Design Patterns: Visitor in Swift – Harness the Power of Polymorphism

Design Patterns: Visitor in Swift – Harness the Power of Polymorphism

Design patterns are a powerful tool for any software engineer, and the Visitor pattern is no exception. It allows us to add functionality to existing classes without having to modify them, making it a great way to use polymorphism effectively. In this blog post, we’ll discuss the Visitor pattern, its implementation in Swift, and some examples of how to use it.

The Visitor pattern is a behavioral design pattern that allows us to separate the logic of an algorithm from its data structure. This is done by creating a separate class that contains all the logic, which is then called by the data structure’s objects when needed. This separation of concerns makes it easier to maintain the code and also allows us to add new functionality without having to modify the existing code.

In Swift, the Visitor pattern is implemented using protocols. We create a protocol that defines the functionality that we want to add to the existing classes. Then, we create a class that implements this protocol and contains the logic that we want to add. Finally, we call this class from the objects of the data structure.

For example, let’s say we have a data structure that contains objects of type `Shape`. We want to add a function that prints out the area of each shape. To do this, we can create a protocol called `AreaVisitor` that defines a function called `visit` which takes a `Shape` as a parameter. Then, we can create a class called `AreaCalculator` that implements this protocol and defines the logic for calculating the area of each shape. Finally, we can call this class from the objects of the data structure, passing in the object as a parameter. This allows us to add the functionality without modifying the existing code.

 
protocol AreaVisitor {
    func visit(shape: Shape)
}

class AreaCalculator: AreaVisitor {
    func visit(shape: Shape) {
        switch shape {
        case let circle as Circle:
            print("The area of the circle is \(circle.radius * circle.radius * .pi)")
        case let rectangle as Rectangle:
            print("The area of the rectangle is \(rectangle.height * rectangle.width)")
        default:
            print("Unknown shape")
        }
    }
}

let shapes = [Circle(radius: 2.0), Rectangle(height: 3.0, width: 4.0)]

let areaCalculator = AreaCalculator()
shapes.forEach { shape in
    areaCalculator.visit(shape: shape)
}

The output of the above code would be:

“`
The area of the circle is 12.5663706143592
The area of the rectangle is 12.0
“`

The Visitor pattern is a great way to add functionality to existing classes without having to modify them. It also allows us to take advantage of polymorphism, as the Visitor class can act on different types of objects without needing to know their specific type. This makes it a powerful tool for any software engineer and should definitely be considered when designing a system.

In summary, the Visitor pattern is a powerful tool for any software engineer. It allows us to add functionality to existing classes without having to modify them, making it a great way to use polymorphism effectively. By creating a separate class that contains all the logic and calling it from the objects of the data structure, we can add new functionality without having to modify the existing code. We can also take advantage of polymorphism, as the Visitor class can act on different types of objects without needing to know their specific type. With all these benefits, the Visitor pattern should definitely be considered when designing a system.

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