如果单元素解码失败，Swift JSONDecode 解码数组将失败

发布于09月23日

在使用SWIFT4和可编码协议时，我遇到了以下问题-似乎无法允许JSONDecoder跳过数组中的元素. 例如，我有以下JSON:

[
    {
        "name": "Banana",
        "points": 200,
        "description": "A banana grown in Ecuador."
    },
    {
        "name": "Orange"
    }
]

和一个可编码的 struct :

struct GroceryProduct: Codable {
    var name: String
    var points: Int
    var description: String?
}

When decoding this json

let decoder = JSONDecoder()
let products = try decoder.decode([GroceryProduct].self, from: json)

结果products为空.这是意料之中的，因为JSON中的第二个对象没有"points"键，而points在GroceryProduct struct 中不是可选的.

问题是如何允许JSONDecoder"跳过"无效对象？

推荐答案

一种 Select 是使用试图解码给定值的包装器类型；如果不成功，则存储nil:

struct FailableDecodable<Base : Decodable> : Decodable {

    let base: Base?

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        self.base = try? container.decode(Base.self)
    }
}

We can then decode an array of these, with your GroceryProduct filling in the Base placeholder:

import Foundation

let json = """
[
    {
        "name": "Banana",
        "points": 200,
        "description": "A banana grown in Ecuador."
    },
    {
        "name": "Orange"
    }
]
""".data(using: .utf8)!


struct GroceryProduct : Codable {
    var name: String
    var points: Int
    var description: String?
}

let products = try JSONDecoder()
    .decode([FailableDecodable<GroceryProduct>].self, from: json)
    .compactMap { $0.base } // .flatMap in Swift 4.0

print(products)

// [
//    GroceryProduct(
//      name: "Banana", points: 200,
//      description: Optional("A banana grown in Ecuador.")
//    )
// ]

然后我们使用.compactMap { $0.base }来过滤出nil个元素(那些在解码时抛出错误的元素).

This will create an intermediate array of [FailableDecodable<GroceryProduct>], which shouldn't be an issue; however if you wish to avoid it, you could always create another wrapper type that decodes and unwraps each element from an unkeyed container:

struct FailableCodableArray<Element : Codable> : Codable {

    var elements: [Element]

    init(from decoder: Decoder) throws {

        var container = try decoder.unkeyedContainer()

        var elements = [Element]()
        if let count = container.count {
            elements.reserveCapacity(count)
        }

        while !container.isAtEnd {
            if let element = try container
                .decode(FailableDecodable<Element>.self).base {

                elements.append(element)
            }
        }

        self.elements = elements
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(elements)
    }
}

然后，您将解码为:

let products = try JSONDecoder()
    .decode(FailableCodableArray<GroceryProduct>.self, from: json)
    .elements

print(products)

// [
//    GroceryProduct(
//      name: "Banana", points: 200,
//      description: Optional("A banana grown in Ecuador.")
//    )
// ]