继对我的 previous question 的回答之后, 我有 2 个协议(protocol)…
protocol Filters: Encodable {
}
protocol QueryParameters: Encodable {
associatedtype T: Filters
var page: Int { get }
var filters: T { get }
}
然后对于类型Transaction
,我有...
struct TransactionFilters: Filters {
var isWithdrawal: Bool
}
struct TransactionParamters<T: Filters>: QueryParameters {
var page: Int
var filters: T
}
到目前为止一切都很好。
接下来我添加一个协议(protocol),Filterable
,我希望任何符合 Filterable
的类型都能够返回参数,就像这样……
protocol Filterable {
func parameters() -> QueryParameters
}
struct Transactions: Filterable {
func parameters() -> QueryParameters {
let transactionFilters = TransactionFilters(isWithdrawal: true)
return TransactionParamters(page: 1, filters: transactionFilters)
}
}
但我最终……
error: protocol 'QueryParameters' can only be used as a generic constraint because it has Self or associated type requirements
这似乎是一个非常简单的要求,但我花了 2 天的时间尝试所有我能想到的组合来让它工作。现在我终于认输了。
我需要做什么来解决这个问题?
最佳答案
正如我在评论中提到的。您的代码中缺少的是 associatedtype
实际上从未成为一种类型。在您的 structs
之一的代码中,您没有将类型分配给 associatedtype
。如果你想要一个通用的 filterable
功能,你可以按照这些思路做一些事情:
// Your generic Filters with required properties
protocol Filters: Encodable {
var isWithdrawal: Bool { get }
init(isWithdrawal: Bool)
}
// Your generic query parameters
protocol QueryParameters: Encodable {
associatedtype F: Filters
var page: Int { get }
var filters: F { get }
init(page: Int, filters: Filters)
}
// Filterable protocol will eventually accept any types conforming to Filters and QueryParameters
protocol Filterable {
associatedtype F: Filters
associatedtype P: QueryParameters
func parameters() -> P
}
// This is your generic Transactions struct
// With this you will be able to pass other types that meet the constraints
struct Transactions<ParameterType: QueryParameters>: Filterable {
typealias P = ParameterType
typealias F = ParameterType.F
func parameters() -> ParameterType {
return P(page: 1, filters: F(isWithdrawal: true))
}
}
你已经完成了通用的东西。现在您可以创建符合您的协议(protocol)的不同模型对象
struct TransactionFilters: Filters {
private(set) var isWithdrawal: Bool // Conforming to filters
}
struct TransactionParameters<FilterType: Filters>: QueryParameters {
// Telling what type is the object that conforms to Filters
typealias F = FilterType
var page: Int
var filters: FilterType
init(page: Int, filters: Filters) {
self.page = page
self.filters = filters as! F
}
}
像这样创建您的交易对象:
let transactions = Transactions<TransactionParameters<TransactionFilters>>()
print(transactions.parameters().page)
print(transactions.parameters().filters.isWithdrawal)
您可以创建更多类型的查询参数和过滤器
struct SomeOtherParameters<FilterType: Filters>: QueryParameters {
// You can do custom stuff in your SomeOtherParameters struct
// e.g. add an offset to page
typealias F = FilterType
var page: Int
var filters: FilterType
init(page: Int, filters: Filters) {
self.page = page + 100
self.filters = filters as! F
}
}
// Your special filter that always returns false
struct SomeOtherFilters: Filters {
init(isWithdrawal: Bool) {}
var isWithdrawal: Bool {
return false
}
}
let transactionsWithDifferentFilters = Transactions<SomeOtherParameters<SomeOtherFilters>>()
// You can combine any types that conform to you declared protocols
let evenMoreTransactions = Transactions<SomeOtherParameters<TransactionFilters>>()
print(evenMoreTransactions.parameters().page)
关于swift - 协议(protocol)只能用作通用约束,因为它具有 Self 或关联类型要求,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/50408628/