如何判断某个方法是否在类中定义？

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定义了？

Answer 1

你发布的代码对于检查方法是否定义工作得很好。 Module#method_defined?是完全正确的选择。 （还有变种Module#public_method_defined?,Module#protected_method_defined?和Module#private_method_defined?。）问题出在您拨打def_method时不存在。 （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 

不过，既然你已经事先知道名字，不使用任何关闭，也没有必要使用Module#define_method，你可以只使用def关键字相反：

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

或者我误解了你的问题，你担心继承？在这种情况下，Module#method_defined?不是正确的选择，因为它遍历整个继承链。在这种情况下，您将不得不使用Module#instance_methods或其一个亲戚Module#public_instance_methods,Module#protected_instance_methods或Module#private_instance_methods，它们采用可选参数告诉他们是否包含来自超类/ mixin的方法。 （请注意，文件是错误的：如果你没有传递参数，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 

这里有一个小的测试套件，它表明我的建议的工作：

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

Object类有方法 “方法”：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 

Answer 3

看那Ruby Object class。它有一个methods函数来获取方法列表和respond_to?来检查特定的方法。所以你需要这样的代码：

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