协议扩展的可编码（或可编码）不符合它 [英] Protocol extending Encodable (or Codable) does not conform to it

问题描述

我有2个协议，过滤器和参数，它们都扩展了 Encodable

I have 2 protocols, Filters and Parameters, both of which extend Encodable

protocol Filters: Encodable {
    var page: Int { get }
}

protocol Parameters: Encodable {
    var type: String { get }
    var filters: Filters { get }
}

因此，我创建了符合这些协议的结构……

I create structs conforming to these protocols, thusly…

struct BankAccountFilters: Filters {
    var page: Int
    var isWithdrawal: Bool
}

struct BankAccountParamters: Parameters {
    let type: String = "Bank"
    var filters: Filters
}

let baf = BankAccountFilters(page: 1, isWithdrawal: true)
let bap = BankAccountParamters(filters: baf)

由于以下原因而失败：

Which fails because…

错误：输入 BankAccountParamters不符合协议'Encodable'

error: type 'BankAccountParamters' does not conform to protocol 'Encodable'

注意：由于过滤器不符合 Encodable，因此无法自动合成 Encodable

note: cannot automatically synthesize 'Encodable' because 'Filters' does not conform to 'Encodable'

过滤器显然确实符合 Encodable （至少它对我来说似乎是这样）。

Filters clearly does conform to Encodable (at least it seems that way to me). Is there a way around this?

推荐答案

如协议不符合自身？，协议不符合自身或它所继承的
a协议。在您的情况下，过滤器是否不符合可编码。

As discussed in Protocol doesn't conform to itself?, a protocol does not conform to itself, or to a protocol that it inherits from. In your case, Filters does not conform to Encodable.

可能的解决方案是制作结构BankAccountParamters 和
协议参数常规：

A possible solution is to make struct BankAccountParamters and protocol Parameters generic:

protocol Filters: Encodable {
    var page: Int { get }
}

protocol Parameters: Encodable {
    associatedtype T: Filters
    var type: String { get }
    var filters: T { get }
}

struct BankAccountFilters: Filters {
    var page: Int
    var isWithdrawal: Bool
}

struct BankAccountParamters<T: Filters>: Parameters {
    let type: String = "Bank"
    var filters: T
}

现在 var个过滤器的类型为 T ，它符合过滤器，然后将其转换为 Encodable 。

Now var filters has type T, which conforms to Filters and consequently, to Encodable.

这将编译并产生预期的结果：

This compiles and produces the expected result:

let baf = BankAccountFilters(page: 1, isWithdrawal: true)
let bap = BankAccountParamters(filters: baf)

let data = try! JSONEncoder().encode(bap)
print(String(data: data, encoding: .utf8)!)
// {"type":"Bank","filters":{"isWithdrawal":true,"page":1}}

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