ruby-on-rails - 如何判断一个方法是否已经定义在一个类中？

Question

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

那么，如何判断一个方法是否已经定义呢？

Answer 1

您发布的代码可以很好地检查方法是否已定义。Module#method_defined?正是正确的选择。（还有变体Module#public_method_defined?和Module#protected_method_defined?。Module#private_method_defined?）问题在于您对 的调用def_method，它不存在。（它被称为Module#define_method）。

这就像一个魅力：

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

但是，由于您已经提前知道名称并且不使用任何闭包，因此无需使用Module#define_method，您可以使用def关键字代替：

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

还是我误解了你的问题，你担心继承？在那种情况下，Module#method_defined?这不是正确的选择，因为它走遍了整个继承链。在这种情况下，您将不得不使用Module#instance_methodsor 它的表亲之一Module#public_instance_methods，Module#protected_instance_methodsor Module#private_instance_methods，它带有一个可选参数，告诉他们是否包含来自超类/混合的方法。（请注意，文档是错误的：如果您不传递任何参数，它将包括所有继承的方法。）

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

这是一个小测试套件，表明我的建议有效：

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

Answer 2

查看Ruby Object 类。它具有methods获取方法列表和respond_to?检查特定方法的功能。所以你想要这样的代码：

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

Answer 3

Object 类具有方法“方法”：文档

 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