#strategy pattern

Strategy Design Pattern Tutorial with Java Coding Example | Strategy Pat...

Hello friends, a new #video on #strategydesignpattern with #Java #programming #coding #example is published on #codeonedigest #youtube channel. Ultimate #tutorial of #strategydesignpattern in #java

I’m reading The Gene: An Intimate History by Siddhartha Mukherjee. As a programmer I’m endlessly fascinated by genetics, and how genes actually function in the business of making life, you know, do stuff and be alive.

The first fourteen chapters are wonderful, but also pretty focused on the science and theory around genes. They are full of observations on how genes function, but not how genes actually produces phenotypes, behaviors and adaptations.

That changes in chapter 15.

E. Coli can feed off of both sugar and lactose. If both are present in the environment, it prefers sugar. I has DNA for both scenarios, encoded right next to each other. The DNA for each case produces a set of proteins that work together to allow it to digest sugar or lactose respectively.

The super interesting thing is that it also produces a protein that detects lactose in the environment. Presumably if the protein detects sugar it doesn’t do anything, but if it detects lactose, then it changes in a way that RNA (as part of protein synthesis) starts producing the lactose digesting proteins instead. 

The really interesting thing to me is that the proteins are a set. E. Coli has two completely different chunks of DNA and switches between them depending on a single signal.

In programming, we’d call this a Strategy Pattern. An entire algorithm is chosen at a focal point. E. Coli could have had genes that just produced both proteins all the time, or possibly produced more complex proteins that could do either jobs. 

Instead it’s very efficient. Only the proteins currently needed are produced. It’s a textbook strategy pattern. I’ve seen much worse code written by humans.

TriadCity's command vocabulary is far larger than the 400 or so commands typed by players.  Every Admin and Builder action is implemented as a command, so there are tens of thousands altogether. In C, you'd probably route user input through a simple switch() statement.  CircleMUD does this.  The parser and command router are tightly coupled inside one master controlling statement which switch()es on the user's first word of input.  Logic can be parceled out to functions, or handled right there inside the switch() statement itself.  This is fine for a couple of hundred commands, but growing it to tens of thousands would be a performance and maintenance disaster. We ensure infinite command routing scalability for TriadCity by combining the Strategy and Hashed Adapter OO patterns. Strategy encapsulates an algorithm as an object, allowing algorithms to be selected and substituted at runtime.  In Java you'll typically implement Strategy via an Interface defining the algorithm's contract.  Classes implementing that Interface define the available algorithms.  In TriadCity we have a Command Interface defining our Strategy contract:

public Interface Command {           public abstract boolean doCommand(<multile-args-go-here>); }

And we have many thousands of classes implementing that Interface, for example, Chat, which implements the doCommand() method to iterate all NPCs and logged-in Players, sending the arguments to the chat channel. The Hashed Adapter pattern achieves scalable routing by stashing these concrete Command instances in a HashMap keyed by command name.  Each concrete Command is a Singleton; we populate the map with command names as keys, instantiated Command Singletons as values.  The resulting routing algorithm is super-simple:

Command command = this.commands.get(commandName); command.doCommand(<multiple-args-passed-here>);

There's other logic wrapped around these lines, for example authorization checking for security. But that's the idea. By using Hashed Adapter this way, routing scales infinitely without performance degradation.  Server processing is very efficient, and for developers there's no nightmare switch() statement of ten million lines to maintain.  By using Strategy, it's very easy to do the file housekeeping necessary to keep tens of thousands of Command implementations easy to navigate.

+) (template) (.NET) (C#)

Abstract Head First Design Patterns是用strategy pattern當作第一個範例，而陳俊杉教授也是用strategy當作授課的第一個pattern，可見strategy的確適合初學者學第一個學習的pattern。 Intent 定義一整族演算法，將每一個演算法封裝起來，可互換使用，更可以在不影響外界的情況下各別抽換所引用的演算法。 其UML表示法 GoF說strategy也稱為policy，我個人喜歡稱它為plugin，因為可以動態的換演算法，如同在eclipse上可以動態的換plugin一樣。 原本在單一class中有一個單一method很單純，如圖Grapher class只有drawShape()這個method，只能畫方形。 但後來『需求改變』，希望Grapher也能畫三角形和圓形，而且日後還可能增加功能，如畫橢圓形，菱形...，當然可以在Grapher陸續加上drawTriangle()，drawCircle()，drawEllipse()，但如此就違反OCP，Grapher須不斷的修改，根據DP第三守則"Identify the aspects of your application that vary and separate them from  what you stays the same"，將『會變』的部份另外包成class，但這些class必須要和原來的class溝通，所以必須訂出『標準』彼此才能溝通，IShape就是彼此溝通的標準，Triangle，Circle，Square則必須實做IShape這個interface，這就是strategy pattern。 我們看看這個架構，若日後還有新的shape加入，Grapher，IShape，Triangle，Circle，Square皆不用修改，符合OCP的closed for modification原則，若要加入新的class，只需實做IShape即可，符合OCP的open for extension原則，所以是非常好維護的架構，事實上，.NET Framework和STL都用了很多strategy pattern。

簡言之，strategy pattern就是將會變動的member function用class包起來，變成object『掛』在原本的class上。

ISO C++ by Interface

 1/*   2(C) OOMusou 2007 http://oomusou.cnblogs.com  3  4Filename    : DP_StrategyPattern_Classic.cpp  5Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++  6Description : Demo how to use Strategy Pattern  7Release     : 03/26/2007 1.0  8              07/10/2007 2.0  9*/ 10#include <iostream> 11using namespace std; 12 13class IDrawStrategy { 14public: 15  virtual void draw() const = 0; 16}; 17 18class Grapher { 19public: 20  Grapher(IDrawStrategy* drawStrategy = 0) : _drawStrategy(drawStrategy) {} 21   22public: 23  void drawShape() const; 24  void setShape(IDrawStrategy* drawStrategy); 25   26protected: 27  IDrawStrategy* _drawStrategy; 28}; 29 30void Grapher::drawShape() const { 31  if (_drawStrategy) 32      _drawStrategy->draw(); 33} 34 35void Grapher::setShape(IDrawStrategy* drawStrategy) { 36  _drawStrategy = drawStrategy; 37} 38 39class Triangle : public IDrawStrategy { 40public: 41  void draw() const; 42}; 43 44void Triangle::draw() const { 45  cout << "Draw Triangle" << endl; 46} 47 48class Circle : public IDrawStrategy { 49public: 50  void draw() const; 51}; 52 53void Circle::draw() const { 54  cout << "Draw Circle" << endl; 55} 56 57class Square : public IDrawStrategy { 58public: 59  void draw() const; 60}; 61 62void Square::draw() const { 63  cout << "Draw Square" << endl; 64} 65 66int main() { 67  Grapher grapher(&Square()); 68  grapher.drawShape(); 69   70  grapher.setShape(&Circle()); 71  grapher.drawShape(); 72}

執行結果

Draw Square Draw Circle

67行和70行可以看到strategy pattern的美，可以動態的換演算法，如同plugin一樣，且若將來擴充其他shape，只需加上新的class實做IDrawStrategy，其他程式都不用再改，符合OCP原則。 C# by Interface

 1/*   2(C) OOMusou 2007 http://oomusou.cnblogs.com  3  4Filename    : DP_StrategyPattern_Classic.cs  5Compiler    : Visual Studio 2005 / C# 2.0  6Description : Demo how to implement Strategy Pattern by C#  7Release     : 07/08/2007 1.0  8*/  9using System; 10 11interface IDrawStrategy { 12  void draw(); 13} 14 15class Grapher { 16  private IDrawStrategy _drawStrategy = null; 17 18  public Grapher() {} 19  public Grapher(IDrawStrategy drawStrategy) { 20    _drawStrategy = drawStrategy; 21  } 22 23  public void drawShape() { 24    if (_drawStrategy != null) 25      _drawStrategy.draw(); 26  } 27 28  public void setShape(IDrawStrategy drawStrategy) { 29    _drawStrategy = drawStrategy; 30  } 31} 32 33class Triangle : IDrawStrategy { 34  public void draw() { 35    Console.WriteLine("Draw Triangle"); 36  } 37} 38 39class Circle : IDrawStrategy { 40  public void draw() { 41    Console.WriteLine("Draw Circle"); 42  } 43} 44 45class Square : IDrawStrategy { 46  public void draw() { 47    Console.WriteLine("Draw Square"); 48  } 49} 50 51class Program { 52  public static void Main() { 53    Grapher grapher = new Grapher(new Square()); 54    grapher.drawShape(); 55 56    grapher.setShape(new Circle()); 57    grapher.drawShape(); 58  } 59}

執行結果

Draw Square Draw Circle

使用interface是最正規的OOP寫法，另外Effective C++的item 35也使用了function pointer來實做strategy pattern，function pointer是C/C++的獨門寫法。 ISO C++ by Function Pointer

 1/*   2(C) OOMusou 2007 http://oomusou.cnblogs.com  3  4Filename    : DP_StrategyPattern_FunctionPointer.cpp  5Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++  6Description : Demo how to use Strategy Pattern by function pointer  7Release     : 03/31/2007 1.0  8            : 07/08/2007 2.0  9            : 07/10/2007 3.0 10*/ 11#include <iostream> 12 13using namespace std; 14 15class Grapher { 16public: 17  typedef void (*pfDraw)(); 18  Grapher(pfDraw draw = 0) : _draw(draw) {} 19   20public: 21  void drawShape() const; 22  void setShape(pfDraw draw); 23   24protected: 25  pfDraw _draw; 26}; 27 28void Grapher::drawShape() const { 29  if (_draw) 30    _draw(); 31} 32 33void Grapher::setShape(pfDraw draw) { 34  _draw = draw; 35} 36 37class Triangle { 38public: 39  static void draw(); 40}; 41 42void Triangle::draw() { 43  cout << "Draw Triangle" << endl; 44} 45 46class Circle { 47public: 48  static void draw(); 49}; 50 51void Circle::draw() { 52  cout << "Draw Circle" << endl; 53} 54 55class Square { 56public: 57  static void draw(); 58}; 59 60void Square::draw() { 61  cout << "Draw Square" << endl; 62} 63 64int main() { 65  Grapher grapher(Square::draw); 66  grapher.drawShape(); 67   68  grapher.setShape(Circle::draw); 69  grapher.drawShape(); 70}

執行結果

Draw Square Draw Circle

說穿了，本來只是本來由interface定義function的signature，現在改由16行的

typedef void (*pfDraw)();

定義pfDraw這個function pointer型別，所有要傳進的的function必須符合這個function pointer型別才可。 既然ISO C++可以用function pointer實現strategy pattern，就讓我想到C#的delegate了。delegate是C#對function pointer和observer pattern的實現，理應可用delegate來實現strategy pattern。 C# by Delegate

 1/*   2(C) OOMusou 2007 http://oomusou.cnblogs.com  3  4Filename    : DP_StrategyPattern_Delegate.cs  5Compiler    : Visual Studio 2005 / C# 2.0  6Description : Demo how to implement Strategy Pattern by C# delegate  7Release     : 07/08/2007 1.0  8*/  9using System; 10 11class Grapher { 12  private DrawDelegate _drawDelegate = null; 13   14  public delegate void DrawDelegate(); 15   16  public Grapher() {} 17  public Grapher(DrawDelegate drawDelegate) { 18    _drawDelegate = drawDelegate; 19  } 20   21  public void drawShape() { 22    if (_drawDelegate != null) 23      _drawDelegate(); 24  } 25   26  public void setShape(DrawDelegate drawDelegate) { 27    _drawDelegate = drawDelegate; 28  } 29} 30 31class Triangle { 32  public static void draw() { 33    Console.WriteLine("Draw Triangle"); 34  } 35} 36 37class Circle { 38  public static void draw() { 39    Console.WriteLine("Draw Circle"); 40  } 41} 42 43class Square { 44  public static void draw() { 45    Console.WriteLine("Draw Square"); 46  } 47} 48 49class Program { 50  public static void Main() { 51    Grapher grapher = new Grapher(Square.draw); 52    grapher.drawShape(); 53     54    grapher.setShape(Circle.draw); 55    grapher.drawShape(); 56  } 57}

執行結果

Draw Square Draw Circle

除此之外，GoF的Design Pattern也展示了使用template實做Strategy Pattern。 ISO C++ by Template

 1/*   2(C) OOMusou 2007 http://oomusou.cnblogs.com  3  4Filename    : DP_StrategyPattern_template.cpp  5Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++  6Description : Demo how to use Strategy Pattern by template  7Release     : 03/31/2007 1.0  8*/  9#include <iostream> 10 11using namespace std; 12 13template <typename T> 14class Grapher { 15private: 16  T _drawTemplate; 17 18public: 19  void drawShape() const; 20}; 21 22template<typename T> 23void Grapher<T>::drawShape() const { 24  _drawTemplate.draw(); 25} 26 27class Triangle { 28public: 29  void draw() const; 30}; 31 32void Triangle::draw() const { 33  cout << "Draw Triangle" << endl; 34} 35 36class Circle { 37public: 38  void draw() const; 39}; 40 41void Circle::draw() const { 42  cout << "Draw Circle" << endl; 43} 44 45class Square { 46public: 47  void draw() const; 48}; 49 50void Square::draw() const { 51  cout << "Draw Square" << endl; 52} 53 54int main() { 55  Grapher<Square> grapher; 56  grapher.drawShape(); 57   58  Grapher<Circle> grapher2; 59  grapher2.drawShape(); 60}

執行結果

Draw Square Draw Circle

同樣是實現多型，Design Pattern的用的是OOP的interface + dynamic binding技術，這是在run-time下完成，優點是在run-time動態改變，缺點是速度較慢；GP用template技術，這是在compile-time下完成，優點是速度較快，缺點是無法在run-time動態改變，由於template方式不需interface，所以整個程式看不到interface，也由於無法run-time改變，所以沒有setShape()，而16行的

T _drawTemplate;

也只是object而非pointer，因為不需run-time的多型，也非function pointer。 46行

Grapher<Square> grapher;

也只能在直接指定strategy，無法動態再改變。 C# 2.0也提供泛型了，所以C#也可以用Generics實現Strategy Pattern。 C# by Generics

 1/*   2(C) OOMusou 2007 http://oomusou.cnblogs.com  3  4Filename    : DP_StrategyPattern_Generics.cs  5Compiler    : Visual Studio 2005 / C# 2.0  6Description : Demo how to implement Strategy Pattern by C# Generics  7Release     : 07/08/2007 1.0  8*/  9using System; 10 11interface IDrawStrategy { 12  void draw(); 13} 14 15class Grapher<T> where T : class, IDrawStrategy, new() { 16  private T _drawStrategy = default(T); 17 18  public Grapher() { 19    _drawStrategy = new T(); 20  } 21 22  public void drawShape() { 23    if (_drawStrategy != null) 24      _drawStrategy.draw(); 25  } 26 27  public void setShape(T drawStrategy) { 28    _drawStrategy = drawStrategy; 29  } 30} 31 32class Triangle : IDrawStrategy { 33  public void draw() { 34    Console.WriteLine("Draw Triangle"); 35  } 36} 37 38class Circle : IDrawStrategy { 39  public void draw() { 40    Console.WriteLine("Draw Circle"); 41  } 42} 43 44class Square : IDrawStrategy { 45  public void draw() { 46    Console.WriteLine("Draw Square"); 47  } 48} 49 50class Program { 51  public static void Main() { 52    Grapher<Square> grapher = new Grapher<Square>(); 53    grapher.drawShape(); 54     55    Grapher<Circle> grapher2 = new Grapher<Circle>(); 56    grapher2.drawShape(); 57  } 58}

執行結果

Draw Square Draw Circle

Remark strategy和template method目的相同，皆對『新需求』的不同演算法提供『擴充』的機制，但手法卻不同，strategy採用object的方式，利用delegation改變演算法，而template method則採用class的繼承方式來改變演算法，也因為strategy採用object方式，所以有run-time改變的可能，但template method採class手法，所以無法run-time改變。 GoF的原文如下

Template methods use inheritance to vary part of an algorithm. Strategies use delegation to vary the entire algorithm.

Known Use 1.eclipse的plugin，可以在不修改eclipse原始碼下，外掛plugin變更eclipse所提供的功能。