如何判斷某個方法是否在類中定義？

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 

那麼，如何判斷一個方法是否定義了？

Answer 1

你發佈的代碼對於檢查方法是否定義工作得很好。 Module#method_defined?是完全正確的選擇。 （還有變種Module#public_method_defined?,Module#protected_method_defined?和Module#private_method_defined?。）問題出在您撥打def_method時不存在。 （它被稱爲Module#define_method）。

這就像一個魅力：

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

不過，既然你已經事先知道名字，不使用任何關閉，也沒有必要使用Module#define_method，你可以只使用def關鍵字相反：

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

或者我誤解了你的問題，你擔心繼承？在這種情況下，Module#method_defined?不是正確的選擇，因爲它遍歷整個繼承鏈。在這種情況下，您將不得不使用Module#instance_methods或其一個親戚Module#public_instance_methods,Module#protected_instance_methods或Module#private_instance_methods，它們採用可選參數告訴他們是否包含來自超類/ mixin的方法。 （請注意，文件是錯誤的：如果你沒有傳遞參數，它將包括所有繼承的方法。）

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

這裏有一個小的測試套件，它表明我的建議的工作：

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類有方法 「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。它有一個methods函數來獲取方法列表和respond_to?來檢查特定的方法。所以你需要這樣的代碼：

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