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调用协议静态方法时无法推断泛型参数

xcode10 protocols generics swift

2

Aaron Bratcher · 技术社区 · 7 年前

使用操场并给出以下定义:

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:

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

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

但是我得到了 Generic parameter 'T' could not be inferred 调用静态方法时出错,我不确定原因。

2 回复 | 直到 7 年前

1

J. Doe 7 年前

我不明白为什么需要通用约束。如果您将协议扩展更改为:

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。

2

1

Code Different 7 年前

好。。。唯一的地方 T 在完成处理程序参数内使用。当你写这个的时候:

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

编译器不知道什么类型 transaction 因此不能专门化泛型函数。提供如下类型的信息:

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

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