swift protocol的几种形式

三个关注点：1、形式；2、实现方式；3、使用方式；

一、基本形式：

形式：内部无泛型类型；

实现：只需指定类型和实现相应的功能即可；

使用：可以用在其他类型出现的任何地方；

protocol Response {

/// The task metrics containing the request / response statistics.

var _metrics: AnyObject? { get set }

mutating func add(_ metrics: AnyObject?)

}

Protocols as Types

Protocols don’t actually implement any functionality themselves. Nonetheless, any protocol you create will become a fully-fledged type for use in your code.

Because it’s a type, you can use a protocol in many places where other types are allowed, including:

• As a parameter type or return type in a function, method, or initializer
• As the type of a constant, variable, or property
• As the type of items in an array, dictionary, or other container

实现类提供：函数的具体实现和存储变量；

对于变量，提供同名的存储变量即可；

二、普通泛型形式：

形式：内部无高阶类型，只包含普通泛型类型；

实现：只需指定类型和实现相应的功能即可；

使用：只能用作泛型类型的约束；

Protocol 'TransformType' can only be used as a generic constraint because it has Self or associated type requirements

public protocol TransformType {

associatedtype Object

associatedtype JSON

func transformFromJSON(_ value: Any?) -> Object?

func transformToJSON(_ value: Object?) -> JSON?

}

open class DataTransform: TransformType {

public typealias Object = Data

public typealias JSON = String

public init() {}

open func transformFromJSON(_ value: Any?) -> Data? {

guard let string = value as? String else{

return nil

}

return Data(base64Encoded: string)

}

open func transformToJSON(_ value: Data?) -> String? {

guard let data = value else{

return nil

}

return data.base64EncodedString()

}

}

三、monad形式：

形式：内部有包含泛型的高阶类型，包含类型构造器，是低阶类型与高阶类型相互转换和引用的桥梁；

使用：只能用作泛型类型的约束；

实现：

1、指定类型和实现相应；

2、对泛型本身进行扩展，实现构造类型变量的赋值；

3、对构造类型进行扩展，实现更多的功能；

4、实现为一个泛型类型？

public protocol ReactiveCompatible {

/// Extended type

associatedtype CompatibleType

/// Reactive extensions.

var rx: Reactive<CompatibleType> { get set }

}

extension ReactiveCompatible {

/// Reactive extensions.

public var rx: Reactive<Self> {

get {

return Reactive(self)

}

set {

// this enables using Reactive to "mutate" base object

}

}

}

extension NSObject: ReactiveCompatible { }

//—————————————————————

public final class Kingfisher<Base> {

public let base: Base

public init(_ base: Base) {

self.base = base

}

}

public protocol KingfisherCompatible {

associatedtype CompatibleType

var kf: CompatibleType { get }

}

public extension KingfisherCompatible {

public var kf: Kingfisher<Self> {

return Kingfisher(self)

}

}

extension Kingfisher where Base: Image {

fileprivate(set) var animatedImageData: Data? {

get {

return objc_getAssociatedObject(base, &animatedImageDataKey) as? Data

}

set {

objc_setAssociatedObject(base, &animatedImageDataKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)

}

}

}

四、内部实现依赖于其它协议

形式：关联类型有其它协议约束

实现：

1、关联类型协议类型的实现：

2、关联类型所定义的变量的构造；

3、主协议的实现（引用关联类型的协议的实现）；

本质：先实现依赖的协议，然后实现本协议

使用：同二

public protocol Sequence {

associatedtype Iterator : IteratorProtocol

public func makeIterator() -> Self.Iterator

// ...

}

public protocol IteratorProtocol {

associatedtype Element

public mutating func next() -> Self.Element?

}

struct _Iterator: IteratorProtocol {

var children: Mirror.Children

init(obj: Any) {

children = Mirror(reflecting: obj).children

}

mutating func next() -> String? {

guard let child = children.popFirst() else { return nil }

return "\(child.label.wrapped) is \(child.value)"

}

}

protocol Sequencible: Sequence { }

extension Sequencible {

func makeIterator() -> _Iterator {

return _Iterator(obj: self)

}

}