名称 | Singleton |
结构 | ![]() |
意图 | 保证一个类仅有一个实例,并提供一个访问它的全局访问点。 |
适用性 |
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// Singleton
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// Intent: "Ensure a class only has one instance, and provide a global
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// point of access to it".
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// For further information, read "Design Patterns", p127, Gamma et al.,
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// Addison-Wesley, ISBN:0-201-63361-2
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/* Notes:
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* If it makes sense to have only a single instance of a class (a so-called
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* singleton), then it makes sense to enforce this (to elimintate potential
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* errors, etc).
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*
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* A class based on the singleton design pattern protects its constructor,
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* so that only the class itself (e.g. in a static method) may instantiate itself.
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* It exposes an Instance method which allows client code to retrieve the
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* current instance, and if it does not exist to instantiate it.
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*/
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namespace Singleton_DesignPattern
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{
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using System;
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class Singleton
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{
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private static Singleton _instance;
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public static Singleton Instance()
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{
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if (_instance == null)
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_instance = new Singleton();
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return _instance;
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}
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protected Singleton(){}
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// Just to prove only a single instance exists
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private int x = 0;
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public void SetX(int newVal) {x = newVal;}
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public int GetX(){return x;}
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}
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/// <summary>
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/// Summary description for Client.
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/// </summary>
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public class Client
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{
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public static int Main(string[] args)
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{
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int val;
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// can't call new, because constructor is protected
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Singleton FirstSingleton = Singleton.Instance();
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Singleton SecondSingleton = Singleton.Instance();
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// Now we have two variables, but both should refer to the same object
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// Let's prove this, by setting a value using one variable, and
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// (hopefully!) retrieving the same value using the second variable
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FirstSingleton.SetX(4);
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Console.WriteLine("Using first variable for singleton, set x to 4");
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val = SecondSingleton.GetX();
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Console.WriteLine("Using second variable for singleton, value retrieved = {0}", val);
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return 0;
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}
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}
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}
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