环境:ruby 1.9.2
有这样一个需求， 给你 任意一个字符串，把它转化为类,网上大多数的 解决办法是 下面三种:
Kernel.const_get(:User) # Object.const_get(:User) eval(’User’) ‘User’.constantize
但是上面三种方法, 这个 User 事先必须是初始化的,不然会报错，如下:
Object.const_get(:User) # uninitialized constant User (NameError)
所以需要加个判断 这个 Class 有没有初始化，没有的话 再set 一个Class
require "rails/all" def Kernel.const_missing(name) Object.const_set(name,Class.new) end #p Kernel.const_get("baoxiaos".to_sym) # wrong constant name baoxiaos (NameError) p Kernel.const_get("baoxiaos".classify.to_sym) # Baoxiao , const的首字母必须大写 p Kernel.constants # [] p Object.constants.include?(:UBaoxiao) # true
刚开始把这个 const 定义在 Object 里:
require "rails/all" def Object.const_missing(name) Object.const_set(name,Class.new) end #p Object.const_get("baoxiaos".to_sym) # wrong constant name baoxiaos (NameError) p Object.const_get("baoxiaos".classify.to_sym)
发现 用在 rails 中后 , rails 处处报错，可能 怪我重写了 Object.const_missing 导致的 ……..
上面方法的原理是 当找不到 const的时候 执行了
Object.const_set(name,Class.new)
那么 name 就变成是 Class 的实例了 , 为什么定义在 Kernel 中的 const_missing ，Object 也可以访问到，需要注意 Object,Kernel 之间的关系,Object 是 inluce Kernel 的,如下:
class Object include Kernel end
SEE:
http://niczsoft.com/2010/01/string-to-class-in-ruby-on-rails/
http://www.ruby-forum.com/topic/96222
环境:ruby 1.9.2 + rails 3.0.3
我们经常会有这样的操作:
user = User.find_by_login("wxianfeng") # => nil user.name # => NoMethodError: undefined method `name' for nil:NilClass
假如 login 为 wxianfeng 不存在 ，会报错:
NoMethodError: undefined method `name' for nil:NilClass
那么建议使用 try 方法避免报错，try 返回的是 nil
user.try(:name) # =>nil
也就相当于
nil.try(:name) # => nil
看下源码: here
其实就是调用了 __send__ 方法 , __send__ 方法 和 send 方法等价 , 只不过 __send__ 方法 为了防止 有已经存在的 send 方法 , nil 的话 调用 NilClass 的 try 方法
另外 发现 github上 try方法已经重新写了 ，如下: here
class Object
# Invokes the method identified by the symbol +method+, passing it any arguments
# and/or the block specified, just like the regular Ruby <tt>Object#send</tt> does.
#
# *Unlike* that method however, a +NoMethodError+ exception will *not* be raised
# and +nil+ will be returned instead, if the receiving object is a +nil+ object or NilClass.
#
# If try is called without a method to call, it will yield any given block with the object.
#
# ==== Examples
#
# Without try
# @person && @person.name
# or
# @person ? @person.name : nil
#
# With try
# @person.try(:name)
#
# +try+ also accepts arguments and/or a block, for the method it is trying
# Person.try(:find, 1)
# @people.try(:collect) {|p| p.name}
#
# Without a method argument try will yield to the block unless the reciever is nil.
# @person.try { |p| "#{p.first_name} #{p.last_name}" }
#--
# +try+ behaves like +Object#send+, unless called on +NilClass+.
def try(*a, &b)
if a.empty? && block_given?
yield self
else
__send__(*a, &b)
end
end
end
class NilClass #:nodoc:
def try(*args)
nil
end
end
其实只是判断了 if a.empty? && block_given? 这种情况 则直接执行block 内容然后返回，效果一样…..
DEMO:
require "active_support/core_ext/object/try" class Klass def send(*args) "helo " + args.join(' ') end def hello(*args) "Hello " + args.join(' ') end def self.foobar(s) "#{s} foobar" end end k = Klass.new # __send__ 为了防止有方法名叫send ， 建议用 __send__ p k.__send__ :hello, "gentle", "readers" #=> "Hello gentle readers" p k.send "gentle", "readers" #=> "Helo gentle readers" # Ruby 里一切皆是对象,类也是对象 # Klass(类) 是 Class 的实例 , Class 是 Object 的实例 , 那么 Klass 也就是 Object 的实例 所以 Klass 可以调用try 方法 p Klass.try(:foobar,"hey") # => "hey foobar" # k 是Klass 的实例,Klass 的父类是 Object , 所以 k 可以调用 try 方法 p k.try(:send,"bla","bla") # => "helo bla bla" # class 得到的是 实例关系 # superclass 得到的是 继承关系 p Klass.superclass # Object p Klass.class # Class p k.class # Klass
另外 这是 对象nil 那如果 没有那个字段了 ， 就会 报 找不到方法的错误
例如:
ruby-1.9.2-p0 > u=User.first User Load (175.8ms) SELECT `users`.* FROM `users` LIMIT 1 => #<User id: 1, login: "entos", name: "", email: "entos@entos.com", crypted_password: "557c88b0713f63397249f4198368e4a57d6d400f", salt: "4e04ef1cf506595ac3edf6a249791c55995b0f8f", remember_token: nil, remember_token_expires_at: nil, activation_code: nil, activated_at: nil, status: 2, suspend_at: nil, avatar_id: nil, orgunit_id: nil, mobile_phone: nil, last_login_at: nil, language: nil, options: nil, created_at: "2011-02-24 02:55:42", updated_at: "2011-02-24 02:55:42"> ruby-1.9.2-p0 > u.hi NoMethodError: undefined method `hi' for #<User:0x9fcfe00>
建议加上 respond_to? 判断
ruby-1.9.2-p0 > u.respond_to? "hi" => false
环境:ruby 1.9.2 + rails 3.0.3 + ubuntu 10.10
params在rails中很常用，特别在表单提交的时候,params 产生的是一个Hash ，里面构造通过 form域的name构造 ,产生不同的 params 内容,今天 在看rails params 实现的时候 发现通过 attr_internal 的方法实现,params方法 的源码:
def params @_params ||= request.parameters end
发现其实是从 request 这个方法得到的,那么request方法又是怎么定义的:
attr_internal :headers, :response, :request
就是 用了 attr_internal 方法
看下 整个 metal.rb文件: here
发现了 response,headers,session(借助delegate委派) ,status,params == 都是通过 attr_internal 实现的,来看看 attr_internal 到底是何须人也 ：
源码： here
class Module
# Declares an attribute reader backed by an internally-named instance variable.
def attr_internal_reader(*attrs)
attrs.each do |attr|
module_eval "def #{attr}() #{attr_internal_ivar_name(attr)} end", __FILE__, __LINE__
end
end
# Declares an attribute writer backed by an internally-named instance variable.
def attr_internal_writer(*attrs)
attrs.each do |attr|
module_eval "def #{attr}=(v) #{attr_internal_ivar_name(attr)} = v end", __FILE__, __LINE__
end
end
# Declares an attribute reader and writer backed by an internally-named instance
# variable.
def attr_internal_accessor(*attrs)
attr_internal_reader(*attrs)
attr_internal_writer(*attrs)
end
alias_method :attr_internal, :attr_internal_accessor
class << self; attr_accessor :attr_internal_naming_format end
self.attr_internal_naming_format = '@_%s'
private
def attr_internal_ivar_name(attr)
Module.attr_internal_naming_format % attr
end
end
发现其实就是通过 module_eval 给 对象 添加了 settet , getter 方法而已,但是命名格式是这样的:
self.attr_internal_naming_format = '@_%s'
DEMO:
require "active_support/core_ext/module/attr_internal" class Foo attr_accessor :sex,:birthday # attr_accessor ruby里封装的method attr_internal :name,:city # attr_internal rails 封装的 def bar name # call getter method # => @_name end end f = Foo.new f.name = 'wxianfeng' p f.instance_variables # => [:@_name] p f.name # => "wxianfeng" p f # => #<Foo:0x8630e18 @_name="wxianfeng"> p f.bar # => "wxianfeng"
所以 attr_internal 和 attr_accessor 其实是 等价的,只不过 从字面意思上看是内部变量(闭包变量的写法) ，attr_internal 希望你 通过方法名来调用，不用 @_%s 这个写法 来调用
所以 其实 一般我们在 controller 用的 request 方法 其实 可以直接这样写 @_request ,
request #=> @_request params # => @_request.parameters params # => @_params headers #=> @_headers status #=> @_status . . .
但是一般 不建议这样写
还发现 这些和 http相关的东西都定义在 metal 模块, metal 是 rails 链接 rack 的中间件,源码中的解释:
ActionController::Metal provides a way to get a valid Rack application from a controller.
Rack 是一个 ruby实现的web server,封装了 http的请求和响应等,例如 rails,sinatra == 都是在 rack 基础上实现的……
有机会很有必要 深入学习下…
SEE:
http://rubyonrailswin.wordpress.com/2007/03/07/actioncontroller-and-what-the-heck-is-attr_internal/
http://www.oschina.net/p/rack
环境:ruby 1.9.2 + ubuntu 10.10
instance_variables 得到当前 对象已经开辟内存空间的实例变量,疑惑在这里
class Foo attr_accessor :sex,:birthday end p Foo.new.instance_variables # => []
刚开始不理解,怎么是 空……..
后来想了想，因为 ruby 是 动态的解释型的语言，如果没有 给实例变量赋值 的话，是不会开辟内存空间的，所以 instance_variables 只能得到已经开辟内存空间的 实例变量，
但是如果是 编译型的静态语言 则不然，例如java ，实例变量 声明了 就会开辟内存空间了
DEMO1:
# ruby version : 1.9.2 class Foo attr_accessor :sex,:birthday # attr_accessor ruby里封装的method end p Foo.new.instance_variables # => [] f = Foo.new p f.sex #=> nil f.sex = 'M' p f.instance_variables #=> [:@sex] p f.inspect # => "#<Foo:0x85ead8c @sex=\"M\">" b = Foo.new b.birthday = nil # 注意赋值为nil,也开辟了内存空间 p b.instance_variables # => [:@birthday] p b.inspect # => "#<Foo:0x9dca358 @birthday=nil>"
DEMO2:
class Foo def initialize @name = 'wxianfeng' end def bar # => as getter method @name end end f = Foo.new p f.instance_variables #=> [:@name] p f.inspect #=> "#<Foo:0x8aa7c08 @name=\"wxianfeng\">" p f.name # => undefined method `name' for #<Foo:0x99adf7c @name="wxianfeng"> p f.bar # => "wxianfeng"
ruby 源码:
# obj.instance_variables => array
#
#
# Returns an array of instance variable names for the receiver. Note
# that simply defining an accessor does not create the corresponding
# instance variable.
#
# class Fred
# attr_accessor :a1
# def initialize
# @iv = 3
# end
# end
# Fred.new.instance_variables #=> ["@iv"]
#
#
def instance_variables
# This is just a stub for a builtin Ruby method.
# See the top of this file for more info.
end
SEE:
http://www.megasolutions.net/ruby/instance_variables-doesnt-return-unassigned-variables-68358.aspx
环境:ruby 1.9.2 + ubuntu 10.10 + rails 3.0.3
我们知道 ruby 中 扩展class ,写公用方法 ,或者 利用命名空间 来模块化 ，都是通过 module 来实现的 ， 今天 看 rails 中 camelize 方法的源码的时候 ， 发现 module 这样写的…..
active_support/inflector/methods.rb
module ActiveSupport # The Inflector transforms words from singular to plural, class names to table names, modularized class names to ones without, # and class names to foreign keys. The default inflections for pluralization, singularization, and uncountable words are kept # in inflections.rb. # # The Rails core team has stated patches for the inflections library will not be accepted # in order to avoid breaking legacy applications which may be relying on errant inflections. # If you discover an incorrect inflection and require it for your application, you'll need # to correct it yourself (explained below). module Inflector extend self # By default, +camelize+ converts strings to UpperCamelCase. If the argument to +camelize+ # is set to <tt>:lower</tt> then +camelize+ produces lowerCamelCase. # # +camelize+ will also convert '/' to '::' which is useful for converting paths to namespaces. # # Examples: # "active_record".camelize # => "ActiveRecord" # "active_record".camelize(:lower) # => "activeRecord" # "active_record/errors".camelize # => "ActiveRecord::Errors" # "active_record/errors".camelize(:lower) # => "activeRecord::Errors" # # As a rule of thumb you can think of +camelize+ as the inverse of +underscore+, # though there are cases where that does not hold: # # "SSLError".underscore.camelize # => "SslError" def camelize(lower_case_and_underscored_word, first_letter_in_uppercase = true) if first_letter_in_uppercase lower_case_and_underscored_word.to_s.gsub(/\/(.?)/) { "::#{$1.upcase}" }.gsub(/(?:^|_)(.)/) { $1.upcase } else lower_case_and_underscored_word.to_s[0].chr.downcase + camelize(lower_case_and_underscored_word)[1..-1] end end # Makes an underscored, lowercase form from the expression in the string. # # Changes '::' to '/' to convert namespaces to paths. # # Examples: # "ActiveRecord".underscore # => "active_record" # "ActiveRecord::Errors".underscore # => active_record/errors # # As a rule of thumb you can think of +underscore+ as the inverse of +camelize+, # though there are cases where that does not hold: # # "SSLError".underscore.camelize # => "SslError" def underscore(camel_cased_word) word = camel_cased_word.to_s.dup word.gsub!(/::/, '/') word.gsub!(/([A-Z]+)([A-Z][a-z])/,'\1_\2') word.gsub!(/([a-z\d])([A-Z])/,'\1_\2') word.tr!("-", "_") word.downcase! word end # Replaces underscores with dashes in the string. # # Example: # "puni_puni" # => "puni-puni" def dasherize(underscored_word) underscored_word.gsub(/_/, '-') end # Removes the module part from the expression in the string. # # Examples: # "ActiveRecord::CoreExtensions::String::Inflections".demodulize # => "Inflections" # "Inflections".demodulize # => "Inflections" def demodulize(class_name_in_module) class_name_in_module.to_s.gsub(/^.*::/, '') end # Creates a foreign key name from a class name. # +separate_class_name_and_id_with_underscore+ sets whether # the method should put '_' between the name and 'id'. # # Examples: # "Message".foreign_key # => "message_id" # "Message".foreign_key(false) # => "messageid" # "Admin::Post".foreign_key # => "post_id" def foreign_key(class_name, separate_class_name_and_id_with_underscore = true) underscore(demodulize(class_name)) + (separate_class_name_and_id_with_underscore ? "_id" : "id") end # Ruby 1.9 introduces an inherit argument for Module#const_get and # #const_defined? and changes their default behavior. if Module.method(:const_get).arity == 1 # Tries to find a constant with the name specified in the argument string: # # "Module".constantize # => Module # "Test::Unit".constantize # => Test::Unit # # The name is assumed to be the one of a top-level constant, no matter whether # it starts with "::" or not. No lexical context is taken into account: # # C = 'outside' # module M # C = 'inside' # C # => 'inside' # "C".constantize # => 'outside', same as ::C # end # # NameError is raised when the name is not in CamelCase or the constant is # unknown. def constantize(camel_cased_word) names = camel_cased_word.split('::') names.shift if names.empty? || names.first.empty? constant = Object names.each do |name| constant = constant.const_defined?(name) ? constant.const_get(name) : constant.const_missing(name) end constant end else def constantize(camel_cased_word) #:nodoc: names = camel_cased_word.split('::') names.shift if names.empty? || names.first.empty? constant = Object names.each do |name| constant = constant.const_defined?(name, false) ? constant.const_get(name) : constant.const_missing(name) end constant end end # Turns a number into an ordinal string used to denote the position in an # ordered sequence such as 1st, 2nd, 3rd, 4th. # # Examples: # ordinalize(1) # => "1st" # ordinalize(2) # => "2nd" # ordinalize(1002) # => "1002nd" # ordinalize(1003) # => "1003rd" def ordinalize(number) if (11..13).include?(number.to_i % 100) "#{number}th" else case number.to_i % 10 when 1; "#{number}st" when 2; "#{number}nd" when 3; "#{number}rd" else "#{number}th" end end end end end
发现 了这样的写法
module A module B extend self ..... end end
extend self 有何作用?
先来看个demo:
module Foo module Bar extend self # self => Foo::Bar def hello p "hello" end end end class Klass include Foo::Bar end Klass.new.hello # "hello" Foo::Bar.hello # "hello" Klass.hello # undefined method `hello' for Klass:Class (NoMethodError)
发现 module 中的方法 可以当作模块方法 直接被Module调用 ， 被include 到class 中后 ， 依然还是 class 的实例方法 ， 恩,不错,以后 像下面 这样的写法， 都要 改改了:
module A def self.foo end end
改成这样:
module A extend self def foo end end
demo里的extend self 其实就是 Foo::Bar.extend(Foo::Bar)
所以 可以 更动态的 写成这样:
module Foo module Bar def hello p "hello" end end end class Klass include Foo::Bar end Foo::Bar.extend(Foo::Bar) Klass.new.hello # "hello" Foo::Bar.hello # "hello" Klass.hello # undefined method `hello' for Klass:Class (NoMethodError)
环境:ruby 1.9.2 + rails 3.0.3 + ubuntu 10.10
项目需要运行中动态生成表 和 Model ， 怎么办 ？
借助 ActiveRecord::Migration 来实现 动态建表 和 字段
可以 借助 Object.const_set 来实现 动态Model
DEMO:
# RUN : rails runner lib/dynamic_table.rb ActiveRecord::Migration.create_table :posts ActiveRecord::Migration.add_column :posts, :title, :string Object.const_set(:Post,Class.new(ActiveRecord::Base)) # => Object.class_eval { const_set(:Post,Class.new(ActiveRecord::Base)) } # p Post.columns p Post.column_names # ["id", "title"] ActiveRecord::Migration.add_column :posts, :body, :text p Post.column_names # ["id", "title"] Object.class_eval { remove_const :Post } Object.const_set(:Post,Class.new(ActiveRecord::Base)) p Post.column_names # ["id", "title", "body"]
动态Model 实质就相当于 Post = Class.new(ActiveRecord::Base) 或者 Post < ActiveRecord::Base
Class.new(ActiveRecord::Base) 参数指定 super_class ， 默认是 Object
可以从ruby源码中看出:
# Class.new(super_class=Object) => a_class # # # Creates a new anonymous (unnamed) class with the given superclass # (or <code>Object</code> if no parameter is given). You can give a # class a name by assigning the class object to a constant. # # # def self.new(super_class=Object) # This is just a stub for a builtin Ruby method. # See the top of this file for more info. end
当给表添加了新的字段后,Model 需要重新 const_set 一次 ,注意 const_set 之前 需要 remove_const 一次 , 不然会出现 已经初始化的警告
see:
http://hildolfur.wordpress.com/2006/10/29/class-reloading-in-ruby/