The Visitor Pattern

31. The Visitor Pattern The Visitor pattern turns the tables on our object-oriented model and creates an external class to act on data in other classes. This is useful when you have a polymorphic operation that cannot reside in the class hierarchy for some reason—for example, because the operation wasn’t considered when the hierarchy was designed or it would clutter the interface of the classes unnecessarily. Motivation While at first it may seem “unclean” to put operations inside one class that should be in another, there are good reasons for doing so. Suppose each of a number of drawing object classes has similar code for drawing itself. The drawing methods may be different, but they probably all use underlying utility functions that we might have to duplicate in each class. Further, a set of closely related functions is scattered throughout a number of different classes, as shown in Figure 31-1. Figure 31-1 – A DrawObject and three of its subclasses Instead, we write a Visitor class that contains all the related draw methods and have it visit each of the objects in succession (Figure 31-2). Figure 31-2 – A Visitor class (Drawer) that visits each of three triangle classes The first question that most people ask about this pattern is “What does visiting mean?” There is only one way that an outside class can gain access to another class, and that is by calling its public methods. In the Visitor case, visiting each class means that you are calling a method already installed for this purpose, called accept. The accept method has one argument: the instance of the visitor. In return, it calls the visit method of the Visitor, passing itself as an argument, as shown in Figure 31-3. visited.accept(v) Visitor v.visit(Me) Figure 31-3 - How the visit and accept methods interact Visited instance Putting it in simple code terms, every object that you want to visit must have the following method. public virtual void accept(Visitor v) { v.visit(this); } In this way, the Visitor object receives a reference to each of the instances, one by one, and can then call its public methods to obtain data, perform calculations, generate reports, or just draw the object on the screen. Of course, if the class does not have an accept method, you can subclass it and add one. When to Use the Visitor Pattern You should consider using a Visitor pattern when you want to perform an operation on the data contained in a number of objects that have different interfaces. Visitors are also valuable if you have to perform a number of unrelated operations on these classes. Visitors are a useful way to add function to class libraries or frameworks for which you either do not have the course or cannot change the source for other technical (or political) reasons. In these latter cases, you simply subclass the classes of the framework and add the accept method to each subclass. On the other hand, as we will see, Visitors are a good choice only when you do not expect many new classes to be added to your program. Sample Code Let’s consider a simple subset of the Employee problem we discussed in the Composite pattern. We have a simple Employee object that maintains a record of the employee’s name, salary, vacation taken, and number of sick days taken. The following is a simple version of this class. public class Employee { int sickDays, vacDays; float salary; string name; public Employee(string name, float salary, int vDays, int sDays) { this.name this.salary = salary; sickDays = sDays; vacDays = vDays; } //----public string getName() { return name; } public int getSickDays() { return sickDays; } public int getVacDays() { return vacDays; } public float getSalary() { return salary; } public virtual void accept(Visitor v) { v.visit(this); } } Note that we have included the accept method in this class. Now let’s suppose that we want to prepare a report on the number of vacation days that all employees have taken so far this year. We could just write some code in the client to sum the results of calls to each Employee’s getVacDays function, or we could put this function into a Visitor. Since C# is a strongly typed language, our base Visitor class needs to have a suitable abstract visit method for each kind of class in your program. In this first simple example, we only have Employees, so our basic abstract Visitor class is just the following. public abstract class Visitor { public abstract void visit(Employee emp); public abstract void visit(Boss bos); } Notice that there is no indication what the Visitor does with each class in either the client classes or the abstract Visitor class. We can, in fact, write a whole lot of visitors that do different things to the classes in our program. The Visitor we are going to write first just sums the vacation data for all our employees. public class VacationVisitor : Visitor { private int totalDays; //----public VacationVisitor() { totalDays = 0; } //----public int getTotalDays() { return totalDays; } //----public override void visit(Employee emp){ totalDays += emp.getVacDays (); } //----public override void visit(Boss bos){ totalDays += bos.getVacDays (); } } Visiting the Classes Now all we have to do to compute the total vacation days taken is go through a list of the employees, visit each of them, and ask the Visitor for the total. for (int i = 0; i< empls.Length; i++) { empls[i].accept(vac); //get the employee } lsVac.Items.Add("Total vacation days=" + vac.getTotalDays().ToString()); Let’s reiterate what happens for each visit. 1. We move through a loop of all the Employees. 2. The Visitor calls each Employee’s accept method. 3. That instance of Employee calls the Visitor’s visit method. 4. The Visitor fetches the vacation days and adds them into the total. 5. The main program prints out the total when the loop is complete. Visiting Several Classes The Visitor becomes more useful when there are a number of different classes with different interfaces and we want to encapsulate how we get data from these classes. Let’s extend our vacation days model by introducing a new Employee type called Boss. Let’s further suppose that at this company, Bosses are rewarded with bonus vacation days (instead of money). So the Boss class has a couple of extra methods to set and obtain the bonus vacation day information. public class Boss : Employee { private int bonusDays; public Boss(string name, float salary, int vdays, int sdays): base(name, salary, vdays, sdays) { } public void setBonusDays(int bdays) { bonusDays = bdays; } public int getBonusDays() { return bonusDays; } public override void accept(Visitor v ) { v.visit(this); } } When we add a class to our program, we have to add it to our Visitor as well, so that the abstract template for the Visitor is now the following. public abstract class Visitor { public abstract void visit(Employee emp); public abstract void visit(Boss bos); } This says that any concrete Visitor classes we write must provide polymorphic visit methods for both the Employee class and the Boss class. In the case of our vacation day counter, we need to ask the Bosses for both regular and bonus days taken, so the visits are now different. We’ll write a new bVacationVisitor class that takes account of this difference. public class bVacationVisitor :Visitor { private int totalDays; public bVacationVisitor() { totalDays = 0; } //----public override void visit(Employee emp) { totalDays += emp.getVacDays(); try { Manager mgr = (Manager)emp; totalDays += mgr.getBonusDays(); } catch(Exception ){} } //----public override void visit(Boss bos) { totalDays += bos.getVacDays(); totalDays += bos.getBonusDays(); } //----public int getTotalDays() { return totalDays; } } Note that while in this case Boss is derived from Employee, it need not be related at all as long as it has an accept method for the Visitor class. It is quite important, however, that you implement a visit method in the Visitor for every class you will be visiting and not count on inheriting this behavior, since the visit method from the parent class is an Employee rather than a Boss visit method. Likewise, each of your derived classes (Boss, Employee, etc.) must have its own accept method rather than calling one in its parent class. This is illustrated in the class diagram in Figure 31-4. Visitor Employee +visit(emp) +visit(bos) +New +getName +getSalary +getSickdays +getVacDays +accept VacationVisitor Boss bVacationVisitor +New +setBonusDays +getBonusDays:Integer +accept Figure 31-4 – The two visitor classes visiting the Boss and Employee classes Bosses Are Employees, Too We show in Figure 31-5 a simple application that carries out both Employee visits and Boss visits on the collection of Employees and Bosses. The original VacationVisitor will just treat Bosses as Employees and get only their ordinary vacation data. The bVacationVisitor will get both. for (int i = 0; i< empls.Length; i++) { empls[i].accept(vac); //get the employee empls[i].accept(bvac); } lsVac.Items.Add("Total vacation days=" + vac.getTotalDays().ToString()); lsVac.Items.Add("Total boss vacation days=" + bvac.getTotalDays().ToString()); The two lines of displayed data represent the two sums that are computed when the user clicks on the Vacations button. Figure 31-5 – A simple application that performs the vacation visits described Catch-All Operations with Visitors In the preceding cases, the Visitor class has a visit method for each visiting class, such as the following. public abstract void visit(Employee emp); public abstract void visit(Boss bos); However, if you start subclassing your visitor classes and adding new classes that might visit, you should recognize that some visit methods might not be satisfied by the methods in the derived class. These might instead “fall through” to methods in one of the parent classes where that object type is recognized. This provides a way of specifying default visitor behavior. Now every class must override accept(v) with its own implementation so the return call v.visit(this) returns an object this of the correct type and not of the superclass’s type. Let’s suppose that we introduce another layer of management into our company: the Manager. Managers are subclasses of Employees, and now they have the privileges formerly reserved for Bosses of extra vacation days. Bosses now have an additional reward—stock options. Now if we run the same program to compute vacation days but do not revise our Visitor to look for Managers, it will recognize them as mere Employees and count only their regular vacation and not their extra vacation days. However, the catch-all parent class is a good thing if subclasses may be added to the application from time to time and you want the visitor operations to continue to run without modification. There are three ways to integrate the new Manager class into the visitor system. You could define a ManagerVisitor or use the BossVisitor to handle both. However, there could be conditions when continually modifying the Visitor structure is not desirable. In that case, you could simply test for this special case in the EmployeeVisitor class. public override void visit(Employee emp) { totalDays += emp.getVacDays(); try { Manager mgr = (Manager)emp; totalDays += mgr.getBonusDays(); } catch(Exception ){} } While this seems “unclean” at first compared to defining classes properly, it can provide a method of catching special cases in derived classes without writing whole new visitor program hierarchies. This “catch-all” approach is discussed in some detail in the book Pattern Hatching (Vlissides 1998). Double Dispatching No discussion on the Visitor pattern is complete without mentioning that you are really dispatching a method twice for the Visitor to work. The Visitor calls the polymorphic accept method of a given object, and the accept method calls the polymorphic visit method of the Visitor. It is this bidirectional calling that allows you to add more operations on any class that has an accept method, since each new Visitor class we write can carry out whatever operations we might think of using the data available in these classes. Why Are We Doing This? You may be asking your self why we are jumping through these hoops when we could call the getVacationDays methods directly. By using this “callback” approach, we are implementing “double dispatching.” There is no requirement that the objects we visit be of the same or even of related types. Further, using this callback approach, you can have a different visit method called in the Visitor, depending on the actual type of class. This is harder to implement directly. Further, if the list of objects to be visited in an ArrayList is a collection of different types, having different versions of the visit methods in the actual Visitor is the only way to handle the problem without specifically checking the type of each class. Traversing a Series of Classes The calling program that passes the class instances to the Visitor must know about all the existing instances of classes to be visited and must keep them in a simple structure such as an array or collection. Another possibility would be to create an Enumeration of these classes and pass it to the Visitor. Finally, the Visitor itself could keep the list of objects that it is to visit. In our simple example program, we used an array of objects, but any of the other methods would work equally well. Consequences of the Visitor Pattern The Visitor pattern is useful when you want to encapsulate fetching data from a number of instances of several classes. Design Patterns suggests that the Visitor can provide additional functionality to a class without changing it. We prefer to say that a Visitor can add functionality to a collection of classes and encapsulate the methods it uses. The Visitor is not magic, however, and cannot obtain private data from classes. It is limited to the data available from public methods. This might force you to provide public methods that you would otherwise not have provided. However, it can obtain data from a disparate collection of unrelated classes and utilize it to present the results of a global calculation to the user program. It is easy to add new operations to a program using Visitors, since the Visitor contains the code instead of each of the individual classes. Further, Visitors can gather related operations into a single class rather than forcing you to change or derive classes to add these operations. This can make the program simpler to write and maintain. Visitors are less helpful during a program’s growth stage, since each time you add new classes that must be visited, you have to add an abstract visit operation to the abstract Visitor class, and you must add an implementation for that class to each concrete Visitor you have written. Visitors can be powerful additions when the program reaches the point where many new classes are unlikely. Visitors can be used very effectively in Composite systems, and the bossemployee system we just illustrated could well be a Composite like the one we used in the Composite chapter. Thought Question An investment firm’s customer records consist of an object for each stock or other financial instrument each investor owns. The object contains a history of the purchase, sale, and dividend activities for that stock. Design a Visitor pattern to report on net end-of-year profit or loss on stocks sold during the year. Programs on the CD-ROM \Visitor\ Visitor example