• Protocol

  Definition: protocol— allow you to specify and work with the interface of a type without knowing the type itself.

• basic example

import Cocoa
protocol TabularDataSource {
    var numberOfRows: Int {get}
    var numberOfColumns: Int {get}
    func labelForRow(row:Int) -> String
    func labelForColumn(column: Int)->String
    func itemForRow (row:Int ,column:Int)->Int
}
func padding(amount:Int) -> String{
    var paddingString = ""
    for _ in 0..<amount{
     paddingString += " "
    }
    return paddingString
}
func printTable(dataSource:TabularDataSource){
    //Create arrays of the row and column labels
    let rowLabels = (0..<dataSource.numberOfRows).map{dataSource.labelForRow($0)}
    let columnLabels = (0..<dataSource.numberOfColumns).map{dataSource.labelForColumn($0)}
    //Create an array of the width of each row
    let rowLabelWidths = rowLabels.map{$0.characters.count}
    //Determine length of longest row
    guard let maxRowLabelWidth = rowLabelWidths.maxElement() else {
        return
    }
    //Create first row containing column headers
    var firstRow:String = padding(maxRowLabelWidth) + " |"
    //keep track of the width of each column
    var columnWidths = [Int]()
    for columnLabel in columnLabels{
        let columnHeader = " \(columnLabel) |"
        firstRow += columnHeader
        columnWidths.append(columnHeader.characters.count)
    }
    print(firstRow)
     for i in 0..<dataSource.numberOfRows{
        //Pad the row label out so they are all the same length
        let paddingAmount = maxRowLabelWidth - rowLabelWidths[i]
        var out = rowLabels[i] + padding(paddingAmount) + " |"
        //Append each item in this row to our string
        for j in 0..<dataSource.numberOfColumns{
            let item = dataSource.itemForRow(i, column: j)
            let itemString = "\(item) |"
            let paddingAmount = columnWidths[j] - itemString.characters.count
            out += padding(paddingAmount) + itemString
        }
        print(out)
    }
}
struct Person {
    let name: String
    let age:Int
    let yearsOfExperience : Int
    
}
struct Department:TabularDataSource{
    let name: String
    var people = [Person]()
    init(name: String){
        self.name = name
    }
    mutating func addPerson(person:Person){
        people.append(person)
    }
    var numberOfRows: Int{
        return people.count
    }
    var numberOfColumns: Int{
        return 2
    }
    func labelForColumn(column: Int) -> String {
        switch column {
        case 0 : return "Age"
        case 1 : return "Years of Experience"
        default: fatalError("Invalid column!")
        }
    }
    func labelForRow(row: Int) -> String {
        return people[row].name
    }
    func itemForRow(row: Int, column: Int) -> Int {
        let person = people[row]
        switch column {
        case 0 : return person.age
        case 1 : return person.yearsOfExperience
        default: fatalError("Invalid column!")
        }
    }    
}
var department =  Department(name: "Engineering")
department.addPerson(Person(name:"Joe",age: 30,yearsOfExperience: 6))
department.addPerson(Person(name:"Karen",age: 40,yearsOfExperience: 18))
department.addPerson(Person(name:"Fred",age: 50,yearsOfExperience: 20))
printTable(department) 

• usage summation

<small> In order to give a method the right data to process ,we let the data to confirm to a designed protocol.When the data come out from the protocol, it can fit for the method and then the method process is well-functioned as before </small>