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)
}
}