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In our example, we will pretend that we'll ask the user what method they'd like to use to cook their food, and then implement that method using the Strategy design pattern. Step1: Creating Abstract Strategy Create an interface with the name ICompression and then copy and paste the following code in it. Design Patterns [2] defines the intent of STRATEGY as “ Define a family of algorithms, encapsulate each one, and make them In software engineering, Behavioural Design Patterns deal with the assignment of responsibilities between objects which in turn make the interaction between the objects easy & loosely coupled. Strategy lets the algorithm vary independently from the clients that use it. StrategyDesignPattern.zip This pattern falls under the category of behavioral pattern and as the name suggests, it allows clients to choose an algorithm from a set of algorithms at run time. Let us discuss the step by step procedure to implement the above example using the Strategy Design Pattern in C#. Strategy pattern defines a family of algorithms, encapsulates each one of them and makes them interchangeable at run time within that family. Strategy パターンは、コンピュータープログラミングの領域において、アルゴリズムを実行時に選択することができるデザインパターンである。, Strategyパターンはアルゴリズムを記述するサブルーチンへの参照をデータ構造の内部に保持する。このパターンの実現には、関数ポインタや関数オブジェクト、デリゲートのほか、オーソドックスなオブジェクト指向言語におけるポリモーフィズムと委譲、あるいはリフレクションによる動的ダック・タイピングなどが利用される。, このパターンは、関数が第一級オブジェクトである言語では暗黙のうちに使用されている。例として後述のPythonコード例を参照のこと。, Strategy パターンは、アプリケーションで使用されるアルゴリズムを動的に切り替える必要がある際に有用である。Strategy パターンはアルゴリズムのセットを定義する方法を提供し、これらを交換可能にすることを目的としている。Strategy パターンにより、アルゴリズムを使用者から独立したまま様々に変化させることができるようになる。, Javaではクラスのメソッドオーバーライドによるポリモーフィズムを使ってStrategyパターンを実現することができる。インターフェイスを用いた例を示す。, Python では関数が第一級オブジェクトであり、このパターンを明示的に定義する必要はない。下記はコールバック関数を用いる GUI プログラミングで見られる例である。, C#はJava同様にクラスやインターフェイスによるポリモーフィズムを用いることもできるが、カスタマイズポイントがひとつのメソッドしかない場合(オブジェクトの他のプロパティやメソッドにアクセスしない場合)は、継承関係を必要としないデリゲートを使うほうが好まれる[1]。, なお、Javaもバージョン8以降であれば、メソッド参照と関数型インターフェイス (functional interface) を用いることで、C#と類似の実装が可能となる。, Strategy パターンの UML クラス図は Bridge パターンのものと同じである。しかし、これら二つのデザインパターンはその意図が同じではない。Strategy パターンは振る舞いに対するパターンであるが、Bridge パターンは構造に対するパターンである。, 一般にコンテキストと戦略との結合は、Bridge パターンにおける抽象化と実装の結合より強固である。, Strategy パターンに従うと、クラスの振る舞いは継承されるべきではなく、インターフェイスを用いてカプセル化するべきである。例として Car クラスを考えると、Car の振る舞いにはブレーキとアクセルがある。, アクセルとブレーキの振る舞いはモデルにより大きく異なるため、良くあるアプローチはこれらの振る舞いをサブクラスとして実装することであるが、このアプローチには大きな問題点がある。すなわち、アクセルとブレーキの振る舞いは、新たな Car モデルごとに宣言されなければならない。これはモデルが少ないときには問題にならないが、モデルの数が増えるにつれ、それらの振る舞いを管理する作業が大幅に増加し、またコードがモデル間で重複することになる。さらに、各コードを詳しく分析しなければそれぞれのモデル用の振る舞いの性質を知ることができない。, これに対して Strategy パターンでは、継承ではなく合成 (composition) を用いる。Strategy パターンにおける振る舞いは別々のインターフェイスと、これらのインターフェイスを実装した抽象クラスとして定義される。具体的なクラスは、これらのインターフェイスをカプセル化する。これにより、振る舞いと、それを用いるクラスがうまく分離できる。振る舞いは、それを用いるクラスに変更を加えずに変更することができ、クラスは大きなコード変更を必要とすることなく、使用する実装を切り替えることで振る舞いを切り替えることができる。振る舞いは設計時にも実行時にも変更することができる。例として、Car オブジェクトのブレーキの振る舞いを、メンバー brakeBehavior を BrakeWithABS から Brake に変えることで変更できる:, これにより設計に優れた柔軟性をもたせることができ、かつ拡張に対して開放的であり変更に対して閉鎖的であるべきとする開放/閉鎖原則 (Open/Closed Principle, OCP) とも調和を保つことができる。, // コンテキストクラスは、具体的な戦略を呼び出すためにこのインターフェイスを使用する。, // ConcreteStrategy を指定して作成され、Strategy オブジェクトへの参照を保持する。, // 具体的な戦略を実装するクラスは、このデリゲートに適合するメソッドを実装する。, // コンテキストクラスは、具体的な戦略を呼び出すためにこのデリゲートを使用する。, // ExecuteStrategyDelegate オブジェクトへの参照を保持する。, // 元々はConcreteStrategyとして明示的に定義していたが、必要なくなっている。, When to Use Delegates Instead of Interfaces (C# Programming Guide) | Microsoft Docs, en:List of object-oriented programming terms, Refactoring: Replace Type Code with State/Strategy, https://ja.wikipedia.org/w/index.php?title=Strategy_パターン&oldid=79799398. As you know, there may be multiple strategies which are applicable for a given problem. But it is not the only part of the puzzle. The strategy pattern is also very practical and could help us refactor and clean up some of our existing code. 中です🌈 Strategy Design Pattern involves the removal of an algorithm from its host class and putting it in a separate class. 【C# / Java / PHP / Python / JS / Flutter / .NET / Docker / UML etc.】, みんなのしあわせのためにソフトウェアをつくる会社です🍀. Paste the following code in it the above example using the strategy design pattern in C++..., called context, holds a reference to a strategy object and delegates executing. One, and make them interchangeable at run time within that family in Java the above example using the pattern! While the program is executing you make an interface that is similar between the designs! Called context, holds a reference to a strategy object and delegates it executing the behavior that.... 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