How to judge whether a method has defined in a class?

问题

class C1
  unless method_defined? :hello  # Certainly, it's not correct. I am asking to find something to do this work.
    def_method(:hello) do
      puts 'Hi Everyone'
    end
  end
end

So, how to judge whether a method has defined or not?

回答1:

The code you posted works just fine for checking whether the method is defined or not. Module#method_defined? is exactly the right choice. (There's also the variants Module#public_method_defined?, Module#protected_method_defined? and Module#private_method_defined?.) The problem is with your call to def_method, which doesn't exist. (It's called Module#define_method).

This works like a charm:

class C1      
  define_method(:hello) do
    puts 'Hi Everyone'
  end unless method_defined? :hello
end

However, since you already know the name in advance and don't use any closure, there is no need to use Module#define_method, you can just use the def keyword instead:

class C1
  def hello
    puts 'Hi Everyone'
  end unless method_defined? :hello
end

Or have I misunderstood your question and you are worried about inheritance? In that case, Module#method_defined? is not the right choice, because it walks the entire inheritance chain. In that case, you will have to use Module#instance_methods or one of its cousins Module#public_instance_methods, Module#protected_instance_methods or Module#private_instance_methods, which take an optional argument telling them whether to include methods from superclasses / mixins or not. (Note that the documentation is wrong: if you pass no arguments, it will include all the inherited methods.)

class C1
  unless instance_methods(false).include? :hello
    def hello
      puts 'Hi Everyone'
    end
  end
end

Here's a little test suite that shows that my suggestion works:

require 'test/unit'
class TestDefineMethodConditionally < Test::Unit::TestCase
  def setup
    @c1 = Class.new do
      def self.add_hello(who)
        define_method(:hello) do
          who
        end unless method_defined? :hello
      end
    end

    @o = @c1.new
  end

  def test_that_the_method_doesnt_exist_when_it_hasnt_been_defined_yet
    assert !@c1.method_defined?(:hello)
    assert !@c1.instance_methods.include?(:hello)
    assert !@o.methods.include?(:hello)
    assert !@o.respond_to?(:hello)
    assert_raise(NoMethodError) { @o.hello }
  end

  def test_that_the_method_does_exist_after_it_has_been_defined
    @c1.add_hello 'one'

    assert @c1.method_defined?(:hello)
    assert @c1.instance_methods.include?(:hello)
    assert @o.methods.include?(:hello)
    assert_respond_to @o, :hello
    assert_nothing_raised { @o.hello }
    assert_equal 'one', @o.hello
  end

  def test_that_the_method_cannot_be_redefined
    @c1.add_hello 'one'

    assert @c1.method_defined?(:hello)
    assert @c1.instance_methods.include?(:hello)
    assert @o.methods.include?(:hello)
    assert_respond_to @o, :hello
    assert_nothing_raised { @o.hello }
    assert_equal 'one', @o.hello

    @c1.add_hello 'two'

    assert @c1.method_defined?(:hello)
    assert @c1.instance_methods.include?(:hello)
    assert @o.methods.include?(:hello)
    assert_respond_to @o, :hello
    assert_nothing_raised { @o.hello }
    assert_equal 'one', @o.hello, 'it should *still* respond with "one"!'
  end
end

回答2:

Look at the Ruby Object class. It has a methods function to get an list of methods and a respond_to? to check for a specific method. So you want code like this:

class C1
  def add_hello
    unless self.respond_to? "hello"
      def hello
        puts 'Hi Everyone'
      end
    end  
  end
end

cone.hello      #This would fail
cone.add_hello  
cone.hello      #This would work

回答3:

The Object class has the method "methods": docs

 class Klass
   def kMethod()
   end
 end
 k = Klass.new
 k.methods[0..9]    #=> ["kMethod", "freeze", "nil?", "is_a?",
                    #    "class", "instance_variable_set",
                    #    "methods", "extend", "__send__", "instance_eval"]
 k.methods.length   #=> 42

来源：https://stackoverflow.com/questions/3403004/how-to-judge-whether-a-method-has-defined-in-a-class