Protocols as types

Any protocol that we define will become a fully-fledged type to use in our code. We can use a protocol as follows:

• A parameter type or return type in a function, method, or initializer
• The type of a constant, variable, or property
• The type of items in an array, dictionary, or another container

Let's look at the following example:

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 example" 
    var anotherProperty: Int = 79799 

    func adjust() { 
        simpleDescription += "Now 100% adjusted..." 
    } 
} 

var aSimpleClass = SimpleClass() 
aSimpleClass.adjust() 
let aDescription = aSimpleClass.simpleDescription 

struct SimpleStructure: ExampleProtocol { 
    var simpleDescription: String = "A simple struct" 
    // Mutating to mark a method that modifies the structure - For      
    classes we do not need to use mutating keyword 
    mutating func adjust() { 
        simpleDescription += "(adjusted)" 
    } 
} 

var aSimpleStruct = SimpleStructure() 
aSimpleStruct.adjust() 
let aSimpleStructDescription = aSimpleStruct.simpleDescription