From: Hideto ISHIBASHI <hideto-i@...4u.or.jp>
Date: 2000-03-28T19:44:35+09:00
Subject: [ruby-list:21671] GoF's Flyweight Pattern

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石橋秀仁です。

"Gang of Four"の"Flyweight"というデザインパターンを試しに
実装してみました。まだいろいろと問題がありそうなので、
御意見ください。

(注)Flyweight Patternとは(引用)
「多数の細かいオブジェクトを効率良くサポートするために共有を利用する」

# ほんとは共有しているobjectを、変更される直前にcloneできれば。
# 複数のオブジェクトから参照されている場合の、副作用の心配が
# 無くなります。例の「dup問題」も解決します。
# インスタンス変数にtrace_varのような操作はできないのでしょうか？

また、これを使ってRuby本のlifegame.rbとgeometry.rbを書換えてみました。
Ruby本p.272の「#（A'）@neighborsの初期化」のように(素直に)記述しても、
「配列が余分にコピーされ」なくなります。

# そのパッチを流してもよろしいですか？＞著者様

--
Hideto "rubyholic" ISHIBASHI

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#  GoF's FLYWEIGHT PATTERN
# Copyright (c) 2000 Hideto ISHIBASHI <hideto-i@rr.iij4u.or.jp>
#
# Flyweight module that enables a large number of small objects to be shared.
#
# 1) It uses parameters of the `initialize' as hash keys.
# 2) Objects created by direct call of "Class#new" are not shared.
# 3) Attributes of flyweights should not be changed after creation.
#   Remember all the objects that have the same key are shared.
#
# The followings are defined in a class/module that includes Flyweight.
#  class instance variable: @__pool__
#  class methods: __create_flyweight__, __remove_flyweight__,
#                 alias_create_flyweight, alias_remove_flyweight

module Flyweight

  def Flyweight.append_features(klass)
    klass.instance_eval %{
      @__pool__ = {}

      def __create_flyweight__(*key)
        if @__pool__.has_key?(key)
          @__pool__[key]
        else
          @__pool__[key] = new_obj = new(*key)
          new_obj
        end
      end

      def __remove_flyweight__(*key)
        @__pool__.delete(key)
      end
    }

    ["create", "remove"].each {|method_name|
      klass.instance_eval %{
        def alias_#{method_name}_flyweight(alias_name)
          alias_name = alias_name.id2name if alias_name.kind_of?(Integer)
          instance_eval %{ alias #\{alias_name\} __#{method_name}_flyweight__ }
        end
      }
    }
  end

end


if __FILE__ == $0

  def ok?(condition)
    raise "NG!" unless condition
  end

  class Foo

    include Flyweight

    alias_create_flyweight :create
    alias_remove_flyweight :remove

    def initialize(str)
      @str = str
    end
    attr_accessor :str           # DANGER! Attributes should not be changed.
    # attr_reader :str           # This is ok.

  end

  foo  = Foo.create("foo")
  foo2 = Foo.create("foo")
  foo3 = Foo.new("foo")

  ok? foo.str == foo2.str
  ok? foo.equal?(foo2)
  ok? foo.str == foo3.str
  #ok? foo.equal?(foo3)          # NG (foo3 is created by Class#new)

  # 2 way to change foo2.str => "foofoo"
  begin
    foo2.str = "foofoo"          # (*a) DANGER!
    ok? foo.str == "foo"         # (*b) NG (foo is changed unexpectedly)
  rescue
    foo.str  = "foo"
  end

  # again with a safe manner
  ok? foo.str == foo2.str
  foo2 = Foo.create("foofoo")    # (*a) SAFE
  ok? foo.str == "foo"           # (*b) OK

end

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