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)

}

}

05-11 16:55