调用协议静态方法时无法推断泛型参数

Question

使用 Playground 并给出以下定义：

import Foundation

protocol MoneyTrakObject {
    var key: String { get set }

    init()
}

extension MoneyTrakObject {
    static func objectFromDB<T: MoneyTrakObject>(for key: String, queue: DispatchQueue? = nil, completion: @escaping (T) -> Void) -> String? {
            // after data is retrieved, call completion closure
            let valueObject = T()
            completion(valueObject)

        return "dbToken"
    }
}


protocol Transaction: MoneyTrakObject {
    var amount: Int { get set }
}


struct BasicTransaction: Transaction {
    var key = UUID().uuidString
    var amount = 0

    init() {}
}

struct RecurringTransaction: Transaction {
    var key = UUID().uuidString
    var amount = 0

    init() {}
}

我希望我能做到这一点：

let token1 = BasicTransaction.objectFromDB(for: "") { (transaction) in
    // use transaction
}

let token2 = RecurringTransaction.objectFromDB(for: "") { (transaction) in
    // use transaction
}

但是，在调用静态方法时，我得到Generic parameter 'T' could not be inferred错误，我不确定为什么。

Answer 1

嗯……唯一使用T地方是在完成处理程序参数中。 当你写这个：

let token1 = BasicTransaction.objectFromDB(for: "") { (transaction) in
    // use transaction
}

编译器不知道transaction是什么类型，因此不能专门化泛型函数。 提供一些这样的类型信息：

let token1 = BasicTransaction.objectFromDB(for: "") { (transaction: Transaction) in
    // use transaction
}

let token2 = BasicTransaction.objectFromDB(for: "") { (transaction: BasicTransaction) in
    // use transaction
}

Answer 2

我不明白你为什么需要通用约束。 如果您将协议的扩展名更改为：

extension MoneyTrakObject {
    static func objectFromDB(for key: String, queue: DispatchQueue? = nil, completion: @escaping (Self) -> Void) -> String? {
        // after data is retrieved, call completion closure
        let valueObject = Self()
        completion(valueObject)

        return "dbToken"
    }
}

你的代码编译得很好。 Self是实际实现类型的占位符。