在处理基于节点的框架（包括序列化和反序列化）的过程中，我发现了一个可以创建忽略类型约束的类实例的情况。为什么我可以在ZNumericProcessor<String>内创建ZSum，但ZNumericProcessor被定义为ZNumericProcessor<T: ZNumeric>，其中String不符合ZNumeric？为什么甚至可以调用初始值设定项，尽管ZSum不确定T是ZNumeric？
通常我认为我甚至不能对ZSum的init进行约束以使其编译。
第二：如果我在ZSum的init中没有约束，我怎么能转换T以便我可以创建一个ZNumericProcessor的实例，如果我确定它是ZNumeric？
代码：
//: Playground - noun: a place where people can play
import UIKit
var str = "Hello, playground"
protocol ZNumeric {
}
extension Double: ZNumeric {
}
class ZInput<T> {
}
class ZNumericProcessor<T: ZNumeric> {
}
class ZNode {
let id: String
var name: String?
required init?<T>(type: T.Type, id: String) {
self.id = id
}
init(name: String?) {
id = NSUUID().UUIDString
}
}
class ZSum: ZNode {
required init?<T: ZNumeric>(type: T.Type, id: String) {
super.init(type: type, id: id)
let p = ZNumericProcessor<T>()
print(p)
if !(T.self is ZNumeric.Type) {
print("could not create ZSum, type is not Numeric")
return nil
}
}
override init(name: String?) {
super.init(name: name)
}
}
class ZFactory {
var nodes: [String: ZNode.Type] = [:]
var types: [String: (type: ZNode.Type, id: String) -> ZNode?] = [:]
func node(node: String, type: String, id: String) -> ZNode? {
let n = nodes[node]
if n == nil {
return nil
}
let t = types[type]
if t == nil {
return nil
}
return t!(type: n!, id: id)
}
func registerNode(name: String, type: ZNode.Type) {
nodes[name] = type
}
func registerType(name: String, processor: (type: ZNode.Type, id: String) -> ZNode?) {
types[name] = processor
}
}
class Serializer {
func node() -> (id: String, node: String, type: String)? {
return nil
}
}
var t = ZFactory()
t.registerNode("ZSum", type: ZSum.self)
t.registerType("Double", processor: { (type: ZNode.Type, id: String) -> ZNode? in return type.init(type: Double.self, id: id) } )
t.registerType("String", processor: { (type: ZNode.Type, id: String) -> ZNode? in return type.init(type: String.self, id: id) } )
var n = t.node("ZSum", type: "Double", id: "ID1")
var m = t.node("ZSum", type: "String", id: "ID2")
编辑：阅读之后：Stackoverflow Thread我试图将其合并但是限制较少的子类initalizer将不会被调用（在ZSum中），尽管已提供。
import UIKit
var str = "Hello, playground"
protocol ZNumeric {
}
protocol ZStringRepresentable {
}
extension Double: ZNumeric {
}
class ZInput<T> {
}
class ZNumericProcessor<T: ZNumeric> {
}
class ZNode {
let id: String
var name: String?
required init?<T>(type: T.Type, id: String) {
self.id = id
}
init(name: String?) {
id = NSUUID().UUIDString
}
}
class ZSum: ZNode {
required init?<T>(type: T.Type, id: String) {
super.init(type: type, id: id)
print("called") // Never happens
}
required init?<T: ZNumeric>(type: T.Type, id: String) {
super.init(type: type, id: id)
if !(T.self is ZNumeric.Type) {
print("could not create ZSum, type is not Numeric") // This is called with String
return nil
}
let pp = ZNumericProcessor<T>()
}
override init(name: String?) {
super.init(name: name)
}
}
class ZFactory {
var nodes: [String: ZNode.Type] = [:]
var types: [String: (type: ZNode.Type, id: String) -> ZNode?] = [:]
func node(node: String, type: String, id: String) -> ZNode? {
let n = nodes[node]
if n == nil {
return nil
}
let t = types[type]
if t == nil {
return nil
}
return t!(type: n!, id: id)
}
func registerNode(name: String, type: ZNode.Type) {
nodes[name] = type
}
func registerType(name: String, processor: (type: ZNode.Type, id: String) -> ZNode?) {
types[name] = processor
}
}
class Serializer {
func node() -> (id: String, node: String, type: String)? {
return nil
}
}
var t = ZFactory()
t.registerNode("ZSum", type: ZSum.self)
t.registerType("Double", processor: { (type: ZNode.Type, id: String) -> ZNode? in return type.init(type: Double.self, id: id) } )
t.registerType("String", processor: { (type: ZNode.Type, id: String) -> ZNode? in return type.init(type: String.self, id: id) } )
var n = t.node("ZSum", type: "Double", id: "ID1")
var m = t.node("ZSum", type: "String", id: "ID2")
编辑2：更简单的版本，同样的效果，即使使用String，也会调用限制性更强的初始化程序，尽管String不是ZNumeric：
protocol ZNumeric {
}
extension Double: ZNumeric {
}
protocol ZInitializable {
init<T>(type: T.Type)
}
class ZNode: ZInitializable {
required init<T>(type: T.Type) {
}
}
class ZSum: ZNode {
required init<T>(type: T.Type) {
super.init(type: type)
}
required init<T: ZNumeric>(type: T.Type) {
super.init(type: type)
print("ZSum ZNumeric:\(type)") // Is called both with String and Double
}
}
func create(node: ZNode.Type) {
let a = node.init(type: Double.self)
let b = node.init(type: String.self)
}
create(ZSum.self)