How to Wrap the Invocation of a Ruby Method by Including a Module

Ruby module to print all method calls and returns in class

You can achieve that by defining a new wrapper method for each public instance method of the class that performs the log logic that you need.

module Loginator
  def logify_me
    self.public_instance_methods.each do |method|
      proxy = Module.new do
        define_method(method) do |*args|
          puts "Method #{method}(#{args.join(', ')}) called"
          # added join to match the exact desired output

          value = super *args

          puts "returns #{value}"

          value
        end
      end
      self.prepend proxy
    end
  end
end

class A
  extend Loginator

  def add(a, b)
    a + b
  end

  def sub(a, b)
    a - b
  end

  logify_me
end

Which yields the following output:

>> A.new.sub(1,2)
Method 'sub' called with args '[1, 2]'
returns -1
=> -1

>> A.new.add(4,7)
Method 'add' called with args '[4, 7]'
returns 11
=> 11

Explanation for the future :)

• define_method(method) :

  Defines an instance method in the receiver. The method parameter can be a Proc, a Method, or an UnboundMethod object. If a block is specified, it is used as the method body. This block is evaluated using instance_eval ref. can be found here

• prepend

  prepend will insert the module at the bottom of the chain, even before the class itself.fourth ref on my post

Section to Edit by @pedrosfdcarneiro

please, add some brief explanation about the second inner module proxy = Module.new do end and why it's important for the return value and the usage of the super.

plus why did you preferred to use the public_instance_methods over instance_methods.

please kindly add some explanation about it because first its a bit unclear to me and secondly for the other noobs out there.
thanks in advance :)

This answer was heavily based on this fantastic answer: https://stackoverflow.com/a/23898539/1781212.

Executing code for every method call in a Ruby module

Like this:

module M
  def self.before(*names)
    names.each do |name|
      m = instance_method(name)
      define_method(name) do |*args, &block|  
        yield
        m.bind(self).(*args, &block)
      end
    end
  end
end

module M
  def hello
    puts "yo"
  end

  def bye
    puts "bum"
  end

  before(*instance_methods) { puts "start" }
end

class C
  include M
end

C.new.bye #=> "start" "bum"
C.new.hello #=> "start" "yo"

When monkey patching an instance method, can you call the overridden method from the new implementation?

EDIT: It has been 9 years since I originally wrote this answer, and it deserves some cosmetic surgery to keep it current.

You can see the last version before the edit here.

---

You can’t call the overwritten method by name or keyword. That’s one of the many reasons why monkey patching should be avoided and inheritance be preferred instead, since obviously you can call the overridden method.

Avoiding Monkey Patching

Inheritance

So, if at all possible, you should prefer something like this:

class Foo
  def bar
    'Hello'
  end
end 

class ExtendedFoo < Foo
  def bar
    super + ' World'
  end
end

ExtendedFoo.new.bar # => 'Hello World'

This works, if you control creation of the Foo objects. Just change every place which creates a Foo to instead create an ExtendedFoo. This works even better if you use the Dependency Injection Design Pattern, the Factory Method Design Pattern, the Abstract Factory Design Pattern or something along those lines, because in that case, there is only place you need to change.

Delegation

If you do not control creation of the Foo objects, for example because they are created by a framework that is outside of your control (like ruby-on-rails for example), then you could use the Wrapper Design Pattern:

require 'delegate'

class Foo
  def bar
    'Hello'
  end
end 

class WrappedFoo < DelegateClass(Foo)
  def initialize(wrapped_foo)
    super
  end

  def bar
    super + ' World'
  end
end

foo = Foo.new # this is not actually in your code, it comes from somewhere else

wrapped_foo = WrappedFoo.new(foo) # this is under your control

wrapped_foo.bar # => 'Hello World'

Basically, at the boundary of the system, where the Foo object comes into your code, you wrap it into another object, and then use that object instead of the original one everywhere else in your code.

This uses the Object#DelegateClass helper method from the delegate library in the stdlib.

“Clean” Monkey Patching

Module#prepend: Mixin Prepending

The two methods above require changing the system to avoid monkey patching. This section shows the preferred and least invasive method of monkey patching, should changing the system not be an option.

Module#prepend was added to support more or less exactly this use case. Module#prepend does the same thing as Module#include, except it mixes in the mixin directly below the class:

class Foo
  def bar
    'Hello'
  end
end 

module FooExtensions
  def bar
    super + ' World'
  end
end

class Foo
  prepend FooExtensions
end

Foo.new.bar # => 'Hello World'

Note: I also wrote a little bit about Module#prepend in this question: Ruby module prepend vs derivation

Mixin Inheritance (broken)

I have seen some people try (and ask about why it doesn’t work here on StackOverflow) something like this, i.e. includeing a mixin instead of prepending it:

class Foo
  def bar
    'Hello'
  end
end 

module FooExtensions
  def bar
    super + ' World'
  end
end

class Foo
  include FooExtensions
end

Unfortunately, that won’t work. It’s a good idea, because it uses inheritance, which means that you can use super. However, Module#include inserts the mixin above the class in the inheritance hierarchy, which means that FooExtensions#bar will never be called (and if it were called, the super would not actually refer to Foo#bar but rather to Object#bar which doesn’t exist), since Foo#bar will always be found first.

Method Wrapping

The big question is: how can we hold on to the bar method, without actually keeping around an actual method? The answer lies, as it does so often, in functional programming. We get a hold of the method as an actual object, and we use a closure (i.e. a block) to make sure that we and only we hold on to that object:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  old_bar = instance_method(:bar)

  define_method(:bar) do
    old_bar.bind(self).() + ' World'
  end
end

Foo.new.bar # => 'Hello World'

This is very clean: since old_bar is just a local variable, it will go out of scope at the end of the class body, and it is impossible to access it from anywhere, even using reflection! And since Module#define_method takes a block, and blocks close over their surrounding lexical environment (which is why we are using define_method instead of def here), it (and only it) will still have access to old_bar, even after it has gone out of scope.

Short explanation:

old_bar = instance_method(:bar)

Here we are wrapping the bar method into an UnboundMethod method object and assigning it to the local variable old_bar. This means, we now have a way to hold on to bar even after it has been overwritten.

old_bar.bind(self)

This is a bit tricky. Basically, in Ruby (and in pretty much all single-dispatch based OO languages), a method is bound to a specific receiver object, called self in Ruby. In other words: a method always knows what object it was called on, it knows what its self is. But, we grabbed the method directly from a class, how does it know what its self is?

Well, it doesn’t, which is why we need to bind our UnboundMethod to an object first, which will return a Method object that we can then call. (UnboundMethods cannot be called, because they don’t know what to do without knowing their self.)

And what do we bind it to? We simply bind it to ourselves, that way it will behave exactly like the original bar would have!

Lastly, we need to call the Method that is returned from bind. In Ruby 1.9, there is some nifty new syntax for that (.()), but if you are on 1.8, you can simply use the call method; that’s what .() gets translated to anyway.

Here are a couple of other questions, where some of those concepts are explained:

• How do I reference a function in Ruby?
• Is Ruby’s code block same as C♯’s lambda expression?

“Dirty” Monkey Patching

alias_method chain

The problem we are having with our monkey patching is that when we overwrite the method, the method is gone, so we cannot call it anymore. So, let’s just make a backup copy!

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  alias_method :old_bar, :bar

  def bar
    old_bar + ' World'
  end
end

Foo.new.bar # => 'Hello World'
Foo.new.old_bar # => 'Hello'

The problem with this is that we have now polluted the namespace with a superfluous old_bar method. This method will show up in our documentation, it will show up in code completion in our IDEs, it will show up during reflection. Also, it still can be called, but presumably we monkey patched it, because we didn’t like its behavior in the first place, so we might not want other people to call it.

Despite the fact that this has some undesirable properties, it has unfortunately become popularized through AciveSupport’s Module#alias_method_chain.

An aside: Refinements

In case you only need the different behavior in a few specific places and not throughout the whole system, you can use Refinements to restrict the monkey patch to a specific scope. I am going to demonstrate it here using the Module#prepend example from above:

class Foo
  def bar
    'Hello'
  end
end 

module ExtendedFoo
  module FooExtensions
    def bar
      super + ' World'
    end
  end

  refine Foo do
    prepend FooExtensions
  end
end

Foo.new.bar # => 'Hello'
# We haven’t activated our Refinement yet!

using ExtendedFoo
# Activate our Refinement

Foo.new.bar # => 'Hello World'
# There it is!

You can see a more sophisticated example of using Refinements in this question: How to enable monkey patch for specific method?

---

Abandoned ideas

Before the Ruby community settled on Module#prepend, there were multiple different ideas floating around that you may occasionally see referenced in older discussions. All of these are subsumed by Module#prepend.

Method Combinators

One idea was the idea of method combinators from CLOS. This is basically a very lightweight version of a subset of Aspect-Oriented Programming.

Using syntax like

class Foo
  def bar:before
    # will always run before bar, when bar is called
  end

  def bar:after
    # will always run after bar, when bar is called
    # may or may not be able to access and/or change bar’s return value
  end
end

you would be able to “hook into” the execution of the bar method.

It is however not quite clear if and how you get access to bar’s return value within bar:after. Maybe we could (ab)use the super keyword?

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  def bar:after
    super + ' World'
  end
end

Replacement

The before combinator is equivalent to prepending a mixin with an overriding method that calls super at the very end of the method. Likewise, the after combinator is equivalent to prepending a mixin with an overriding method that calls super at the very beginning of the method.

You can also do stuff before and after calling super, you can call super multiple times, and both retrieve and manipulate super’s return value, making prepend more powerful than method combinators.

class Foo
  def bar:before
    # will always run before bar, when bar is called
  end
end

# is the same as

module BarBefore
  def bar
    # will always run before bar, when bar is called
    super
  end
end

class Foo
  prepend BarBefore
end

and

class Foo
  def bar:after
    # will always run after bar, when bar is called
    # may or may not be able to access and/or change bar’s return value
  end
end

# is the same as

class BarAfter
  def bar
    original_return_value = super
    # will always run after bar, when bar is called
    # has access to and can change bar’s return value
  end
end

class Foo
  prepend BarAfter
end

old keyword

This idea adds a new keyword similar to super, which allows you to call the overwritten method the same way super lets you call the overridden method:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  def bar
    old + ' World'
  end
end

Foo.new.bar # => 'Hello World'

The main problem with this is that it is backwards incompatible: if you have method called old, you will no longer be able to call it!

Replacement

super in an overriding method in a prepended mixin is essentially the same as old in this proposal.

redef keyword

Similar to above, but instead of adding a new keyword for calling the overwritten method and leaving def alone, we add a new keyword for redefining methods. This is backwards compatible, since the syntax currently is illegal anyway:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  redef bar
    old + ' World'
  end
end

Foo.new.bar # => 'Hello World'

Instead of adding two new keywords, we could also redefine the meaning of super inside redef:

class Foo
  def bar
    'Hello'
  end
end 

class Foo
  redef bar
    super + ' World'
  end
end

Foo.new.bar # => 'Hello World'

Replacement

redefining a method is equivalent to overriding the method in a prepended mixin. super in the overriding method behaves like super or old in this proposal.

Decorating a method in Ruby

Here's a simple example of how to wrap methods with a validator:

class Foo
  def foo(a, b, c)
    p a, b, c
  end

  def bar(a, b)
    p a, b
  end

  validate(:foo, :bar) {|*args, &blk| args.reduce(:+) == 6 } 
end

The Module#validate method takes a list of method names, and a block with the validation logic for those methods.

foo = Foo.new

foo.foo(1, 2, 3)
# 1
# 2
# 3

foo.bar(2, 4)
# 2
# 4

foo.foo(2, 3, 4)
# [2, 3, 4] (Validator::ValidationError)

foo.bar(2, 3)
# [2, 3] (Validator::ValidationError)

As you can see, the validator rejected the argument list in the last two calls because they didn't pass the condition in the block.

This is the "magic" which makes it all happen, which isn't actually that magic. We generate a module which overrides the methods we want to validate with a version that raises an exception if the validation fails and then simply calls super. Then we prepend that module to the class/module that we are currently in, i.e. the one that the call to the validate method is in. And that's it, basically.

I chose to also be a good Ruby citizen and wrap the whole thing in a Refinement, so you need to say

using Validator

to actually activate it.

module Validator
  class ValidationError < ArgumentError; end

  refine Module do
    def validate(*methods, &validator)
      prepend(Module.new do
        methods.each do |method|
          define_method(method) do |*args, &blk|
            raise ValidationError, args.inspect unless yield *args
            super(*args, &blk)
          end
        end
      end)
    end
  end
end

Get included module name in module's method

I've never had to dynamically define module extension methods before, it's unusually difficult to manage on a class but you can always manage this using an intermediate module to help package and import the methods wherever you want:

module Helper
  def self.extended(base)
    base_name = base.to_s

    extension = Module.new

    extension.send(:define_method, :module_name) do
      base_name
    end

    base.send(:extend, extension)
  end
end

module A
  extend Helper
end

module B
  extend Helper
end

A.module_name
# => "A"

B.module_name
# => "B"

This is probably a lot less messy than defining module instance variables. Closures, in practice, tend to be a lot cleaner.

How do I properly use the Ruby DelegateClass to wrap YAML::Store?

To go over some of your questions regarding delegation:

• Forwardable:
  This module when extended allows you to specify specific methods that should be delegated to a designated Object (most often an instance variable) Example:

require 'forwardable'
class A 
  attr_reader :obj
  extend Forwardable 
  def_delegator :@obj, :<<
  
  def initialize(val) 
    @obj = val
  end 
end 

a = A.new([])
a << 1
#=> [1]
a.obj 
#=> [1]

• Delegator: Inheriting from this class allows methods to be delegated to an Object passed via new. This Object is internalized upon instantiation using the __setobj__ method and calls are forwarded via method_missing; however the caveat here is that it uses 2 methods that are not defined by default(__setobj__ and __getobj__) and you would need to define them in the inheriting class

• SimpleDelegator: Inherits from Delegator and shares its personality in most regards; however it predefines the 2 methods previously discussed. Generally inheriting from SimpleDelegator is preferred over inheriting directly from the Delegator class for its ease of use.

• DelegateClass: Is similar to SimpleDelegator however it creates a new anonymous class which defines all the instance methods of the class passed in as an argument and then your inheriting class inherits from this anonymous class directly which allows method via inheritance rather than using method_missing for delegation. e.g.

class A < Delegator;end
class B < DelegateClass(String);end
A.ancestors 
#=> [A, Delegator, #<Module:0x00007fffcc1cc9c8>, BasicObject]
B.ancestors
#=> [B, #<Class:0x00007fffcc5603c8>, Delegator, #<Module:0x00007fffcc1cc9c8>, BasicObject]
A.instance_methods - Object.instance_methods
#=> [:__getobj__, :__setobj__, :method_missing, :marshal_dump, :marshal_load]
B.instance_methods - Object.instance_methods
#=> Array of all the public the instance methods of String plus the above 

This is generally preferable when you are expecting the class to be instantiated with a specific type of Object because the Module injection avoids the full inheritance chain traversal that comes in to play with method_missing. This does not mean that you cannot instantiate this class with another object (however if you do and that object defines methods not defined in the class argument passed to DelegateClass it will fall back to the default method_missing behavior)

Now to your Code
Your first error is the call to super. The Delegator expects 1 argument which an instance of the Object being delegated to (YAML::Store in your case) however you have defined 2 arguments (neither of which represents the object you wish to delegate to) and when you call super both these arguments are forwarded on, thus the error.

Removing thread_safe works because you now only have a single argument but your delegated object in this case is actually the String "store.yml"

For Example the following modification should work (Similar to the example provided in the Source Code

module ExampleDelegator
  class MultiWriter < DelegateClass(YAML::Store)
    attr_reader :store

    def initialize file_name="store.yml", thread_safe=true
      @store = YAML::Store.new(file_name,thread_safe)
      super(@store)
    end
  end
end

Your second issue is that you are calling write on the CollaboratorWithData object which does not define this method and is not otherwise delegating.

While I am not 100% sure what your intent is here as there are some other oddities like attr_accessor in the Module body which is then being included in the global space and as it stands I do not see a true reason for a Delegator becuase you could just use the Object directly via the yaml_store instance variable being set to an instance of YAML::Store (as you already mentioned) but you can rewrite your code as follows to use delegation if you wish

require 'delegate'
require 'yaml/store'
require 'forwardable'

module ExampleDelegator
  class CollaboratorWithData
    extend Forwardable
    def_delegators :@yaml_store, :read, :write

    attr_reader :yaml_store
    attr_accessor :data
    
    def initialize(file_name="store.yml", thread_safe=true)
      @yaml_store = MultiWriter.new(YAML::Store.new(file_name,thread_safe))
      @data = {}
    end
    
    def some_data
      {a: 1, b:2, c: [1, 2, 3]}
    end
  end

  class MultiWriter < DelegateClass(YAML::Store)
    def write **kwargs
      transaction { kwargs.each { |k, v| @store[k] = v } }
    end
    def read *keys
      transaction(read_only=true) { keys.map { |k| @store[k] } }
    end
  end
end

How to execute a method only once in Ruby? Are there static variables?

There is an idiom in ruby: x ||= y.

def something
  @something ||= calculate_something
end

private

def calculate_something
  # some long process
end

But there is a problem with this idiom if your 'long running utility' may return a false value (false or nil), since the ||= operator will still cause the right side to be evaluated.
If you expect false values then use an additional variable, in a way similar to the proposed by DigitalRoss:

def something
  return @something if @something_calculated
  @something = calculate_something
  @something_calculated = true
  return @something
end

Don't try to save a line of code by setting the @something_calculated variable first, an then running calculate_something. If your calculate function raises an exception your function will always return nil and will never call the calculate again.

More generally, in Ruby you use instance variables. Note however, that they are visible in all the methods of given object - they are not local to the method.
If you need a variable shared by all instances, define the method in the class object, and in every instance call self.class.something

class User
  def self.something
    @something ||= calculate_something
  end

  def self.calculate_something
    # ....
  end

  def something
    self.class.something
  end
end

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