Design Patterns in Swift: Visitor Pattern Explained
Design patterns are a set of best practices that software engineers use to create maintainable and reusable code. In the world of software development, design patterns are widely used for many different purposes, such as creating a framework for problem solving, managing complexity, and improving code readability. In this article, we will explore one of the most popular design patterns used in the Swift programming language – the Visitor Pattern.
The Visitor Pattern is a behavioral design pattern that allows us to separate an algorithm from an object structure on which it operates. This pattern can be used when we need to perform operations on objects of different types. By using the Visitor Pattern, we can easily add new features or operations on existing objects without changing the objects’ structure.
The Visitor Pattern is composed of two main components: a Visitor and an Element. The Visitor is an abstract class that defines an interface for performing operations on the elements of the object structure. The Element is an interface that defines an accept() method, which takes a visitor as an argument.
In Swift, we can implement the Visitor Pattern by defining a protocol for the Visitor and a protocol for the Element. The Visitor protocol should define methods for performing operations on the elements of the object structure. The Element protocol should define an accept() method that takes a visitor as an argument.
Let’s take a look at an example of how we can use the Visitor Pattern in Swift. Suppose we have an object structure that represents a collection of shapes, such as circles, rectangles, and squares. We can define a protocol called Shape that represents the elements of our object structure:
protocol Shape {
func accept(visitor: Visitor)
}
We can then define a protocol called Visitor that defines methods for performing operations on the elements of our object structure:
protocol Visitor {
func visit(circle: Circle)
func visit(rectangle: Rectangle)
func visit(square: Square)
}
Next, we can define classes for each of the shapes in our object structure:
class Circle: Shape {
func accept(visitor: Visitor) {
visitor.visit(circle: self)
}
}
class Rectangle: Shape {
func accept(visitor: Visitor) {
visitor.visit(rectangle: self)
}
}
class Square: Shape {
func accept(visitor: Visitor) {
visitor.visit(square: self)
}
}
Finally, we can define a concrete visitor class that implements the Visitor protocol:
class AreaVisitor: Visitor {
var totalArea = 0.0
func visit(circle: Circle) {
totalArea += Double.pi * circle.radius * circle.radius
}
func visit(rectangle: Rectangle) {
totalArea += rectangle.width * rectangle.height
}
func visit(square: Square) {
totalArea += square.side * square.side
}
}
The AreaVisitor class implements the Visitor protocol and defines methods for calculating the area of each shape in our object structure. To use the AreaVisitor class, we simply need to create an instance of it and pass it to the accept() methods of each of the shapes in our object structure.
In this article, we have explored the Visitor Pattern and how it can be used to separate an algorithm from an object structure on which it operates. We saw how to implement the Visitor Pattern in Swift by defining a protocol for the Visitor and a protocol for the Element. Finally, we looked at an example of how the Visitor Pattern can be used to calculate the area of a collection of shapes.