没有泛型约束的协议中的泛型数组

Question

如何在符合类型指定数组类型的协议中指定泛型属性？

Info: Lets say we want to create a protocol which defines an array property where the type of the array is unknown. The conforming types specify the type of this array. This is a possible solution:

protocol Wallet {
    associatedtype Value
    var moneyStack: [Value] {get set}
}

struct BitcoinWallet: Wallet {
    var moneyStack: [String]
}

struct EthereumWallet: Wallet {
    var moneyStack: [Int]
}

The problem with this approach is that we can't access the moneyStack property:

let bitcoinWallet = BitcoinWallet(moneyStack: ["A","B"])
let etheureumWallet = EthereumWallet(moneyStack: [1,2])

let wallets: [Wallet] = [bitcoinWallet, etheureumWallet]

for wallet in wallets {
    print(wallet.moneyStack) // not possible
}

We can try another approach where the type of the array is any:

protocol Wallet {
    var moneyStack: [Any] {get set}
}

struct BitcoinWallet: Wallet {
    var moneyStack: [Any]
}

struct EthereumWallet: Wallet {
    var moneyStack: [Any]
}

This is a possible solution but an array which holds [Any] is too generic. I can't specify the type. Example:

let bitcoinWallet = BitcoinWallet(moneyStack: ["A","B"])
let ethereumWallet = EthereumWallet(moneyStack: [1,2])

var wallets: [Wallet] = [bitcoinWallet, ethereumWallet]

for wallet in wallets {
    print(wallet.moneyStack.count) // this works
    print(wallet.moneyStack[0]) // this works
}

let badBitcoinWallet = BitcoinWallet(moneyStack: [12.4, 312312.123]) // BitcoinWallets aren't allowed to hold doubles!

wallets.append(badBitcoinWallet) // This shouldn't be allowed!

第一个方案就是我想要的。每个结构中都指定了类型，但 Swift 抱怨我不能使用通用约束。我要解决的主要问题是我想要一个包含不同类型钱包的数组，它们都有自己不同类型的数组。

我原以为协议会让这件事变得简单，但事实并非如此。我的架构有问题吗？我什至应该使用协议吗？也许子类化可以解决这个问题？

Answer 1

您不能在数组中存储 2 个不同类型的对象（Wallet<String> 和 Wallet<Int> 是不同的类型）。

但你可以尝试这样的事情：

protocol WalletProtocol {
    func printMoneyStack()
}

class Wallet<T> {
    var moneyStack: [T] = []
    init(moneyStack: [T]) {
        self.moneyStack = moneyStack
    }
}

class BitcoinWallet: Wallet<String>, WalletProtocol {
    func printMoneyStack() {
        print(moneyStack)
    }
}

class EthereumWallet: Wallet<Int>, WalletProtocol {
    func printMoneyStack() {
        print(moneyStack)
    }
}


let bitcoinWallet = BitcoinWallet(moneyStack: ["A","B"])
let etheureumWallet = EthereumWallet(moneyStack: [1,2])

let wallets: [WalletProtocol] = [bitcoinWallet, etheureumWallet]

for wallet in wallets {
    wallet.printMoneyStack()
}

没有泛型约束的协议中的泛型数组

Generic array in protocol without generic constraint

generics

ios

swift

swift-protocols