Observer を Ruby で
Observer は、 振る舞いに関するデザインパターンの一つで、 オブジェクトが別のオブジェクトに状態の変化を通知できるようにします。
Observer パターンは、 サブスクライバー・インターフェースを実装するいかなるオブジェクトのイベント通知の申し込みと停止をする方法を提供します。
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使用例： Observer パターンは、 Ruby コードではよく見かけます。 特に、 GUI コンポーネントで。 他のオブジェクトで起きるイベントに、 そのクラスに結合することなく、 反応する方法を提供します。
見つけ方： このパターンは、 オブジェクトをリストに保存するサブスクリプション・メソッドと、 そのリスト中のオブジェクトの更新メソッドの呼び出しにより、 識別できます。
概念的な例
この例は、 Observer デザインパターンの構造を説明するためのものです。 以下の質問に答えることを目的としています：
- どういうクラスからできているか？
- それぞれのクラスの役割は？
- パターンの要素同士はどう関係しているのか？
main.rb: 概念的な例
# The Subject interface declares a set of methods for managing subscribers. class Subject # Attach an observer to the subject. def attach(observer) raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end # Detach an observer from the subject. def detach(observer) raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end # Notify all observers about an event. def notify raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end end # The Subject owns some important state and notifies observers when the state # changes. class ConcreteSubject < Subject # For the sake of simplicity, the Subject's state, essential to all # subscribers, is stored in this variable. attr_accessor :state # @!attribute observers # @return [Array<Observer>] attr_accessor :observers private :observers def initialize @observers = [] end # List of subscribers. In real life, the list of subscribers can be stored # more comprehensively (categorized by event type, etc.). # @param [Observer] observer def attach(observer) puts 'Subject: Attached an observer.' @observers << observer end # @param [Observer] observer def detach(observer) @observers.delete(observer) end # The subscription management methods. # Trigger an update in each subscriber. def notify puts 'Subject: Notifying observers...' @observers.each { |observer| observer.update(self) } end # Usually, the subscription logic is only a fraction of what a Subject can # really do. Subjects commonly hold some important business logic, that # triggers a notification method whenever something important is about to # happen (or after it). def some_business_logic puts "\nSubject: I'm doing something important." @state = rand(0..10) puts "Subject: My state has just changed to: #{@state}" notify end end # The Observer interface declares the update method, used by subjects. class Observer # Receive update from subject. def update(_subject) raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end end # Concrete Observers react to the updates issued by the Subject they had been # attached to. class ConcreteObserverA < Observer # @param [Subject] subject def update(subject) puts 'ConcreteObserverA: Reacted to the event' if subject.state < 3 end end class ConcreteObserverB < Observer # @param [Subject] subject def update(subject) return unless subject.state.zero? || subject.state >= 2 puts 'ConcreteObserverB: Reacted to the event' end end # The client code. subject = ConcreteSubject.new observer_a = ConcreteObserverA.new subject.attach(observer_a) observer_b = ConcreteObserverB.new subject.attach(observer_b) subject.some_business_logic subject.some_business_logic subject.detach(observer_a) subject.some_business_logic output.txt: 実行結果
Subject: Attached an observer. Subject: Attached an observer. Subject: I'm doing something important. Subject: My state has just changed to: 1 Subject: Notifying observers... ConcreteObserverA: Reacted to the event Subject: I'm doing something important. Subject: My state has just changed to: 10 Subject: Notifying observers... ConcreteObserverB: Reacted to the event Subject: I'm doing something important. Subject: My state has just changed to: 2 Subject: Notifying observers... ConcreteObserverB: Reacted to the event 
