Adapter

Adapters are used to convert the interface of a given class, known as an Adaptee, into another interface, called the Target. Operations on the Target are called by a Client, and those operations are adapted by the Adapter and passed on to the Adaptee.

In Swift, Adapters can often be formed through the use of protocols. In the following example, a Client able to communicate with the Target is provided with the ability to perform functions of the Adaptee class through the use of an adapter.

// The functionality to which a Client has no direct access
class Adaptee {
    func foo() {
        // ...
    }
}

// Target's functionality, to which a Client does have direct access
protocol TargetFunctionality {
    func fooBar() {}
}

// Adapter used to pass the request on the Target to a request on the Adaptee
extension Adaptee: TargetFunctionality {
    func fooBar() {
        foo()
    }
}

Example flow of a one-way adapter: Client -> Target -> Adapter -> Adaptee

Adapters can also be bi-directional, and these are known as two-way adapters. A two-way adapter may be useful when two different clients need to view an object differently.