Swift: Developing iOS Applications

Extensions

Up until this point, we had to define our entire custom type in a single file. However, it is sometimes useful to separate out part of our custom types into different files, or even just in the same file. To achieve this, Swift provides a feature called extensions. Extensions allow us to add additional functionality to existing types from anywhere.

This functionality is limited to additional functions and additional computed properties:

extension Building {
    var report: String {
        return "This building is \(self.squareFootage) sq ft"
    }

    func isLargerThanOtherBuilding(building: Building) -> Bool {
        return self.squareFootage > building.squareFootage
    }
}

Note that, to define an extension, we use the extension keyword, followed by the type that ...