Design Patterns in Swift: Visitor Pattern Explained
The Visitor pattern is a powerful design pattern commonly used in software development. It allows for adding new operations to existing objects, without modifying the object itself. This type of design pattern is useful in situations where you need to add additional behaviour to an object without having to modify the object’s class. In this article, we will explore how to use the Visitor pattern in Swift programming language.
The Visitor pattern is a behavioral design pattern which enables us to separate an algorithm from an object structure on which it operates. The pattern allows us to define a new operation without changing the classes of the elements on which it operates. The visitors are known as the “visitor” objects and the elements they operate on are known as the “element” objects.
To better understand the Visitor pattern, let’s take a look at a simple example. Consider a class structure with two classes: a base class called “Shape” and two subclasses called “Circle” and “Square”. Each of these classes has a method called “draw()” which draws a shape on the screen.
Now, let’s say we want to add a new operation to our Shape class, such as calculating the area of the shape. We can do this without modifying the Shape class itself by using the Visitor pattern. We can create a new Visitor class called “AreaVisitor” which will calculate the area of the shapes. The AreaVisitor class will have a method called “visit()” which will accept an instance of the Shape class as a parameter.
The “visit()” method will then check the type of the Shape instance and call the appropriate method to calculate the area. For example, if the Shape instance is an instance of the Circle class, the AreaVisitor will call the “calculateCircleArea()” method to calculate the area of the circle. Similarly, if the Shape instance is an instance of the Square class, the AreaVisitor will call the “calculateSquareArea()” method to calculate the area of the square.
Let’s take a look at some code to see how this works. First, let’s define the Shape class:
class Shape {
func draw() {
// draw the shape
}
}
Next, let’s define the Circle and Square classes which extend the Shape class:
class Circle: Shape {
var radius: Double
init(radius: Double) {
self.radius = radius
}
override func draw() {
// draw the circle
}
}
class Square: Shape {
var sideLength: Double
init(sideLength: Double) {
self.sideLength = sideLength
}
override func draw() {
// draw the square
}
}
Finally, let’s define the AreaVisitor class:
class AreaVisitor {
func visit(shape: Shape) {
if let circle = shape as? Circle {
calculateCircleArea(circle: circle)
} else if let square = shape as? Square {
calculateSquareArea(square: square)
}
}
private func calculateCircleArea(circle: Circle) {
let area = Double.pi * (circle.radius * circle.radius)
print("Area of circle is: \(area)")
}
private func calculateSquareArea(square: Square) {
let area = square.sideLength * square.sideLength
print("Area of square is: \(area)")
}
}
Now that we have defined our classes, let’s see how we can use them. First, let’s create an instance of the Circle class with a radius of 5.0:
let circle = Circle(radius: 5.0)
Next, let’s create an instance of the AreaVisitor class:
let areaVisitor = AreaVisitor()
Finally, let’s call the “visit()” method of the AreaVisitor class, passing in the Circle instance as a parameter:
areaVisitor.visit(shape: circle)
This will call the “calculateCircleArea()” method of the AreaVisitor class and print out the area of the circle.
The Visitor pattern is a powerful pattern which enables us to add new operations to existing objects without modifying the objects themselves. It is particularly useful when dealing with complex class structures, as it allows us to separate the algorithm from the object structure. In this article, we have explored how to use the Visitor pattern in Swift programming language.