Adding requirements to protocols 

Empty protocols can be useful as we've seen previously, but protocols are meant to provide requirements.

Protocols can require different aspects of objects:

• Properties, their type, and accessibility
• Methods
• Initializers

protocol DemoProtocol {
    var aRequiredProperty: String { get set }
    var aReadonlyProperty: Double { get }
    static var aStaticProperty: Int { get set }

    init(requiredProperty: String)

    func doSomething() -> Bool 
}

The previous DemoProtocol protocol exposes all of the requirements a conforming class should provide:

• aRequiredProperty should be String that can be read and written. 
• aReadonlyProperty should be Double, but should only be at least readable. Conforming classes can implement it as let or as a computed property.
• aStaticProperty is a static Int member; conforming classes expose them as vars.
• Conforming classes should provide an initializer that takes String as an argument.
• Conforming classes should expose a non mutating function doSomething() that returns Bool.

You can implement this protocol with a class or a structure or even an enumeration.