Codable in Swift 4.0

https://medium.com/@sarunw/codable-in-swift-4-0-1a12e38599d8

can it replace JSON encode/decode lib out there?

• 它可以取代JSON编码/解码lib吗？

After I watched WWDC 2017 and heard about Codable I’m thinking of replacing my current JSON encode/decoder in my projects or at least use it in a new one.

• 在我观看WWDC 2017并听说Codable之后，我正在考虑在我的项目中替换当前的JSON编码/解码器，或者至少在新项目中使用它。

I’m happy to see Apple finally come up with this encoder/decoder built into Swift standard lib, since its such a mandatory tasks nowdays and for me I haven’t seen a clear winner in this area. I try to avoid using third part library as much as possible, so I’m really excited to explore its possibility and limitation.

• 我很高兴看到Apple终于推出了内置于Swift标准库中的这种编码器/解码器，因为它现在是如此强制性的任务，而且对于我来说，我还没有看到这个领域的明显赢家。 我尽量避免使用第三方库，所以我很高兴能够探索它的可能性和局限性。

What I looking for in encode/decode?

• 我在编码/解码中寻找什么？

1. Swift optional type supported

• 支持Swift可选类型

The most important thing I need is a Swift optional type supported, this is very cruciel for me, without this it is a deal-breaker.

• 我需要的最重要的是支持Swift可选类型，对我来说这是非常棘手的，如果没有它，这是一个交易破坏者。

Luckily Codable support this. If you have following User Object.

• 幸运的是Codable支持这一点。 如果您有以下用户对象。

struct User: Codable {
    var firstName: String
    var lastName: String
    var middleName: String?
}

These JSON strings are valid.

• 这些JSON字符串有效。

{
    "firstName": "John",
    "lastName": "Doe",
    "middleName": null
}
{
    "firstName": "John",
    "lastName": "Doe"
}

2. Be able to rename properties

• 能够重命名属性

If you have ever work with REST API you will see that most JSON keys doesn’t use CamelCase naming, but snake_case. Codable also support rename property keys and its very easy to do so.

• 如果您曾经使用REST API，您将看到大多数JSON密钥不使用CamelCase命名，而是使用snake_case。 Codable也支持重命名属性键，它很容易实现。

All you need to do is adding a nested enumeration named CodingKeys that conforms to the CodingKey protocol.

• 您需要做的就是添加一个符合CodingKey协议的名为CodingKeys的嵌套枚举。

From above example we can rename it like this.

• 从上面的例子我们可以像这样重命名它。

struct User: Codable {
    var firstName: String
    var lastName: String
    var middleName: String?
    enum CodingKeys: String, CodingKey {
         case firstName = "first_name"
         case lastName = "last_name"
         case middleName = "middle_name"
    }
}

And you will be able to decode this JSON

• 并且您将能够解码此JSON

{
    "first_name": "John",
    "last_name": "Doe",
    "middle_name": null
}
{
    "first_name": "John",
    "last_name": "Doe"
}

3. Custom mapping between JSON and Swift structure.

• JSON和Swift结构之间的自定义映射。

There are cases where we have no control over how JSON look like, be able to have different Swift structure than JSON is nice-to-have feature.

• 在某些情况下，我们无法控制JSON的外观，能够拥有与JSON不同的Swift结构，这是一个很好的功能。

I will test on 2 common cases. 我将测试2种常见情况

1. Flatten out JSON nested property.

• 展平JSON嵌套属性。

2. Make nested structure from flat JSON.

• 从平面JSON创建嵌套结构。

4. Flatten out JSON

• 展平JSON

Let say you have User JSON that contain nested billingAddress property.

• 假设您有User JSON包含嵌套的billingAddress属性。

{
    "name": "John",
    "billingAddress": {
        "district": "District",
        "subDistrict": "Sub District",
        "country": "Country",
        "postalCode": "Postal Code"
    }
}

But somehow you want to layout Swift User like this.

• 但不知何故，你想要像这样布局Swift用户。

struct User: Codable {
    var name: String
    var district: String
    var subDistrict: String
    var country: String
    var postalCode: String
}

You need to define two enumerations that each list the complete set of coding keys used on a particular level.

• 您需要定义两个枚举，每个枚举列出在特定级别上使用的完整编码密钥集。

struct User: Codable {
    ....
    enum CodingKeys: String, CodingKey {
        case name
        case billingAddress
    }
    enum BillingAddressKeys: String, CodingKey {
        case district
        case subDistrict
        case country
        case postalCode
    }
}

Since this isn’t direct mapping we need to implementing Decodable's required initializer, init(from:):

• 由于这不是直接映射，我们需要实现Decodable所需的初始化器init（from :)：

init(from decoder: Decoder) throws {
   let values = try decoder.container(keyedBy: CodingKeys.self)
   name = try values.decode(String.self, forKey: .name)
   let billingAddress = try values.nestedContainer(keyedBy:     BillingAddressKeys.self, forKey: .billingAddress)
   district = try billingAddress.decode(String.self, forKey: .district)
   subDistrict = try billingAddress.decode(String.self, forKey: .subDistrict)
   country = try billingAddress.decode(String.self, forKey: .country)
   postalCode = try billingAddress.decode(String.self, forKey: .postalCode)
}

And same apply to Encodable protocol, a custom encode(to:):

• 同样适用于Encodable协议，自定义编码（至:)：

func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(name, forKey: .name)
    var billingAddress = container.nestedContainer(keyedBy: BillingAddressKeys.self, forKey: .billingAddress)
    try billingAddress.encode(district, forKey: .district)
    try billingAddress.encode(subDistrict, forKey: .subDistrict)
    try billingAddress.encode(country, forKey: .country)
    try billingAddress.encode(postalCode, forKey: .postalCode)
}

5. Nested structure

• 嵌套结构

This is opposite of what we just did, we got JSON like this.

• 这与我们刚刚做的相反，我们得到了这样的JSON。

{
    "name": "John",
    "district": "District",
    "subDistrict": "Sub District",
    "country": "Country",
    "postalCode": "Postal Code"
}

And want this Swift structure

• 并希望这个Swift结构

struct User: Codable {
    var name: String
    var billingAddress: BillingAddress
}
struct BillingAddress: Codable {
    var district: String
    var subDistrict: String
    var country: String
    var postalCode: String
}

Following are what we need to implement

• 以下是我们需要实施的内容

struct User: Codable {
    ....
    enum CodingKeys: String, CodingKey {
        case name
        case billingAddress
        case district
        case subDistrict
        case country
        case postalCode
    }
}
init(from decoder: Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    name = try values.decode(String.self, forKey: .name)
    billingAddress = try BillingAddress(from: decoder)
}
func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)    
    try container.encode(name, forKey: .name)
    try container.encode(billingAddress.district, forKey: .district)
    try container.encode(billingAddress.subDistrict, forKey: .subDistrict)
    try container.encode(billingAddress.country, forKey: .country)
    try container.encode(billingAddress.postalCode, forKey: .postalCode)
}

Conclusion

Codable pass all my criteria, it can do what I needed with easy to understand syntax. The only aspect that I didn’t touch is performance (I think it would be good). My conclusion is I definitely use it for my next project.

• Codable通过了我的所有标准，它可以通过易于理解的语法完成我需要的工作。 我没有触及的唯一方面是性能（我认为这会很好）。 我的结论是我肯定将它用于我的下一个项目。