I started looking into Ruby, since I am looking to a more dynamic alternative to Java.
I like how you can modify a class in Ruby after it's definition, for example like this:
class A
def print
"A"
end
end
class B < A
def print
super + "B"
end
end
class A
alias_method :print_orig, :print
def print
print_orig + "+"
end
end
puts B.new.print # A+B
Now I try to do the same with mixins:
class A
def print
"A"
end
end
class B < A
def print
super + "B"
end
end
module Plus
alias_method :print_orig, :print
def print
print_orig + "+"
end
end
A.extend(Plus) # variant 1
B.extend(Plus) # variant 2
class A # variant 3
include Plus
end
class B # variant 4
include Plus
end
puts B.new.print
However none of the variants produce the expected result. BTW, the expected result is the following: I want to be able to 'patch' class A with a mixin, in order to modify its behavior. I want to use mixins, since I want to 'patch' several classes with the same behavior.
Is it possible to do what I want? If yes, how?
Your module code doesn't work because it is executed in wrong context. You need to execute it in context of A, but it is instead evaluated in context of Plus. This means, you need to change self from Plus to A.
Observe:
class A
def print
"A"
end
end
class B < A
def print
super + "B"
end
end
module Plus
self # => Plus
def self.included base
self # => Plus
base # => A
base.class_eval do
self # => A
alias_method :print_orig, :print
def print
print_orig + "+"
end
end
end
end
A.send :include, Plus
B.new.print # => "A+B"
You can't really use Mixins in this way. You're generating a conflict between the class and its mixin. Mixins implicitly resolve the conflict by linearization. Bottom line is: In case of conflict, the class's method is preferred over the mixin. To fix that, you can use Sergio' Tulentsev's approach and have the mixin change its base class aggressively.
Or, you can add methods reflectively. Consider this example, which I've stolen from Mark's blog.
class Talker
[:hello, :good_bye].each do |arg|
method_name = ("say_" + arg.to_s).to_sym
send :define_method, method_name do
puts arg
end
end
end
t = Talker.new
t.say_hello
t.say_good_bye
I'm not sure that's the right title for this question, but I don't know how else to ask it. I have classes that need to be registered globally so they can be called later. I have most of it working except for a very important part. When the child inherits from the parent class, it registers a new instance, but when the on_message class method is called, I can't figure out how to set the instance variables that I need.
class MyExtension < ExtensionBase
on_message '/join (.+)' do |username|
# this will be a callback function used later
end
end
class ExtensionBase
def self.inherited(child)
MainAppModule.registered_extensions << child.new
end
def self.on_message(string, &block)
# these need to be set on child instance
#regex = Regexp.new(string)
#on_message_callback = block
end
def exec(message)
args = #regex.match(message).captures
#on_message_callback.call(args)
end
end
# somewhere else in the code, I find the class that I need...
MainAppModule.registered_extensions.each do |child|
puts child.regex.inspect # this is nil and I dont want it to be
if message =~ child.regex
return child.exec(message)
end
end
How can I design this so that the #regex will be set so I can access it within the loop?
I finally found a solution that works, and I have added now the whole code that is executable. Just store the code e.g. in file callexample.rb and call it by ruby callexample.rb
The main difference of my solution to the question is that the call to on_message now creates the instance with the relevant arguments and registers the created instance. Therefore I have deleted the method inherited because I don't need it any more.
I have added some puts statements to demonstrate in which order the code works.
class MainAppModule ## Added class
##registered_extensions = []
def self.registered_extensions; ##registered_extensions; end
end
class ExtensionBase
attr_reader :regex
def self.on_message(string, &block)
MainAppModule.registered_extensions << self.new(string, block)
end
def initialize(string, block)
#regex = Regexp.new(string)
#on_message_callback = block
end
def exec(message)
args = #regex.match(message).captures
#on_message_callback.call(args)
end
end
class MyExtension < ExtensionBase
on_message '/join (.+)' do |username|
# this will be a callback function used later
puts "Callback of #{self} called."
"returnvalue"
end
end
# somewhere else in the code, I find the class that I need...
MainAppModule.registered_extensions.each do |child|
puts "Value of regex: #{child.regex}" # this is no more nil
message = '/join something'
if message =~ child.regex
puts "On match evalue 'child.exec(message)' to: #{child.exec(message)}"
end
end
I would like to write a method in ruby that takes a class with certain methods and modifies its behavior by adding methods or changing how existing methods work. I would like to do this in a way that doesn't modify the base class so basically I want a function that takes a class and returns a new modified class without harming the initial class. I'm pretty sure this is possible but I'm not sure where to start.
You have a couple options:
you can use x = Class.new(Parent) { def meth; puts "hello"; super; puts "bye"; end } to dynamically define a class and override methods (& define new ones)
you can use a Delegator
So for instance, if you wanted to dynamically create classes that logged certain method calls:
class Class
def logging_subclass(*methods)
Class.new(self) do
methods.each do |method|
define_method(method) do |*args,&blk|
puts "calling #{method}"
ret = super(*args,&blk)
puts "#{method} returned #{ret.inspect}"
ret
end
end
end
end
end
class One
def foo
"I'm foo!"
end
end
# this prints nothing
One.new.foo #=> returns :foo
# this prints:
# > calling foo
# > foo returned "I'm foo!"
One.logging_subclass(:foo).new.foo #=> returns :foo
Note that you need ruby 1.9 to support capturing do |&blk| (capturing blocks in block arguments).
I'd suggest using inheritance or a mixin; in my opinion, the use of a mixin would be a wiser idea though using inheritance is easier for a newbie.
Remember, you can always inherit from the class and change behavior or wrap it with new code as desired.
class Mammal
def speak
"..."
end
end
class Cat < Mammal
def speak
"meow"
end
end
class Lion < Cat
def speak
"get ready for a big " + super + "!"
end
end
module Asexual_Critter
def reproduce(critter_list)
puts "*poink!*"
critter_list << self.clone
end
end
class Mutated_Kitty < Cat
include Asexual_Critter # inane example I know, but functional...
end
Just remember that if you want to play with this not to do:
critters = [Mutated_Kitty.new]
begin
critters.each { |c| c.reproduce(critters) }
end while critters.length > 0
Or else you'll be in for a long wait until you run out of RAM, or perhaps segfault.
class A
def a
puts 'in #a'
end
end
class B < A
def a
b()
end
def b
# here i want to call A#a.
end
end
class B < A
alias :super_a :a
def a
b()
end
def b
super_a()
end
end
There's no nice way to do it, but you can do A.instance_method(:a).bind(self).call, which will work, but is ugly.
You could even define your own method in Object to act like super in java:
class SuperProxy
def initialize(obj)
#obj = obj
end
def method_missing(meth, *args, &blk)
#obj.class.superclass.instance_method(meth).bind(#obj).call(*args, &blk)
end
end
class Object
private
def sup
SuperProxy.new(self)
end
end
class A
def a
puts "In A#a"
end
end
class B<A
def a
end
def b
sup.a
end
end
B.new.b # Prints in A#a
If you don't explicitly need to call A#a from B#b, but rather need to call A#a from B#a, which is effectively what you're doing by way of B#b (unless you're example isn't complete enough to demonstrate why you're calling from B#b, you can just call super from within B#a, just like is sometimes done in initialize methods. I know this is kind of obvious, I just wanted to clarify for any Ruby new-comers that you don't have to alias (specifically this is sometimes called an "around alias") in every case.
class A
def a
# do stuff for A
end
end
class B < A
def a
# do some stuff specific to B
super
# or use super() if you don't want super to pass on any args that method a might have had
# super/super() can also be called first
# it should be noted that some design patterns call for avoiding this construct
# as it creates a tight coupling between the classes. If you control both
# classes, it's not as big a deal, but if the superclass is outside your control
# it could change, w/o you knowing. This is pretty much composition vs inheritance
end
end
I'm not sure of the best idiom for C style call-backs in Ruby - or if there is something even better ( and less like C ). In C, I'd do something like:
void DoStuff( int parameter, CallbackPtr callback )
{
// Do stuff
...
// Notify we're done
callback( status_code )
}
Whats a good Ruby equivalent? Essentially I want to call a passed in class method, when a certain condition is met within "DoStuff"
The ruby equivalent, which isn't idiomatic, would be:
def my_callback(a, b, c, status_code)
puts "did stuff with #{a}, #{b}, #{c} and got #{status_code}"
end
def do_stuff(a, b, c, callback)
sum = a + b + c
callback.call(a, b, c, sum)
end
def main
a = 1
b = 2
c = 3
do_stuff(a, b, c, method(:my_callback))
end
The idiomatic approach would be to pass a block instead of a reference to a method. One advantage a block has over a freestanding method is context - a block is a closure, so it can refer to variables from the scope in which it was declared. This cuts down on the number of parameters do_stuff needs to pass to the callback. For instance:
def do_stuff(a, b, c, &block)
sum = a + b + c
yield sum
end
def main
a = 1
b = 2
c = 3
do_stuff(a, b, c) { |status_code|
puts "did stuff with #{a}, #{b}, #{c} and got #{status_code}"
}
end
This "idiomatic block" is a very core part of everyday Ruby and is covered frequently in books and tutorials. The Ruby information section provides links to useful [online] learning resources.
The idiomatic way is to use a block:
def x(z)
yield z # perhaps used in conjunction with #block_given?
end
x(3) {|y| y*y} # => 9
Or perhaps converted to a Proc; here I show that the "block", converted to a Proc implicitly with &block, is just another "callable" value:
def x(z, &block)
callback = block
callback.call(z)
end
# look familiar?
x(4) {|y| y * y} # => 16
(Only use the above form to save the block-now-Proc for later use or in other special cases as it adds overhead and syntax noise.)
However, a lambda can be use just as easily (but this is not idiomatic):
def x(z,fn)
fn.call(z)
end
# just use a lambda (closure)
x(5, lambda {|y| y * y}) # => 25
While the above approaches can all wrap "calling a method" as they create closures, bound Methods can also be treated as first-class callable objects:
class A
def b(z)
z*z
end
end
callable = A.new.method(:b)
callable.call(6) # => 36
# and since it's just a value...
def x(z,fn)
fn.call(z)
end
x(7, callable) # => 49
In addition, sometimes it's useful to use the #send method (in particular if a method is known by name). Here it saves an intermediate Method object that was created in the last example; Ruby is a message-passing system:
# Using A from previous
def x(z, a):
a.__send__(:b, z)
end
x(8, A.new) # => 64
Happy coding!
Explored the topic a bit more and updated the code.
The following version is an attempt to generalize the technique, although remaining extremely simplified and incomplete.
I largely stole - hem, found inspiration in - the implementation of callbacks of DataMapper, which seems to me quite complete and beatiful.
I strongly suggest to have a look at the code # http://github.com/datamapper/dm-core/blob/master/lib/dm-core/support/hook.rb
Anyway, trying to reproduce the functionality using the Observable module was quite engaging and instructive.
A few notes:
method added seems to be require because the original instance methods are not available at the moment of registering the callbacks
the including class is made both observed and self-observer
the example is limited to the instance methods, does not support blocks, args and so on
code:
require 'observer'
module SuperSimpleCallbacks
include Observable
def self.included(klass)
klass.extend ClassMethods
klass.initialize_included_features
end
# the observed is made also observer
def initialize
add_observer(self)
end
# TODO: dry
def update(method_name, callback_type) # hook for the observer
case callback_type
when :before then self.class.callbacks[:before][method_name.to_sym].each{|callback| send callback}
when :after then self.class.callbacks[:after][method_name.to_sym].each{|callback| send callback}
end
end
module ClassMethods
def initialize_included_features
#callbacks = Hash.new
#callbacks[:before] = Hash.new{|h,k| h[k] = []}
#callbacks[:after] = #callbacks[:before].clone
class << self
attr_accessor :callbacks
end
end
def method_added(method)
redefine_method(method) if is_a_callback?(method)
end
def is_a_callback?(method)
registered_methods.include?(method)
end
def registered_methods
callbacks.values.map(&:keys).flatten.uniq
end
def store_callbacks(type, method_name, *callback_methods)
callbacks[type.to_sym][method_name.to_sym] += callback_methods.flatten.map(&:to_sym)
end
def before(original_method, *callbacks)
store_callbacks(:before, original_method, *callbacks)
end
def after(original_method, *callbacks)
store_callbacks(:after, original_method, *callbacks)
end
def objectify_and_remove_method(method)
if method_defined?(method.to_sym)
original = instance_method(method.to_sym)
remove_method(method.to_sym)
original
else
nil
end
end
def redefine_method(original_method)
original = objectify_and_remove_method(original_method)
mod = Module.new
mod.class_eval do
define_method(original_method.to_sym) do
changed; notify_observers(original_method, :before)
original.bind(self).call if original
changed; notify_observers(original_method, :after)
end
end
include mod
end
end
end
class MyObservedHouse
include SuperSimpleCallbacks
before :party, [:walk_dinosaure, :prepare, :just_idle]
after :party, [:just_idle, :keep_house, :walk_dinosaure]
before :home_office, [:just_idle, :prepare, :just_idle]
after :home_office, [:just_idle, :walk_dinosaure, :just_idle]
before :second_level, [:party]
def home_office
puts "learning and working with ruby...".upcase
end
def party
puts "having party...".upcase
end
def just_idle
puts "...."
end
def prepare
puts "preparing snacks..."
end
def keep_house
puts "house keeping..."
end
def walk_dinosaure
puts "walking the dinosaure..."
end
def second_level
puts "second level..."
end
end
MyObservedHouse.new.tap do |house|
puts "-------------------------"
puts "-- about calling party --"
puts "-------------------------"
house.party
puts "-------------------------------"
puts "-- about calling home_office --"
puts "-------------------------------"
house.home_office
puts "--------------------------------"
puts "-- about calling second_level --"
puts "--------------------------------"
house.second_level
end
# => ...
# -------------------------
# -- about calling party --
# -------------------------
# walking the dinosaure...
# preparing snacks...
# ....
# HAVING PARTY...
# ....
# house keeping...
# walking the dinosaure...
# -------------------------------
# -- about calling home_office --
# -------------------------------
# ....
# preparing snacks...
# ....
# LEARNING AND WORKING WITH RUBY...
# ....
# walking the dinosaure...
# ....
# --------------------------------
# -- about calling second_level --
# --------------------------------
# walking the dinosaure...
# preparing snacks...
# ....
# HAVING PARTY...
# ....
# house keeping...
# walking the dinosaure...
# second level...
This simple presentation of the use of Observable could be useful: http://www.oreillynet.com/ruby/blog/2006/01/ruby_design_patterns_observer.html
So, this may be very "un-ruby", and I am not a "professional" Ruby developer, so if you guys are going to smack be, be gentle please :)
Ruby has a built-int module called Observer. I have not found it easy to use, but to be fair I did not give it much of a chance. In my projects I have resorted to creating my own EventHandler type (yes, I use C# a lot). Here is the basic structure:
class EventHandler
def initialize
#client_map = {}
end
def add_listener(id, func)
(#client_map[id.hash] ||= []) << func
end
def remove_listener(id)
return #client_map.delete(id.hash)
end
def alert_listeners(*args)
#client_map.each_value { |v| v.each { |func| func.call(*args) } }
end
end
So, to use this I expose it as a readonly member of a class:
class Foo
attr_reader :some_value_changed
def initialize
#some_value_changed = EventHandler.new
end
end
Clients of the "Foo" class can subscribe to an event like this:
foo.some_value_changed.add_listener(self, lambda { some_func })
I am sure this is not idiomatic Ruby and I am just shoehorning my C# experience into a new language, but it has worked for me.
If you are willing to use ActiveSupport (from Rails), you have a straightforward implementation
class ObjectWithCallbackHooks
include ActiveSupport::Callbacks
define_callbacks :initialize # Your object supprots an :initialize callback chain
include ObjectWithCallbackHooks::Plugin
def initialize(*)
run_callbacks(:initialize) do # run `before` callbacks for :initialize
puts "- initializing" # then run the content of the block
end # then after_callbacks are ran
end
end
module ObjectWithCallbackHooks::Plugin
include ActiveSupport::Concern
included do
# This plugin injects an "after_initialize" callback
set_callback :initialize, :after, :initialize_some_plugin
end
end
I know this is an old post, but I found it when tried to solve a similar problem.
It's a really elegant solution, and most importantly, it can work with and without a callback.
Let's say we have the Arithmetic class which implements basic operations on them — addition and subtraction.
class Arithmetic
def addition(a, b)
a + b
end
def subtraction(a, b)
a - b
end
end
And we want to add a callback for each operation which will do something with the input data and result.
In the below example we will implement the after_operation method which accepts the Ruby block which will be executed after an operation.
class Arithmetic
def after_operation(&block)
#after_operation_callback = block
end
def addition(a, b)
do_operation('+', a, b)
end
def subtraction(a, b)
do_operation('-', a, b)
end
private
def do_operation(sign, a, b)
result =
case sign
when '+'
a + b
when '-'
a - b
end
if callback = #after_operation_callback
callback.call(sign, a, b, result)
end
result
end
end
Using with callback:
callback = -> (sign, a, b, result) do
puts "#{a} #{sign} #{b} = #{result}"
end
arithmetic = Arithmetic.new
arithmetic.after_operation(&callback)
puts arithmetic.addition(1, 2)
puts arithmetic.subtraction(3, 1)
Output:
1 + 2 = 3
3
3 - 1 = 2
2
I often implement callbacks in Ruby like in the following example. It's very comfortable to use.
class Foo
# Declare a callback.
def initialize
callback( :on_die_cast )
end
# Do some stuff.
# The callback event :on_die_cast is triggered.
# The variable "die" is passed to the callback block.
def run
while( true )
die = 1 + rand( 6 )
on_die_cast( die )
sleep( die )
end
end
# A method to define callback methods.
# When the latter is called with a block, it's saved into a instance variable.
# Else a saved code block is executed.
def callback( *names )
names.each do |name|
eval <<-EOF
##{name} = false
def #{name}( *args, &block )
if( block )
##{name} = block
elsif( ##{name} )
##{name}.call( *args )
end
end
EOF
end
end
end
foo = Foo.new
# What should be done when the callback event is triggered?
foo.on_die_cast do |number|
puts( number )
end
foo.run
I know this is an old post, but others that come across this may find my solution helpful.
http://chrisshepherddev.blogspot.com/2015/02/callbacks-in-pure-ruby-prepend-over.html