Decoupling Algorithms from Object Structures with the Visitor Design Pattern

The Visitor design pattern is a behavioral design pattern that allows you to separate an algorithm from an object structure on which it operates. This can be particularly useful when the algorithm needs to be applied to multiple, different types of objects. In this blog post, we'll take a look at how the Visitor design pattern is implemented in the Swift programming language.

To begin with, let's consider a simple example. Suppose we have a class hierarchy consisting of Document, Paragraph, and BulletPoint classes, which represent documents, paragraphs, and bullet points, respectively. The Document class contains a list of Paragraph objects, and each Paragraph object contains a list of BulletPoint objects.

Now suppose we want to implement a function that prints out the contents of a Document object in a formatted way. We could do this by adding a `printDetails()` method to the Document class and implementing it like so:

While this approach works, it has a couple of drawbacks. First, it couples the Document class to the print() method, which means that any changes to the print() method will require changes to the Document class as well. Second, if we want to add additional functionality to the print() method (e.g. to print the contents of a Paragraph object in a different way), we'll have to modify the print() method itself.

To solve these problems, we can use the Visitor design pattern. In this pattern, the print() method is separated from the Document class and implemented as a separate PrintVisitor class, which accepts a Document object as an argument and applies the appropriate algorithm to it.

Here's how we would implement the PrintVisitor class in Swift:

Now, to use the PrintVisitor class to print the contents of a Document object, we simply need to create an instance of the PrintVisitor class and call the visit() method, passing in the Document object as an argument.

The Visitor Design Pattern is a powerful tool for separating algorithms from object structures in Swift. It allows for a more flexible and modular approach to object-oriented programming, and can be used to implement complex behaviors in a clean and organized manner.