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10: Behavioral Patterns: Visitor, Interpreter, and Memento
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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 } 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 // Added to make the elements Visitor-ready public abstract void Accept(IVisitor visitor); } class Element : IElement { public Element Next {get; set;} public Element Link {get; set;} public Element ( ) {} public Element (Element next) { Next = next; } // Added to make the elements Visitor-ready public override void Accept(IVisitor visitor) { visitor.Visit(this); } } class ElementWithLink : Element { public ElementWithLink (Element link, Element next) { Next = next; Link = link; } // Added to make the elements Visitor-ready public override void Accept(IVisitor visitor) { visitor.Visit(this); } }
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// Visitor interface interface IVisitor { void Visit (Element element); void Visit (ElementWithLink element); } // Visitor class CountVisitor : IVisitor { public int Count {get; set;} public void CountElements(Element element) { element.Accept(this); if (element.Link!=null) CountElements(element.Link); if (element.Next!=null) CountElements(element.Next); } //Elements with links are not counted public void Visit(ElementWithLink element) { Console.WriteLine("Not counting"); } // Only Elements are counted public void Visit(Element element) { Count++;
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62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 } } // Client class Client { static void Main( ) { // Set up the object structure Element objectStructure = new Element( new Element( new ElementWithLink( new Element( new Element( new ElementWithLink( new Element(null), new Element( null)))), new Element( new Element( new Element(null)))))); Console.WriteLine ("Count the Elements"); CountVisitor visitor = new CountVisitor( ); visitor.CountElements(objectStructure); Console.WriteLine("Number of Elements is: "+visitor.Count); } } /* Count the Elements Found Element Found Element Number of Elements is: 9 */
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Now, we can set up Visitor s communication with the object structure as follows: 1. Add an IVisitor interface listing Visit methods for each type in the structure and make the Visitor implement it:
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// Visitor interface interface IVisitor { void Visit (Element element); void Visit (ElementWithLink element); }
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2. In the Visitor class, add implementations for each of these methods, for example:
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// Only Elements are counted public void Visit(Element element) { Count++; }
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10: Behavioral Patterns: Visitor, Interpreter, and Memento
3. In the ObjectStructure s namespace, add an interface with one method called Accept, and make the top level type implement it:
abstract class IElement { // Added to make the elements Visitor-ready public abstract void Accept(IVisitor visitor); }
4. In each type of the structure, add an Accept method of the following identical form:
// Added to make the elements Visitor-ready public override void Accept(IVisitor visitor) { visitor.Visit(this); }
Once all this is in place, the Visitor pattern s mechanism can get into action. Within the recursive method CountElements (line 48 52), an Accept method is called on an object of type Element.
The Elements are unaware of what visitors to expect, but they do have to supply the hooks to accept visits.
Because of the object hierarchy, the actual object could also be ElementWithLink. At runtime, the appropriate Accept statement in the corresponding class will be called. These small methods simply bounce the call back to the correct Visit method in the CountVisitor (line 23 goes to line 55 and line 34 goes to line 60). In this way, the Visitor pattern uses a well-known method called double-dispatch. Now, consider how to extend the example. We can add another Visitor with its own Visit methods and use the same Accept methods already build into the structure. That is good news. But what is not such good news, is that we had to add the Accept methods in the first place. It is possible that we might not have been able to do so (because the code was not accessible). For this reason, we are going to consider an alternative way of finding the right Visit method, without going into the structure to add the Accept methods: reflection. Put together, the reflective methods and types outlined in the sidebar form a ReflectiveVisit method in the CountVisitor:
public void ReflectiveVisit(Element element) { // Use reflection to find and invoke the correct Visit method Type[] types = new Type[] {element.GetType( )}; MethodInfo methodInfo = this.GetType( ).GetMethod("Visit", types); if (methodInfo != null) methodInfo.Invoke(this, new object[] {element}); else Console.WriteLine("Unexpected Visit"); }
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