The Swift Programming Language (Swift 2) -6 Protocols and Extensions

//: ## Protocols and Extensions
//:
//: Use protocol to declare a protocol.
//:
protocol ExampleProtocol {
     var simpleDescription: String { get }
     mutating func adjust()
}


//: Classes, enumerations, and structs can all adopt protocols.
//:
class SimpleClass: ExampleProtocol {
     var simpleDescription: String = "A very simple class."
     var anotherProperty: Int = 69105
     func adjust() {
          simpleDescription += "  Now 100% adjusted."
     }
}
var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription

struct SimpleStructure: ExampleProtocol {
     var simpleDescription: String = "A simple structure"
     mutating func adjust() {
          simpleDescription += " (adjusted)"
     }
}
var b = SimpleStructure()
b.adjust()
let bDescription = b.simpleDescription

//: > Experiment:
//: > Write an enumeration that conforms to this protocol.
//:
//: Notice the use of the mutating keyword in the declaration of SimpleStructure to mark a method that modifies the structure. The declaration of SimpleClass doesn’t need any of its methods marked as mutating because methods on a class can always modify the class.
//:
//: Use extension to add functionality to an existing type, such as new methods and computed properties. You can use an extension to add protocol conformance to a type that is declared elsewhere, or even to a type that you imported from a library or framework.
//:
extension Int: ExampleProtocol {
    var simpleDescription: String {
        return "The number (self)"
    }
    mutating func adjust() {
        self += 42
    }
 }
print(7.simpleDescription)

//: > Experiment:
//: > Write an extension for the Double type that adds an absoluteValue property.
//:
//: You can use a protocol name just like any other named type—for example, to create a collection of objects that have different types but that all conform to a single protocol. When you work with values whose type is a protocol type, methods outside the protocol definition are not available.
//:
let protocolValue: ExampleProtocol = a
print(protocolValue.simpleDescription)
// print(protocolValue.anotherProperty)  // Uncomment to see the error

//: Even though the variable protocolValue has a runtime type of SimpleClass, the compiler treats it as the given type of ExampleProtocol. This means that you can’t accidentally access methods or properties that the class implements in addition to its protocol conformance.
//:

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