Guard 2.0 is a major update and comes with several API changes. Most of these changes are actually new methods (the old methods are deprecated). Following is the exhaustive list of these changes (mostly useful for Guard plugins authors).

• Guard.plugin(filter): Allows to return the first plugin matching the given filter. Filtering works exactly as for Guard.plugins(filter = nil).
• Guard.group(filter): Allows to return the first group matching the given filter. Filtering works exactly as for Guard.groups(filter = nil).

• Guard.guards(filter = nil) is deprecated and replaced by Guard.plugins(filter = nil)
• Guard.add_guard(name, options = {}) is deprecated and replaced by Guard.add_plugin(name, options = {})
• Guard.get_guard_class(name, fail_gracefully = false) is deprecated and replaced by Guard::PluginUtil.new(name).plugin_class(:fail_gracefully => fail_gracefully)
• Guard.locate_guard(name) is deprecated and replaced by Guard::PluginUtil.new(name).plugin_location
• Guard.guard_gem_names is deprecated and replaced by Guard::PluginUtil.plugin_names

To remove the confusion between Guard and its many plugins (guard-rspec, guard-pow, guard-livereload etc.), Guard::Guard is renamed Guard::Plugin. Guard::Guard is deprecated so if you're a plugin maintainer, in your next (major) release that will depend on Guard ~> 2.0 you must make your plugin inherit from Guard::Plugin instead of Guard::Guard.

Also, please be sure to change the #initialize signature from #initialize(watchers = [], options = {}) to #initialize(options = {}) (watchers are now passed in the options argument directly).

For instance, for guard-rspec:

module Guard
  class RSpec < Guard
    def initialize(watchers = [], options = {})
      super
      # rest of the implementation...
    end
  end
end

should become:

module Guard
  class RSpec < Plugin
    def initialize(options = {})
      super
      # you can still access the watchers with options[:watchers]
      # rest of the implementation...
    end
  end
end

• Guard::Dsl.evaluate_guardfile(options) is deprecated and replaced by Guard::Guardfile::Evaluator.new(options).evaluate_guardfile

• Guard::Guardfile.create_guardfile(options) is deprecated and replaced by Guard::Guardfile::Generator.new(options).create_guardfile
• Guard::Guardfile.initialize_template(plugin_name) is deprecated and replaced by Guard::Guardfile::Generator.new.initialize_template(plugin_name)
• Guard::Guardfile.initialize_all_templates is deprecated and replaced by Guard::Guardfile::Generator.new.initialize_all_templates

• Guard::Guardfile.duplicate_definitions? has been removed and replaced by Guard::Guardfile::Evaluator#guardfile_include?(plugin_name)

Notifiers are now classes with a common interface inherited from Guard::Notifier::Base:

• [required] #notify(message, opts = {})
• [optional] .supported_hosts
• [optional] .available?(opts = {})
• [optional] .gem_name

Breaking change: The signature of #notify in the notifiers has changed from #notify(type, title, message, image, options = {}) to notify(message, opts = {}) (:type, :title and :image must now be passed in the opts hash.

Even though the notifiers system (including the notifiers #notify methods signature) has been completely rewritten, this shouldn't create any issues since individual notifiers' interface shouldn't be called directly. You should always use the Guard::Notifier.notify method instead.