职责链模式(Chain of Responsibility Pattern)
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一、 职责链(Chain of Responsibility)模式
从击鼓传花谈起
击鼓传花便是责任链模式的应用。责任链可能是一条直线、一个环链或者一个树结构的一部分。
二、 责任链模式的结构
责任链模式涉及到的角色如下所示:
抽象处理者(Handler)角色:定义出一个处理请求的接口。如果需要,接口可以定义出一个方法,以设定和返回对下家的引用。这个角色通常由一个抽象类或接口实现。
具体处理者(ConcreteHandler)角色:具体处理者接到请求后,可以选择将请求处理掉,或者将请求传给下家。由于具体处理者持有对下家的引用,因此,如果需要,具体处理者可以访问下家。
三、 责任链模式的示意性源代码
using System;
// "Handler"
abstract class Handler
{
// Fields
protected Handler successor;
// Methods
public void SetSuccessor( Handler successor )
{
this.successor = successor;
}
abstract public void HandleRequest( int request );
}
// "ConcreteHandler1"
class ConcreteHandler1 : Handler
{
// Methods
override public void HandleRequest( int request )
{
if( request >= 0 && request < 10 )
Console.WriteLine("{0} handled request {1}",
this, request );
else
if( successor != null )
successor.HandleRequest( request );
}
}
// "ConcreteHandler2"
class ConcreteHandler2 : Handler
{
// Methods
override public void HandleRequest( int request )
{
if( request >= 10 && request < 20 )
Console.WriteLine("{0} handled request {1}",
this, request );
else
if( successor != null )
successor.HandleRequest( request );
}
}
// "ConcreteHandler3"
class ConcreteHandler3 : Handler
{
// Methods
override public void HandleRequest( int request )
{
if( request >= 20 && request < 30 )
Console.WriteLine("{0} handled request {1}",
this, request );
else
if( successor != null )
successor.HandleRequest( request );
}
}
/**//// <summary>
/// Client test
/// </summary>
public class Client
{
public static void Main( string[] args )
{
// Setup Chain of Responsibility
Handler h1 = new ConcreteHandler1();
Handler h2 = new ConcreteHandler2();
Handler h3 = new ConcreteHandler3();
h1.SetSuccessor(h2);
h2.SetSuccessor(h3);
// Generate and process request
int[] requests = { 2, 5, 14, 22, 18, 3, 27, 20 };
foreach( int request in requests )
h1.HandleRequest( request );
}
}
四、 纯的与不纯的责任链模式
一个纯的责任链模式要求一个具体的处理者对象只能在两个行为中选择一个:一个是承担责任,二是把责任推给下家。不允许出现某一个具体处理者对象在承担了一部分责任后又把责任向下传的情况。
在一个纯的责任链模式里面,一个请求必须被某一个处理者对象所接收;在一个不纯的责任链模式里面,一个请求可以最终不被任何接收端对象所接收。纯的责任链模式的例子是不容易找到的,一般看到的例子均是不纯的责任链模式的实现。
五、 责任链模式的实际应用案例
using System;
// "Handler"
abstract class Approver
{
// Fields
protected string name;
protected Approver successor;
// Constructors
public Approver( string name )
{
this.name = name;
}
// Methods
public void SetSuccessor( Approver successor )
{
this.successor = successor;
}
abstract public void ProcessRequest( PurchaseRequest request );
}
// "ConcreteHandler"
class Director : Approver
{
// Constructors
public Director ( string name ) : base( name ) {}
// Methods
override public void ProcessRequest( PurchaseRequest request )
{
if( request.Amount < 10000.0 )
Console.WriteLine( "{0} {1} approved request# {2}",
this, name, request.Number);
else
if( successor != null )
successor.ProcessRequest( request );
}
}
// "ConcreteHandler"
class VicePresident : Approver
{
// Constructors
public VicePresident ( string name ) : base( name ) {}
// Methods
override public void ProcessRequest( PurchaseRequest request )
{
if( request.Amount < 25000.0 )
Console.WriteLine( "{0} {1} approved request# {2}",
this, name, request.Number);
else
if( successor != null )
successor.ProcessRequest( request );
}
}
// "ConcreteHandler"
class President : Approver
{
// Constructors
public President ( string name ) : base( name ) {}
// Methods
override public void ProcessRequest( PurchaseRequest request )
{
if( request.Amount < 100000.0 )
Console.WriteLine( "{0} {1} approved request# {2}",
this, name, request.Number);
else
Console.WriteLine( "Request# {0} requires " +
"an executive meeting!", request.Number );
}
}
// Request details
class PurchaseRequest
{
// Member Fields
private int number;
private double amount;
private string purpose;
// Constructors
public PurchaseRequest( int number,
double amount, string purpose )
{
this.number = number;
this.amount = amount;
this.purpose = purpose;
}
// Properties
public double Amount
{
get{ return amount; }
set{ amount = value; }
}
public string Purpose
{
get{ return purpose; }
set{ purpose = value; }
}
public int Number
{
get{ return number; }
set{ number = value; }
}
}
/**//// <summary>
/// ChainApp Application
/// </summary>
public class ChainApp
{
public static void Main( string[] args )
{
// Setup Chain of Responsibility
Director Larry = new Director( "Larry" );
VicePresident Sam = new VicePresident( "Sam" );
President Tammy = new President( "Tammy" );
Larry.SetSuccessor( Sam );
Sam.SetSuccessor( Tammy );
// Generate and process different requests
PurchaseRequest rs = new PurchaseRequest( 2034, 350.00, "Supplies" );
Larry.ProcessRequest( rs );
PurchaseRequest rx = new PurchaseRequest( 2035, 32590.10, "Project X" );
Larry.ProcessRequest( rx );
PurchaseRequest ry = new PurchaseRequest( 2036, 122100.00, "Project Y" );
Larry.ProcessRequest( ry );
}
}
六、 责任链模式的实现
责任链模式并不创建责任链。责任链的创建必须由系统的其它部分创建出来。
责任链模式降低了请求的发送端和接收端之间的耦合,使多个对象都有机会处理这个请求。一个链可以是一条线,一个树,也可以是一个环。如下图所示,责任链是一个树结构的一部分。
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