協定
UIPickerViewDelegate,UIPickerViewDataSource,UITableView
協定僅宣告方法、屬性，不會直接定義內容，而是使其他服從此協定的class或struct來實作某些方法，或擁有某些屬性。

定義協定

//服從此協定的類別一定要實作giveMeSoyMilk方法
protocol SoyMilkGetable{
    func giveMeSoyMilk()
}

使類別（Brunch）服從協定（SoyMilkGetable）

//Point:服從協定需時作協定中的方法
class Brunch:SoyMilkGetable{
    func giveMeSoyMilk() {
        print("SoyMilk")
    }
}

試用看看

let aBrunch = Brunch()
aBrunch.giveMeSoyMilk() //Print:SoyMilk

以PickerView為例

protocol PickerViewDataSource{
    func howManyComponent()
    func howManyRow()
}

protocol PickerViewDelegate{
    func titleToShow()
    func didSelect()
}

//Point:一個類別可以服從多個協定，但是只能作為某一個類別的子類別
class ViewController:PickerViewDataSource,PickerViewDelegate{
    func howManyComponent() {
        print("2")
    }
    func howManyRow() {
        print("10")
    }
    func titleToShow() {
        print("hello")
    }
    func didSelect() {
        print("select")
    }
}

class PickerView{
    var dataSource:PickerViewDataSource?
    var delegate:PickerViewDelegate?
    func howManyComponentIHave(){
        dataSource?.howManyComponent()
    }
    func howManyRowIHave(){
        dataSource?.howManyRow()
    }
    func whatToShow(){
        delegate?.titleToShow()
    }
    func afterSelect(){
        delegate?.didSelect()
    }
}

let aPickerView = PickerView()
let aViewController = ViewController()
aPickerView.dataSource = aViewController
aPickerView.delegate = aViewController
aPickerView.howManyComponentIHave()
aPickerView.howManyRowIHave()

(未完待續）