Ruby知識積累2

• Block:

  •   Proc.new
        my_proc = Proc.new { puts "tweet" }
        my_proc.call
      lambda
        my_proc = lambda { puts "tweet" }
        my_proc.call
      ->
        my_proc = -> { puts "tweet" }
        my_proc.call
      Parameter:
        e.g.
          class Tweet
            def post(success, error)
              if authenticate?(@user, @password)
                success.call
              else
                error.call
              end
            end
          end
    
    
          tweet = Tweet.new('Ruby Bits!')
          success = -> { puts "Sent!" }
          error = -> { raise 'Auth Error' }
          tweet.post(success, error)
        e.g.
          tweets = ["First tweet", "Second tweet"]
          printer = lambda { |tweet| puts tweet }
          tweets.each(&printer) # &操作符將lambda轉換爲block
      block_given?
        e.g.
          class Timeline
            attr_accessor :tweets
            def print
              if block_given?
                tweets.each { |tweet| puts yield tweet }
              else
                puts tweets.join(', ')
              end
            end
          end
    
    
          timeline = Timeline.new
          timeline.tweets = ["One", "Two"]
          timeline.print
          timeline.print { |tweet|
            "tweet: #{tweet}"
          }
        e.g.
          class Tweet
            def initialize
              yield self if block_given?
            end
          end
          Tweet.new do |tweet|
            tweet.status = "Set in initialize!"
            tweet.created_at = Time.now
          end
      closure(閉包):
        def tweet_as(user)
          lambda { |tweet| puts "#{user}: #{tweet}" }
        end
        gregg_tweet = tweet_as("andy")
        greeg_tweet.call("Mind blowing!")
    
    
      Symbol#to_proc
        tweets.map { |e| tweet.user  }
        tweets.map(&:user)

• Struct:

  •   e.g.
        類：
          class Tweet
            attr_accessor :user, :status
    
    
            def initialize(user, status)
              @user, @status = user, status
            end
    
    
            def to_s
              "#{user}: #{status}"
            end
          end
        結構體：
          Tweet = Struct.new(:user, :status) do
            def to_s
              "#{user}: #{status}"
            end
          end

• Alias_method:

  • Bad:
        class Timeline
          def initialize(tweets = [])
            @tweets = tweets
          end
    
    
          def tweets
            @tweets
          end
    
    
          def contents
            @tweets
          end
        end
      Good:
        class Timeline
          def initialize(tweets = [])
            @tweets = tweets
          end
    
    
          attr_reader :tweets
          alias_method :contents, :tweets
        end

• Define_method:

  • e.g.
        native:
          class Tweet
            def draft
              @status = :draft
            end
    
    
            def posted
              @status = :posted
            end
    
    
            def deleted
              @status = :deleted
            end
          end
        change:
          class Tweet
            status = [:draft, :posted, :deleted]
            status.each do |status|
              define_method status do
                @status = status
              end
            end
          end

• Send:(常用於訪問protected和private方法)

  • e.g.
        class Timeline
          def initialize(tweets)
            @tweets = tweets
          end
    
    
          def contents
            @tweets
          end
    
    
          private
            def direct_message
              #...
            end
        end
        tweets = ['Compiling!', 'Bundling...']
        timeline = Timeline.new(tweets)
        timeline.contents
        timeline.send(:contents)
        timeline.send("contents")
        timeline.send(:direct_message)
        timeline.public_send("direct_message")   # only can access public methods

• Method:

  • e.g.
        class Timeline
          def initialize(tweets)
            @tweets = tweets
          end
    
    
          def contents
            @tweets
          end
    
    
          def show_tweet(index)
            puts @tweets[index]
          end
        end
        tweets = ['Compiling!', 'Bundling...']
        timeline = Timeline.new(tweets)
        content_method = timeline.method(:contents)
        content_method.call
        show_tweet = timeline.method(:show_tweet)
        show_tweet.call(0)
        (0..1).each(&show_tweet)      #相當於show_tweet(0)和show_tweet(1)

• Self:

  • e.g.
        puts "Outside the class: #{self}"    # main
        class Tweet
          puts "Inside the class: #{self}"   # Tweet
          def self.find(keyword)
            # ...
          end
        end

• Class_eval(在指定類中執行):

  • e.g.
        class Tweet
          attr_accessor :status
          attr_reader :created_at
          def initialize(status)
            @status = status
            @created_at = Time.now
          end
        end
    
    
        Tweet.class_eval do
          attr_accessor :user
        end
        tweet = Tweet.new("Learning class_eval")
        tweet.user = "doit"

• Create a log_method:

  •   log_method以類名和方法名作爲參數，調用class_eval來執行類中的代碼。使用alias_method來保存原先的方法，使用define_method來重定義方法，對該方法的調用過程進行記錄，使用send來調用原先的方法。
      class MethodLogger
        def log_method(klass, method_name)
          klass.class_eval do
            alias_method "#{method_name}_original", method_name
            define_method method_name do |*args, &block|
              puts "#{Time.now}: Called #{method_name}"
              send "#{method_name}_original", *args, &block
            end
          end
        end
      end
      e.g.
        class Tweet
          def say_hi
            puts "hi"
          end
        end
        logger = MethodLogger.new
        logger.log_method(Tweet, :say_hi)
        Tweet.new.say_hi

• Instance_eval:

  •  e.g.
        class Tweet
          attr_accessor :user, :status
        end
        tweet = Tweet.new
        tweet.instance_eval do
          self.status = "Changing the tweet's status"
        end
      Good:
        class Tweet
          attr_accessor :user, :status
          def initialize
            yield self if block_given?
          end
        end
        Tweet.new do |tweet|
          tweet.status = "I was set in the initialize block!"
          tweet.user = "Gregg"
        end
      Better:
        class Tweet
          attr_accessor :user, :status
          def initialize(&block)
            instance_eval(&block) if block_given?
          end
        end

• Method_missing:

  • 當無法找到方法時調用
      e.g.
        class Tweet
          def method_missing(method_name, *args)
            logger.warn "You tried to call #{method_name} with these arguments: #{args}"
            super   # ruby's default method handling raise a NoMethodError
          end
        end
      e.g.(對一些方法進行委託)
        class Tweet
          DELEGATED_METHODS = [:username, :avatar]
    
    
          def initialize(user)
            @user = user
          end
    
    
          def method_missing(method_name, *args)
            if DELEGATED_METHODS.include?(method_name)
              @user.send(method_name, *args)
            else
              super
            end
          end
        end
      e.g.(將未知方法，全部委託到user)
        require 'delegate'
        class Tweet < SimpleDelegator
          def initialize(user)
            super(user)
          end
        end
      e.g.
        class Tweet
          def initialize(text)
            @text = text
          end
    
    
          def to_s
            @text
          end
    
    
          def method_missing(method_name, *args)
            match = method_name.to_s.match(/^hash_(\w+)/)
            if match
              @text << " #" + match[1]
            else
              super
            end
          end
        end
      e.g.(define_method)
        class Tweet
          def initialize(text)
            @text = text
          end
    
    
          def to_s
            @text
          end
    
    
          def method_missing(method_name, *args)
            match = method_name.to_s.match(/^hash_(\w+)/)
            if match
              self.class.class_eval do
                define_method(method_name) do
                  @text << " #" + match[1]
                end
              end
              send(method_name)
            else
              super
            end
          end
        end

• Response_to?:

  • e.g.
        class Tweet
          ...
          def respond_to?(method_name)
            method_name =~ /^hash_\w+/ ||super
          end
        end
    
    
        tweet = Tweet.new
        tweet.respond_to?(:to_s)
        tweet.respond_to?(:hash_ruby)
        tweet.method(:hash_ruby)  # NameError:undefined method

• Response_to_missing?:(Ruby 1.9.3)

  •   e.g.
        class Tweet
          ...
          def respond_to_missing?(method_name)
            method_name =~ /^hash_\w+/ ||super
          end
        end
    
    
        tweet = Tweet.new
        tweet.method(:hash_ruby)  # return a Method object

• Implement a simple DSL:

  • DSL:領域專用語言
      e.g.
        class Tweet
          def initialize(user)
            @user = user
            @tweets = []
            @annotations = {}
          end
    
    
          def submit_to_twitter
            tweet_text = @tweet.join(' ')
            if tweet_text.length <= 140
              puts "#{@user}: #{tweet_text}"
              puts @annotations.inspect unless @annotations.empty?
            else
              raise "your tweet is too long."
            end
          end
          def text(str)
            @tweet << str
            self
          end
          def mention(*users)
            users.each do |user|
              @tweet << "@" + user
            end
            self
          end
          def hashtag(str)
            @tweet << "#" + str
            self
          end
          def link(str)
            @tweet << str
          end
    
    
          def method_missing(method, *args)
            @annotations[method] = args.join(', ')
          end
        end
    
    
        def tweet_as(user, text=nil,&block)
          tweet = Tweet.new(user)
          tweet.text(text) if text
          tweet.instance_eval(&block) if block_given?
          tweet.submit_to_twitter
        end
    
    
        tweet_as 'marken' do
          mention 'china','usa'
          text('I made a DSL!').hashtag('hooray').hashtag('ruby')
          link 'http://www.somewhere.com'
        end