大佬教程收集整理的这篇文章主要介绍了具有约束关联类型错误的Swift协议“类型不可转换”,大佬教程大佬觉得挺不错的,现在分享给大家,也给大家做个参考。
复杂层来自符合Manager的类型,应该能够生成一个具体的Reader实例,它生成一个特定类型的Value(Value1或Value2).
通过我对Manager1的具体实现,我希望它始终生成Reader1,而Reader1又生成Value1的实例.
有人可以解释原因
当错误的行改为(现在)返回nil时,所有编译都很好,但现在我无法实例化Reader1或Reader2.
import Foundation protocol Value { var value: Int { get } } protocol Reader { typealias ReaderValueType: Value func value() -> ReaderValueType } protocol Manager { typealias ManagerValueType: Value func read<ManagerReaderType: Reader where ManagerReaderType.ReaderValueType == ManagerValueType>() -> ManagerReaderType? } struct Value1: Value { let value: Int = 1 } struct Value2: Value { let value: Int = 2 } struct Reader1: Reader { func value() -> Value1 { return Value1() } } struct Reader2: Reader { func value() -> Value2 { return Value2() } } class Manager1: Manager { typealias ManagerValueType = Value1 let v = ManagerValueType() func read<ManagerReaderType: Reader where ManagerReaderType.ReaderValueType == ManagerValueType>() -> ManagerReaderType? { return Reader1()// Error: "Reader1 is not convertible to ManagedReaderType?" Try swapping to return nil which does compile. } } let manager = Manager1() let v = manager.v.value let a: Reader1? = manager.read() a.dynamicType
let manager = Manager1() let reader1: Reader1? = manager.read() // ManagerReaderType is of type Reader1 let reader2: Reader2? = manager.read() // ManagerReaderType is of type Reader2
如果你返回nil它可以转换为任何可选类型,所以它总是有效.
作为替代方案,您可以返回特定类型的Reader类型:
protocol Manager { // this is SIMILAR TO the Generator of a SequenCEType which has the Element type // but it consTraints the ManagerReaderType to one specific Reader typealias ManagerReaderType: Reader func read() -> ManagerReaderType? } class Manager1: Manager { func read() -> Reader1? { return Reader1() } }
由于缺少“真正的”泛型,这是使用协议的最佳方法(不支持以下内容):
// this would perfectly match your requirements protocol Reader<T: Value> { fun value() -> T } protocol Manager<T: Value> { func read() -> Reader<T>? } class Manager1: Manager<Value1> { func read() -> Reader<Value1>? { return Reader1() } }
所以最好的解决方法是使Reader成为泛型类,Reader1和Reader2子类是它的特定泛型类型:
class Reader<T: Value> { func value() -> T { // or provide a dummy value fatalError("implement me") } } // a small change in the function signature protocol Manager { typealias ManagerValueType: Value func read() -> Reader<ManagerValueType>? } class Reader1: Reader<Value1> { override func value() -> Value1 { return Value1() } } class Reader2: Reader<Value2> { override func value() -> Value2 { return Value2() } } class Manager1: Manager { typealias ManagerValueType = Value1 func read() -> Reader<ManagerValueType>? { return Reader1() } } let manager = Manager1() // you have to cast it,otherwise it is of type Reader<Value1> let a: Reader1? = manager.read() as! Reader1?
以上是大佬教程为你收集整理的具有约束关联类型错误的Swift协议“类型不可转换”全部内容,希望文章能够帮你解决具有约束关联类型错误的Swift协议“类型不可转换”所遇到的程序开发问题。
如果觉得大佬教程网站内容还不错,欢迎将大佬教程推荐给程序员好友。
本图文内容来源于网友网络收集整理提供,作为学习参考使用,版权属于原作者。
如您有任何意见或建议可联系处理。小编QQ:384754419,请注明来意。