HasGuardedHandlers

HasGuardedHandlers allows an object's API to provide flexible handler registration, storage and matching to arbitrary events.

Installation

gem install has-guarded-handlers

Usage

require 'has_guarded_handlers'

class A
  include HasGuardedHandlers
end

a = A.new
a.register_handler :event, :type => :foo do |event|
  puts "Handled the event of type #{event.type} with value #{event.value}"
end

Event = Class.new Struct.new(:type, :value)

a.trigger_handler :event, Event.new(:foo, 'bar')

Register a handler for a particular named channel:

a.register_handler(:event) { ... }
# or
a.register_handler(:event, :type => :foo) { ... }

a.trigger_handler :event, :foo

Register a global handler for all channels:

a.register_handler { ... }
# or
a.register_handler(nil, :type => :foo) { ... }

a.trigger_handler :event, :foo

Register a temporary handler, which is deleted once triggered:

a.register_tmp_handler(:event) { ... } # This will only fire once
a.trigger_handler :event, :foo

Handlers are triggered in order of priority, followed by order of declaration. By default, all handlers are registered with priority 0, and are thus executed in the order declared:

a.register_handler { ... } # This is triggered first
a.register_handler { ... } # This is triggered second
...

a.trigger_handler :event, :foo

You may specify a handler priority in order to change this order. Higher priority is executed first:

a.register_handler(:event) { ... } # This is triggered second
a.register_handler_with_priority(:event, 10) { ... } # This is triggered first
...

a.trigger_handler :event, :foo

You may specify a priority for a temporary handler:

a.register_handler_with_options(:event, {:tmp => true, :priority => 10}, :foo => :bar) { ... }

Handler chaining

When multiple handlers match the event, the return value of each handler will determine if the handler chain continues. A truthy return value will cause the handler to swallow the event and halt the handler chain. A falsy return value will continue the chain.

It is possible to explicitly pass to the next handler by throwing :pass in your handler:

a.register_handler(:event) { throw :pass }
a.register_handler(:event) { ... } # This will be executed

a.trigger_handler :event, :foo

or indeed explicitly halt the handler chain by throwing :halt in the handler:

a.register_handler(:event) { throw :halt }
a.register_handler(:event) { ... } # This will not be executed

a.trigger_handler :event, :foo

What are guards?

Guards are a concept borrowed from Erlang. They help to better compartmentalise handlers.

There are a number of guard types and one bit of special syntax. Guards act like AND statements. Each condition must be met if the handler is to be used.

# Equivalent to saying (stanza.chat? && stanza.body)
message :chat?, :body

The different types of guards are:

# Class / Module
#   Checks that the event is of the type specified
#   Equivalent to event.is_a? Foo
register_handler Foo

# Symbol
#   Checks for a non-false reply to calling the symbol on the event
#   Equivalent to event.chat?
register_handler :chat?

# Hash with any value (:body => 'exit')
#   Calls the key on the event and checks for equality
#   Equivalent to event.body == 'exit'
register_handler :body => 'exit'

# Hash with regular expression (:body => /exit/)
#   Calls the key on the event and checks for a match
#   Equivalent to event.body.match /exit/
register_handler :body => /exit/

# Hash with array value (:name => [:gone, :forbidden])
#   Calls the key on the event and check for inclusion in the array
#   Equivalent to [:gone, :forbidden].include?(event.name)
register_handler :name => [:gone, :fobidden]

# Hash with array key ([:[], :name] => :gone)
#   Calls the first element of the key on the event, passing the other elements as arguments
#   and checks the value matches
#   Equivalent to event[:name] == :gone
register_handler [:[], :name] => :gone

# Proc
#   Calls the proc passing in the event
#   Checks that the ID is modulo 3
register_handler proc { |m| m.id % 3 == 0 }

# Array
#   Use arrays with the previous types effectively turns the guard into
#   an OR statement.
#   Equivalent to event.body == 'foo' || event.body == 'baz'
register_handler [{:body => 'foo'}, {:body => 'baz'}]

Links:

Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history.
    • If you want to have your own version, that is fine but bump version in a commit by itself so I can ignore when I pull
  • Send me a pull request. Bonus points for topic branches.

Copyright

Copyright (c) 2011 Ben Langfeld, Jeff Smick. MIT licence (see LICENSE.md for details).