Design Patterns: Visitor in Swift: Learn to Implement and Master It
Design patterns are an important tool for software developers. They are a set of solutions to common software design problems, which have been identified and documented over the years. One of the most popular design patterns is the Visitor pattern. This pattern allows a developer to separate an algorithm from an object structure on which it operates. In this article, we will learn how to implement the Visitor pattern in Swift, and how to master it.
The Visitor pattern was first described by the Gang of Four (GoF) in their book Design Patterns: Elements of Reusable Object-Oriented Software. The pattern is used to separate an algorithm from an object structure on which it operates. The idea is that instead of having an algorithm embedded within an object structure, the algorithm can be applied separately. This allows the object structure to remain unchanged while new algorithms can be added or existing ones modified without changing the object structure.
To understand how the Visitor pattern works in Swift, let’s take a look at a simple example. Suppose we have a class called Student, which contains information about a student such as name, age, and grade. We also have a class called Course, which contains information about a course such as name, teacher, and number of credits. Our goal is to create an algorithm that can calculate the total number of credits a student has taken.
In this scenario, we could use the Visitor pattern to separate the algorithm from the object structure. We would create a Visitor class, which would contain the algorithm to calculate the total number of credits a student has taken. Then, we would create a method in the Student class, which would accept the visitor object and call its calculateTotalCredits method.
Let’s see how this would look in Swift code:
class Student {
var name: String
var age: Int
var grade: Int
init(name: String, age: Int, grade: Int) {
self.name = name
self.age = age
self.grade = grade
}
func acceptVisitor(visitor: CreditCalculatorVisitor) {
visitor.calculateTotalCredits(student: self)
}
}
class Course {
var name: String
var teacher: String
var credits: Int
init(name: String, teacher: String, credits: Int) {
self.name = name
self.teacher = teacher
self.credits = credits
}
}
protocol CreditCalculatorVisitor {
func calculateTotalCredits(student: Student) -> Int
}
class CreditCalculator: CreditCalculatorVisitor {
private var courses = [Course]()
func addCourse(course: Course) {
courses.append(course)
}
func calculateTotalCredits(student: Student) -> Int {
var totalCredits = 0
for course in courses {
if course.teacher == student.name {
totalCredits += course.credits
}
}
return totalCredits
}
}
let john = Student(name: "John", age: 21, grade: 3)
let math = Course(name: "Math", teacher: "John", credits: 4)
let physics = Course(name: "Physics", teacher: "John", credits: 5)
let creditCalculator = CreditCalculator()
creditCalculator.addCourse(course: math)
creditCalculator.addCourse(course: physics)
let totalCredits = john.acceptVisitor(visitor: creditCalculator)
print("John has taken a total of \(totalCredits) credits.")
// Output: John has taken a total of 9 credits.
In this example, we created a Student class, a Course class, and a CreditCalculatorVisitor protocol. We then created a CreditCalculator class which implements the protocol. This class contains the algorithm to calculate the total number of credits a student has taken. Finally, we created an instance of the Student class and an instance of the CreditCalculator class and used the acceptVisitor method of the Student class to call the calculateTotalCredits method of the CreditCalculator class.
As you can see, the Visitor pattern can be a powerful tool for separating an algorithm from an object structure. It allows us to keep the object structure unchanged while adding or modifying algorithms without having to change the object structure itself. By mastering the Visitor pattern, we can create more flexible and maintainable code.