Swift泛型函子无法将类型的值转换为预期的参数类型
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 {
答
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.