Design Pattern-Strategy Pattern 设计模式之策略模式
Strategy Pattern
Ø Definition
Ø UML diagram
Ø Participants
Ø Sample code in C#
1. Definition
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
2. UML diagram
3. Participants
The classes and/or objects participating in this pattern are:
- Strategy (SortStrategy)
- declares an interface common to all supported algorithms. Context uses this interface to call the algorithm defined by a ConcreteStrategy
- ConcreteStrategy (QuickSort, ShellSort, MergeSort)
- implements the algorithm using the Strategy interface
- Context (SortedList)
- is configured with a ConcreteStrategy object
- maintains a reference to a Strategy object
- may define an interface that lets Strategy access its data.
4. Sample code in C #
This structural code demonstrates the Strategy pattern which encapsulates functionality in the form of an object. This allows clients to dynamically change algorithmic strategies.
| // Strategy pattern -- Structural example |
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using System;
namespace DoFactory.GangOfFour.Strategy.Structural { /// <summary> /// MainApp startup class for Structural /// Strategy Design Pattern. /// </summary> class MainApp { /// <summary> /// Entry point into console application. /// </summary> static void Main() { Context context;
// Three contexts following different strategies context = new Context(new ConcreteStrategyA()); context.ContextInterface();
context = new Context(new ConcreteStrategyB()); context.ContextInterface();
context = new Context(new ConcreteStrategyC()); context.ContextInterface();
// Wait for user Console.ReadKey(); } }
/// <summary> /// The 'Strategy' abstract class /// </summary> abstract class Strategy { public abstract void AlgorithmInterface(); }
/// <summary> /// A 'ConcreteStrategy' class /// </summary> class ConcreteStrategyA : Strategy { public override void AlgorithmInterface() { Console.WriteLine( "Called ConcreteStrategyA.AlgorithmInterface()"); } }
/// <summary> /// A 'ConcreteStrategy' class /// </summary> class ConcreteStrategyB : Strategy { public override void AlgorithmInterface() { Console.WriteLine( "Called ConcreteStrategyB.AlgorithmInterface()"); } }
/// <summary> /// A 'ConcreteStrategy' class /// </summary> class ConcreteStrategyC : Strategy { public override void AlgorithmInterface() { Console.WriteLine( "Called ConcreteStrategyC.AlgorithmInterface()"); } }
/// <summary> /// The 'Context' class /// </summary> class Context { private Strategy _strategy;
// Constructor public Context(Strategy strategy) { this._strategy = strategy; }
public void ContextInterface() { _strategy.AlgorithmInterface(); } } } |
| Output Called ConcreteStrategyA.AlgorithmInterface() |
This real-world code demonstrates the Strategy pattern which encapsulates sorting algorithms in the form of sorting objects. This allows clients to dynamically change sorting strategies including Quicksort, Shellsort, and Mergesort.
Hide code
| // Strategy pattern -- Real World example |
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using System; using System.Collections.Generic;
namespace DoFactory.GangOfFour.Strategy.RealWorld { /// <summary> /// MainApp startup class for Real-World /// Strategy Design Pattern. /// </summary> class MainApp { /// <summary> /// Entry point into console application. /// </summary> static void Main() { // Two contexts following different strategies SortedList studentRecords = new SortedList();
studentRecords.Add("Samual"); studentRecords.Add("Jimmy"); studentRecords.Add("Sandra"); studentRecords.Add("Vivek"); studentRecords.Add("Anna");
studentRecords.SetSortStrategy(new QuickSort()); studentRecords.Sort();
studentRecords.SetSortStrategy(new ShellSort()); studentRecords.Sort();
studentRecords.SetSortStrategy(new MergeSort()); studentRecords.Sort();
// Wait for user Console.ReadKey(); } }
/// <summary> /// The 'Strategy' abstract class /// </summary> abstract class SortStrategy { public abstract void Sort(List<string> list); }
/// <summary> /// A 'ConcreteStrategy' class /// </summary> class QuickSort : SortStrategy { public override void Sort(List<string> list) { list.Sort(); // Default is Quicksort Console.WriteLine("QuickSorted list "); } }
/// <summary> /// A 'ConcreteStrategy' class /// </summary> class ShellSort : SortStrategy { public override void Sort(List<string> list) { //list.ShellSort(); not-implemented Console.WriteLine("ShellSorted list "); } }
/// <summary> /// A 'ConcreteStrategy' class /// </summary> class MergeSort : SortStrategy { public override void Sort(List<string> list) { //list.MergeSort(); not-implemented Console.WriteLine("MergeSorted list "); } }
/// <summary> /// The 'Context' class /// </summary> class SortedList { private List<string> _list = new List<string>(); private SortStrategy _sortstrategy;
public void SetSortStrategy(SortStrategy sortstrategy) { this._sortstrategy = sortstrategy; }
public void Add(string name) { _list.Add(name); }
public void Sort() { _sortstrategy.Sort(_list);
// Iterate over list and display results foreach (string name in _list) { Console.WriteLine(" " + name); } Console.WriteLine(); } } } |
|
QuickSorted list |