前几天跟Yuan Ye Pair的时候发现了这个方法
1. 经典的Visitor模式(循环依赖)
public interface Document
{
void Accept(DocumentVisitor visitor);
}
public class WordDocument : Document
{
public void Accept(DocumentVisitor visitor)
{
visitor.Visit(this);
}
}
public class ExcelDocument : Document
{
public void Accept(DocumentVisitor visitor)
{
visitor.Visit(this);
}
}
public interface DocumentVisitor
{
void Visit(WordDocument wordDocument);
void Visit(ExcelDocument excelDocument);
}
这里面 Document依赖于DocumentVisitor, DocumentVisitor依赖于WordDocument和ExcelDocument, 而WrodDocument等又依赖于Document
这样 Document->DocumentVisitor->WordDocument->Document->...循环依赖
2. 经典的解依赖Visitor模式(Down Cast)
public interface Document
{
void Accept(DocumentVisitor visitor);
}
public class WordDocument : Document
{
public void Accept(DocumentVisitor visitor)
{
if (visitor is WordDocumentVisitor)
{
((WordDocumentVisitor)visitor).Visit(this);
}
}
}
public class ExcelDocument : Document
{
public void Accept(DocumentVisitor visitor)
{
if (visitor is ExcelDocumentVisitor)
{
((ExcelDocumentVisitor)visitor).Visit(this);
}
}
}
public interface DocumentVisitor
{
//空接口, 依赖在这里断掉了
}
public class ExcelDocumentVisitor : DocumentVisitor
{
public void Visit(ExcelDocument excelDocument)
{
}
}
public class WordDocumentVisitor : DocumentVisitor
{
public void Visit(WordDocument wordDocument)
{
}
}
3. 消除无聊Accept()方法的Visitor模式
上面的Accept方法几乎千篇一律, 遵循相同的模式. 里面无非就是类型计算, 可以利用C#的泛型支持将之消除
public interface Document
{
void Accept(DocumentVisitor visitor);
}
public abstract class VisitableDocument<T> : Document where T : VisitableDocument<T>
{
public void Accept(DocumentVisitor visitor)
{
var interfaces = visitor.GetType().GetInterfaces();
foreach (var type in interfaces)
{
if (type.GetGenericArguments().Contains(typeof(T)))
{
((DocumentVisitor<T>)visitor).Visit((T)this);
}
}
}
}
public class WordDocument : VisitableDocument<WordDocument>
{
//不需要自己实现Accept, 基类已经实现
}
public class ExcelDocument : VisitableDocument<ExcelDocument>
{
//不需要自己实现Accept, 基类已经实现
}
public interface DocumentVisitor
{
//空接口
}
public interface DocumentVisitor<T> : DocumentVisitor where T : VisitableDocument<T>
{
//泛型, 依赖在这里断掉了
void Visit(T document);
}
public class PrintDocumentVisitor : DocumentVisitor<WordDocument>, DocumentVisitor<ExcelDocument>
{
public void Visit(WordDocument wordDocument)
{
}
public void Visit(ExcelDocument excelDocument)
{
}
}