Swift 中的面向协议编程：是否优于面向对象编程？

public mutating func next() -> Any? {
    if index + 1 > count {
        index = 0
	// 确保没有 self 的成员变量被捕获
        let enumeratedObject = enumerable
        var localState = state
        var localObjects = objects
        
        (count, useObjectsBuffer) = withUnsafeMutablePointer(to: &localObjects) {
            let buffer = AutoreleasingUnsafeMutablePointer<AnyObject?>($0)
            return withUnsafeMutablePointer(to: &localState) { (statePtr: UnsafeMutablePointer<NSFastEnumerationState>) -> (Int, Bool) in
                let result = enumeratedObject.countByEnumerating(with: statePtr, objects: buffer, count: 16)
                if statePtr.pointee.itemsPtr == buffer {
		    // 大多数 cocoa 类会返回它们自己的内部指针缓存，不使用默认的路径获取值。也有例外的情况，比如 NSDictionary 和 NSSet。
		    return (result, true)
                } else {
                    // 这里是通常情形，比如 NSArray。
		    return (result, false)
                }
            }
        }
        
        state = localState // 重置 state 的值
        objects = localObjects // 将对象指针拷贝回 self 
        
        if count == 0 { return nil }
    }
    defer { index += 1 }
    if !useObjectsBuffer {
        return state.itemsPtr![index]
    } else {
        switch index {
        case 0: return objects.0!.takeUnretainedValue()
        case 1: return objects.1!.takeUnretainedValue()
        case 2: return objects.2!.takeUnretainedValue()
        case 3: return objects.3!.takeUnretainedValue()
        case 4: return objects.4!.takeUnretainedValue()
        case 5: return objects.5!.takeUnretainedValue()
        case 6: return objects.6!.takeUnretainedValue()
        case 7: return objects.7!.takeUnretainedValue()
        case 8: return objects.8!.takeUnretainedValue()
        case 9: return objects.9!.takeUnretainedValue()
        case 10: return objects.10!.takeUnretainedValue()
        case 11: return objects.11!.takeUnretainedValue()
        case 12: return objects.12!.takeUnretainedValue()
        case 13: return objects.13!.takeUnretainedValue()
        case 14: return objects.14!.takeUnretainedValue()
        case 15: return objects.15!.takeUnretainedValue()
        default: fatalError("Access beyond storage buffer")
        }
    }
}

var delegate: LogoDownloaderDelegate?

protocol Top {
    var protocolName: String { get }
}

protocol Middle: Top {

}

protocol Bottom: Middle {

}

struct TopStruct : Top {
    var protocolName: String = "TopStruct"
}

struct MiddleStruct : Middle {
    var protocolName: String = "MiddleStruct"
}

struct BottomStruct : Bottom {
    var protocolName: String = "BottomStruct"
}

let top = TopStruct()
let middle = MiddleStruct()
let bottom = BottomStruct()

var topStruct: Top
topStruct = bottom
print("\(topStruct)\n")
// 输出 "BottomStruct(protocolName: "BottomStruct")"

topStruct = middle
print("\(topStruct)\n")
// 输出 "MiddleStruct(protocolName: "MiddleStruct")"

topStruct = top
print("\(topStruct)\n")
// 输出 "TopStruct(protocolName: "TopStruct")"

let protocolStructs:[Top] = [top,middle,bottom]

for protocolStruct in protocolStructs {
    print("\(protocolStruct)\n")
}

BottomStruct(protocolName: "BottomStruct")

MiddleStruct(protocolName: "MiddleStruct")

TopStruct(protocolName: "TopStruct")

TopStruct(protocolName: "TopStruct")

MiddleStruct(protocolName: "MiddleStruct")

BottomStruct(protocolName: "BottomStruct")

protocol SimpleViewWithBorder {}

// 安全的："addBorder" 方法只会被添加到 UIView 的实例。
extension SimpleViewWithBorder where Self : UIView {
    func addBorder() -> Void {
        layer.borderColor = UIColor.green.cgColor
        layer.borderWidth = 10.0
    }
}

class SimpleUIViewWithBorder : UIView, SimpleViewWithBorder {
}

@IBAction func addViewButtonTapped(_ sender: Any) {
    let customFrame0 = CGRect(x: 110, y: 100, width: 100, height: 100)
    let customView0 = SimpleUIViewWithBorder(frame: customFrame0)
    customView0.addBorder()
    self.view.addSubview(customView0)

protocol ViewWithBackground {
    var customBackgroundColor: UIColor { get }
}

extension ViewWithBackground where Self : UIView {
    func addBackgroundColor() -> Void {
        backgroundColor = customBackgroundColor
    }
}

class UIViewWithBackground : UIView, ViewWithBackground {
    let customBackgroundColor: UIColor = .blue
}

let customFrame1 = CGRect(x: 110, y: 210, width: 100, height: 100)
let customView1 = UIViewWithBackground(frame: customFrame1)
customView1.addBackgroundColor()
self.view.addSubview(customView1)

protocol ViewWithBorder {
    var borderColor: UIColor { get }
    var borderThickness: CGFloat { get }
    init(borderColor: UIColor, borderThickness: CGFloat, frame: CGRect)
}

extension ViewWithBorder where Self : UIView {
    func addBorder() -> Void {
        layer.borderColor = borderColor.cgColor
        layer.borderWidth = borderThickness
    }
}

class UIViewWithBorder : UIView, ViewWithBorder {
    let borderColor: UIColor
    let borderThickness: CGFloat

    // UIView 的必要初始化方法
    required init(borderColor: UIColor, borderThickness: CGFloat, frame: CGRect) {
        self.borderColor = borderColor
        self.borderThickness = borderThickness
        super.init(frame: frame)
    }

    // UIView 的必要初始化方法
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

let customFrame2 = CGRect(x: 110, y: 320, width: 100, height: 100)
let customView2 = UIViewWithBorder(borderColor: .red, borderThickness: 10.0, frame: customFrame2)
customView2.addBorder()
self.view.addSubview(customView2)

extension UIView {
    func addBorder() {  ...  }
    func addBackgroundColor() {  ...  }
}

protocol SimpleViewWithBorder {}

extension SimpleViewWithBorder where Self : UIView {
    func addBorder() -> Void {
        layer.borderColor = UIColor.green.cgColor
        layer.borderWidth = 10.0
    }
}

class SimpleUIViewWithBorder : UIView, SimpleViewWithBorder {
    // 覆盖 extension 中的默认实现
    func addBorder() -> Void {
        layer.borderColor = UIColor.darkGray.cgColor
        layer.borderWidth = 20.0
    }
}

protocol ListSubscript {
    associatedtype AnyType
    
    var elements : [AnyType] { get }
}

extension ListSubscript {
    subscript(i: Int) -> Any {
        return elements[i]
    }
}

protocol ListPrintForwards {
    associatedtype AnyType

    var elements : [AnyType] { get }
}

extension ListPrintForwards {
    func showList() {
        if elements.count > 0 {
            var line = ""
            var index = 1

                        for element in elements {
                line += "\(element) "
                index += 1
            }
            print("\(line)\n")
        } else {
            print("EMPTY\n")
        }
    }
}

protocol ListPrintBackward {
    associatedtype AnyType

    var elements : [AnyType] { get }
}

extension ListPrintBackwards {
    func showList() {
        if elements.count > 0 {
            var line = ""
            var index = 1

            for element in elements.reversed() {
                line += "\(element) "
                index += 1
            }
            print("\(line)\n")
        } else {
            print("EMPTY\n")
        }
    }
}

protocol ListCount {
    associatedtype AnyType

    var elements : [AnyType] { get }
}

extension ListCount {
    func count() -> Int {
        return elements.count
    }
}

protocol List {
    associatedtype AnyType

    var elements : [AnyType] { get set }

    mutating func remove() -> AnyType

    mutating func add(_ element: AnyType)
}

extension List {
    mutating func add(_ element: AnyType) {
        elements.append(element)
    }
}

protocol FIFO : List, ListCount, ListPrintForwards, ListSubscript {

}

extension FIFO {
    mutating func remove() -> AnyType {
        if elements.count > 0 {
            return elements.removeFirst()
        } else {
            return "******EMPTY******" as! AnyType
        }
    }
}

struct Queue<AnyType>: FIFO {
    var elements: [AnyType] = []
}

var queue = Queue<Any>()
queue.add("Bob")
queue.showList()
queue.add(1)
queue.showList()
queue.add(3.0)
_ = queue[0] // 该下标输出 "Bob"
_ = queue.count()
queue.showList()
queue.remove()
queue.showList()
queue.remove()
queue.showList()
queue.remove()
queue.showList()
_ = queue.count()

protocol LIFO : List, ListCount, ListPrintBackwards, ListSubscript {
}

extension LIFO {
    mutating func remove() -> AnyType {
        if elements.count > 0 {
            return elements.removeLast()
        } else {
            return "******EMPTY******" as! AnyType
        }
    }    
}

struct Stack<AnyType>: LIFO {
    var elements: [AnyType] = []
}

var stack = Stack<Any>()
stack.add("Bob")
stack.showList()
stack.add(1)
stack.showList()
stack.add(3.0)
_ = stack[0] // 该下标输出 3
_ = stack.count()
stack.showList()
stack.remove()
stack.showList()
stack.remove()
stack.showList()
stack.remove()
stack.showList()
_ = stack.count()

Bob

Bob 1

Bob 1 3.0

1 3.0

3.0

EMPTY

Bob

1 Bob

3.0 1 Bob

1 Bob

Bob

EMPTY