
garfield.before_six { .. }

This feature was added in 0.3.2.

Installation

In your Gemfile, do

gem "hooks"

Anybody using it?

• Hooks is already used in Apotomo, a hot widget framework for Rails.
• The datamappify gem uses hooks and the author Fred Wu contributed to this gem!

Similar libraries

• http://github.com/nakajima/aspectory
• http://github.com/auser/backcall
• http://github.com/mmcgrana/simple_callbacks

License

Copyright (c) 2013-2015, Nick Sutterer

Released under the MIT License.