Swift泛型函子无法将类型的值转换为预期的参数类型 [英] Swift generic func cannot convert value of type to expected argument type

问题描述

我尝试创建通用函数

func importArray<T: ImportableUniqueObject>(from exercisesDict: [[String: Any]], transaction: BaseDataTransaction) -> [T] {

    if let managedObject = try? transaction.fetchOne(From<T>()){

        transaction.delete(managedObject)
    }

    let managedObjects = try! transaction.importUniqueObjects(
        Into<T>(),
        sourceArray: jsonObjects)


    return managedObjects

    }

所以第一部分效果很好:

So first part works good:

if let managedObject = try? transaction.fetchOne(From<T>()){

，但是秒不起作用:

let managedObjects = try! transaction.importUniqueObjects(
        Into<T>(),
        sourceArray: jsonObjects)

编译器说

Cannot convert value of type 'Into<T>' to expected argument type 'Into<_>'

这是func的构造方式:

This is how func is constructed:

public func importUniqueObjects<D: ImportableUniqueObject, S: Sequence>(
        _ into: Into<D>,
        sourceArray: S,
        preProcess: @escaping (_ mapping: [D.UniqueIDType: D.ImportSource]) throws -> [D.UniqueIDType: D.ImportSource] = { $0 }) throws -> [D] where S.Iterator.Element == D.ImportSource {

推荐答案

那是编译器错误. Xcode 10发布时遇到了相同的问题. 将您的方法调整为以下内容:

That's a compiler bug. Had the same issue when Xcode 10 came out. Adapt your method to the following:

func importArray<T: ImportableUniqueObject>(from exercisesDict: [[String: Any]], transaction: BaseDataTransaction) -> [T] where T.ImportSource == [String: Any] {
    let managedObjects = try? transaction.importUniqueObjects(Into<T>(), sourceArray: jsonObjects)
}

尽管我建议在导入时不要强行尝试.

Though I recommend do not do force-try when importing.

另请参见: https://bugs.swift.org/browse/SR-8945

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