EagerBeaver

Overview

EagerBeaver provides an interface for adding #method_missing-related abilities to a class or module.

Key Features

• Method matchers can be added dynamically and cumulatively, reducing the risk of accidentally altering or removing previously-added functionality.
• Matched methods are automatically reflected in calls to #respond_to? and #method, DRY-ing up code by centralizing method-matching logic.
• Matched methods are automatically added to the including class/module and invoked. Subsequent calls won't trigger #method_missing.
• When a method cannot be matched, super's #method_missing is automatically invoked.

Installation

Add this line to your application's Gemfile:

gem 'eager_beaver'

And then execute:

$ bundle

Or install it yourself as:

$ gem install eager_beaver

Usage

Inclusion

Any class or module which includes EagerBeaver will gain the add_method_matcher pseudo-keyword, which [indirectly] yields an EagerBeaver::MethodMatcher to the given block:

require 'eager_beaver'

class NeedsMethods
  include EagerBeaver

  add_method_matcher do |mm|
    ...
  end
end

In this case, the resulting MethodMatcher is added to the end of a MethodMatcher list associated with NeedsMethods.

Each MethodMatcher needs two things: a lambda for matching missing method names and a lambda for creating the code for any method names it matches:

  add_method_matcher do |mm|
    mm.match = lambda { ... }
    mm.new_method_code = lambda { ...}
  end
end

Matching

The match lambda should return a true value if the missing method name is one can be handled by the MethodMatcher. The following example will match missing methods of the form #make_<attr_name>:

    mm.match = lambda {
      /\Amake_(\w+)\z/ =~ context.missing_method_name
      context.attr_name = Regexp.last_match ? Regexp.last_match[1] : nil
      return Regexp.last_match
    }

Context

As the example shows, each MethodMatcher contains a context which provides:

• the name of the missing method (context.missing_method_name)
• the original method receiver instance (context.original_receiver)
• a place to stash information (dynamically-generated accessors context.<attr_name> and context.<attr_name>=)

This context is shared between the match and new_method_code lambdas, and is reset between uses of each MethodMatcher.

Code Generation

The new_method_code lambda should return a string which will create the missing method in NeedsMethods:

    mm.new_method_code = lambda {
      code = %Q{
        def #{context.missing_method_name}(arg)
          puts "method \##{context.missing_method_name} has been called"
          puts "\##{context.missing_method_name} was originally called on #{context.original_receiver}"
          puts "#{context.attr_name} was passed from matching to code generation"
          puts "the current call has arguments: \#{arg}"
          return "result = \#{arg}"
        end
      }
      return code
    }

As the example shows, it is perfectly reasonable to take advantage of work done by the match lambda (in this case, the parsing of <attr_name>).

After the generated code is inserted into NeedsMethods, the missing method call is resent to the original receiver.

Complete Example

require 'eager_beaver'

class NeedsMethods
  include EagerBeaver

  add_method_matcher do |mm|
    mm.match = lambda {
      /\Amake_(\w+)\z/ =~ context.missing_method_name
      context.attr_name = Regexp.last_match ? Regexp.last_match[1] : nil
      return Regexp.last_match
    }

    mm.new_method_code = lambda {
      code = %Q{
        def #{context.missing_method_name}(arg)
          puts "method \##{context.missing_method_name} has been called"
          puts "\##{context.missing_method_name} was originally called on #{context.original_receiver}"
          puts "#{context.attr_name} was passed from matching to code generation"
          puts "the current call has arguments: \#{arg}"
          return "result = \#{arg}"
        end
      }
      return code
    }
  end
end

Execution

Given the NeedsMethods class in the example above, let's work through the following code:

nm1 = NeedsMethods.new
puts nm1.make_thingy(10)
puts nm1.make_widget("hi")

nm2 = NeedsMethods.new
puts nm2.make_thingy(20)
puts nm2.make_widget("hello")

nm2.dont_make_this

As instances of NeedsMethods, nm1 and nm2 will automatically handle methods of the form #make_<attr_name>.

The line:

puts nm1.make_thingy(10)

will trigger nm1's #method_missing, which NeedsMethods implements thanks to EagerBeaver. Each MethodMatcher associated with EagerBeaver is run against the method name make_thingy, and sure enough one matches. This causes the following methods to be inserted to NeedsMethods:

  def make_thingy(arg)
    puts "method #make_thingy has been called"
    puts "#make_thingy was originally called on #<NeedsMethods:0x007fa1bc17f498>"
    puts "thingy was passed from matching to code generation"
    puts "the current call has arguments: #{arg}"
    return "result = #{arg}"
  end

and when #make_thingy is resent to nm1, the existing method is called and outputs:

method #make_thingy has been called
#make_thingy was originally called on #<NeedsMethods:0x007fa1bc17f498>
thingy was passed from matching to code generation
the current call has arguments: 10
result = 10

Similarly, the line:

puts nm1.make_widget("hi")

generates the code:

  def make_widget(arg)
    puts "method #make_widget has been called"
    puts "#make_widget was originally called on #<NeedsMethods:0x007fa1bc17f498>"
    puts "widget was passed from matching to code generation"
    puts "the current call has arguments: #{arg}"
    return "result = #{arg}"
  end

and outputs:

method #make_widget has been called
#make_widget was originally called on #<NeedsMethods:0x007fa1bc17f498>
widget was passed from matching to code generation
the current call has arguments: hi
result = hi

Note that the following lines do NOT trigger #method_missing because both methods have already been added to NeedsMethods:

puts nm2.make_thingy(20)
puts nm2.make_widget("hello")

This can be seen by examining the identity of the original receiver in the output:

method #make_thingy has been called
#make_thingy was originally called on #<NeedsMethods:0x007fa1bc17f498>
thingy was passed from matching to code generation
the current call has arguments: 20
result = 20

method #make_widget has been called
#make_widget was originally called on #<NeedsMethods:0x007fa1bc17f498>
widget was passed from matching to code generation
the current call has arguments: hello
result = hello

String substitutions which were part of the generated code body (emphasized) reflect the circumstances of the first set of method calls, as opposed to those which reflect the current call's argument.

Finally, the call:

nm2.dont_make_this

will cause NeedsMethods to examine all of its MethodMatchers and finally call super's #method_missing. Because no superclass of NeedsMethods handles #dont_make_this, the output is:

undefined method `dont_make_this' for #<NeedsMethods:0x007f8e2b991f90> (NoMethodError)

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request