How to dynamically create instance methods at runtime?

Question

[ruby 1.8]

Assume I have:

dummy "string" do
    puts "thing" 
end

Now, this is a call to a method which has as input arguments one string and one block. Nice.

Now assume I can have a lot of similar calls (different method names, same arguments). Example:

otherdummy "string" do
    puts "thing"
end

Now because they do the same thing, and they can be hundreds, I don't want create an instance method for each one in the wanted class. I would like rather find a smart way to define the method dynamically at runtime based on a general rule.

Is that possible? Which techniques are commonly used?

Thanks

Answer 1

I'm particularly fond of using method_missing, especially when the code you want to use is very similar across the various method calls. Here's an example from this site - whenever somebody calls x.boo and boo doesn't exist, method_missing is called with boo, the arguments to boo, and (optionally) a block:

class ActiveRecord::Base
  def method_missing(meth, *args, &block)
    if meth.to_s =~ /^find_by_(.+)$/
      run_find_by_method($1, *args, &block)
    else
      super # You *must* call super if you don't handle the
            # method, otherwise you'll mess up Ruby's method
            # lookup.
    end
  end

  def run_find_by_method(attrs, *args, &block)
    # Make an array of attribute names
    attrs = attrs.split('_and_')

    # #transpose will zip the two arrays together like so:
    #   [[:a, :b, :c], [1, 2, 3]].transpose
    #   # => [[:a, 1], [:b, 2], [:c, 3]]
    attrs_with_args = [attrs, args].transpose

    # Hash[] will take the passed associative array and turn it
    # into a hash like so:
    #   Hash[[[:a, 2], [:b, 4]]] # => { :a => 2, :b => 4 }
    conditions = Hash[attrs_with_args]

    # #where and #all are new AREL goodness that will find all
    # records matching our conditions
    where(conditions).all
  end
end

define_method also looks like it would work for you, but I have less experience with it than method_missing. Here's the example from the same link:

%w(user email food).each do |meth|
  define_method(meth) { @data[meth.to_sym] }
end