I'm new in this pattern , could please someone help me in it?
I got an Object like this :
public class Object
{
public string Name { get; set; }
public object Value { get; set; }
public List<Object> Childs { get; set; }
}
Here is an JSON example:
{
"Name": "Method",
"Value": "And",
"Childs": [{
"Name": "Method",
"Value": "And",
"Childs": [{
"Name": "Operator",
"Value": "IsEqual",
"Childs": [{
"Name": "Name",
"Value": "5",
"Childs": []
}]
},
{
"Name": "Operator",
"Value": "IsEqual",
"Childs": [{
"Name": "Name",
"Value": "6",
"Childs": []
}]
}]
},
{
"Name": "Operator",
"Value": "IsEqual",
"Childs": [{
"Name": "Name",
"Value": "3",
"Childs": []
}]
}]
}
My question how to make Visitor Pattern in order to get this final string:
(Name IsEqual 3)And((Name IsEqul 5)And(Name IsEqual 6))
To implement visitor pattern you need two simple interfaces
IVisitablewith anAcceptmethod having theIVisitoras the parameter.IVisitorwith manyVisitmethods for each implementation ofIVisitable
So basic idea of the visitor pattern is to change the behavior dynamically according to the type of implementation.
For your case the thing you want to visit (the visitable) is the Object class which apparently does not have different derivatives and you want to change the behavior according to a property value not the type. So Visitor Pattern is not what you really need here and I highly recommend you to consider the answers with the recursive method.
But if you really want to use visitor pattern here, it may look something like this.
interface IVisitable { void Accept(IVisitor visitor); }
interface IVisitor {
void VisitAnd(Object obj);
void VisitEquals(Object obj);
}
Since the Object class is a simple POCO I assume you won't want to implement an interface and add a method into this class. So you'll need an adapter object which adapts Object to IVisitable
class VisitableObject : IVisitable {
private Object _obj;
public VisitableObject(Object obj) { _obj = obj; }
public void Accept(IVisitor visitor) {
// These ugly if-else are sign that visitor pattern is not right for your model or you need to revise your model.
if (_obj.Name == "Method" && _obj.Value == "And") {
visitor.VisitAnd(obj);
}
else if (_obj.Name == "Method" && _obj.Value == "IsEqual") {
visitor.VisitEquals(obj);
}
else
throw new NotSupportedException();
}
}
}
public static ObjectExt {
public static IVisitable AsVisitable(this Object obj) {
return new VisitableObject(obj);
}
}
And finally the visitor implementation may look like this
class ObjectVisitor : IVisitor {
private StringBuilder sb = new StringBuilder();
public void VisitAnd(Object obj) {
sb.Append("(");
var and = "";
foreach (var child in obj.Children) {
sb.Append(and);
child.AsVisitable().Accept(this);
and = "and";
}
sb.Append(")");
}
public void VisitEquals(Object obj) {
// Assuming equal object must have exactly one child
// Which again is a sign that visitor pattern is not bla bla...
sb.Append("(")
.Append(obj.Children[0].Name);
.Append(" Equals ");
.Append(obj.Children[0].Value);
.Append(")");
}
}
The JSON clearly represents a token tree (possibly produced by a parser).
Visitor pattern use polymorphism.
In order to be used by a Visitor pattern, you must deserialize it to obtain objects with the different Visit behavior :
- MethodToken
- OperatorToken
- NameToken
Then IVisitor should implement Visit method for each:
public interface IVisitor
{
void Visit(MethodToken token) { /* */ }
void Visit(OperatorToken token) { /* */ }
void Visit(NameToken token) { /* */ }
}
public interface IVisitable
{
void Accept(IVisitor visitor);
}
public class MethodToken : IVisitable
{
public void Accept(IVisitor visitor)
{
visitor.Visit(this);
}
}
Additional remark:
Object is a really poor name especially in C# as Object is the base class for every classes, not to mention the conflict, it doesn't convey any special meaning ... What about token ?
public class Token
{
public string Name { get; set; }
public object Value { get; set; }
public List<Token> Childs { get; set; }
}
Purpose of Visitor
You shouldn't use a screwdriver if you don't know when/why to use it (by the way it can be dangerous).
Visitor pattern is useful to avoid 'ugly'/hard to maintain/painful to read dozen switch cases or the even worse if else if else while giving you the strong type checking advantage. It also helps to keep related code (high cohesion) in one class (the Visitor). Of course, once implemented, the tree of objects (here tokens) can be visited by several kind of visitors as long as they implement the IVisitor interface.
In your case, you must first convert each Token to a strongly subtype of Token (through Dictionary mapping to avoid any if/switch or custom deserialization)
First of all you have wrong order in the result.Second, somethimes you miss brackets in the result.Final it should be:
(((Name IsEqual 5) And (Name IsEqual 6)) And (Name IsEqual 3))
To complete this task you should use recursive function.
static IEnumerable<string> ReturnString(Obj val)
{
foreach (Obj node in val.Childs)
yield return ConvertToString(node);
}
static string ConvertToString(Obj val)
{
switch(val.Name)
{
case "Operator":
{
return string.Format("({0} {1} {2})", val.Childs[0].Name, val.Value, val.Childs[0].Value);
}
case "Method":
{
IEnumerable<string> coll = ReturnString(val);
StringBuilder final = new StringBuilder();
final.Append("(");
IEnumerator<string> e = coll.GetEnumerator();
e.MoveNext();
final.Append(string.Format("{0}", e.Current, val.Value));
while (e.MoveNext())
{
final.Append(string.Format(" {0} {1}", val.Value, e.Current));
}
final.Append(")");
return final.ToString();
}
case "Name":
return Convert.ToString(val.Value);
}
return "-";
}
Below is your example in the code:
string s = ConvertToString(new Obj
{
Name = "Method",
Value = "And",
Childs = new List<Obj>
{
new Obj()
{
Name = "Method",
Value = "And",
Childs = new List<Obj>
{
new Obj()
{
Name = "Operator",
Value = "IsEqual",
Childs = new List<Obj>
{
new Obj()
{
Name="Name",
Value="5",
Childs=null
}
}
},
new Obj()
{
Name = "Operator",
Value = "IsEqual",
Childs = new List<Obj>
{
new Obj()
{
Name="Name",
Value="6",
Childs=null
}
}
}
}
},
new Obj()
{
Name = "Operator",
Value = "IsEqual",
Childs = new List<Obj>
{
new Obj()
{
Name="Name",
Value="3",
Childs=null
}
}
}
}
});
This might not be what you want. But one way to create the output that you want without using the Visitor pattern is add the following method to the Object class, like this:
public string Format()
{
if (Name == "Operator")
{
if(Childs == null || Childs.Count != 1)
throw new Exception("Invalid Childs");
Object chlid = Childs[0];
return chlid.Name + " IsEqual " + chlid.Value;
}
if (Name == "Method")
{
if(Childs == null || Childs.Count == 0)
throw new Exception("Invalid Childs");
var str = " " + Value + " ";
return string.Join(str, Childs.Select(x => "(" + x.Format() + ")"));
}
throw new Exception("Format should only be invoked on Operator/Method");
}
来源：https://stackoverflow.com/questions/32933122/implementing-visitor-pattern-in-c-sharp