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Swift Codable指南：从JSON到模型对象的飞行教程

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"《Swift Codable指南》是一份PDF文档资源，由Mattt Zmuda、Lauren Mendez和Morgan Tarling合作编写，由Read Evaluate Press, LLC出版，发行地在波特兰，俄勒冈。该文档旨在帮助读者深入了解Swift编程中的Codable功能，这是一个用于序列化和反序列化数据的关键工具。第1章，"FirstFlight"，引导读者通过实际操作，学习如何将JSON数据解码成模型对象（Model Object），以及如何将这个对象编码回JSON。这一章强调了基础的编码与解码过程，适合初学者理解Codable的基本原理。章节2，"Holding Patterns"，可能涉及在数组形式下进行编码和解码的练习，展示了如何处理不同类型的数据结构和数组中的 Codable 对象。第3章，"Taking the Controls"，深入探讨如何在控制数据流时灵活运用Codable，可能包括自定义编码器和解码器，以及如何处理复杂数据类型。第4章和5章分别介绍了如何利用iTunes Search API构建一个音乐商店应用和使用UserDefaults构建一个航班服务应用，展示了Codable在实际项目中的应用场景，特别是与网络请求和用户数据管理的结合。第6章进一步提升挑战，通过Core Data介绍如何构建一个行李扫描应用，这表明Codable不仅可以与Swift的标准数据存储方式协作，还能与其他框架集成。第7章则专门讲解如何实现MessagePack编码器，这是一种高效且跨平台的数据序列化格式，有助于提高应用程序的性能和兼容性。整个文档不仅提供理论指导，还通过实例演示和练习，帮助读者掌握Swift Codable的核心技能，并准备好应对可能出现的错误处理，确保编码和解码过程的健壮性。通过这些章节的学习，读者能够获得全面的Codable使用策略，为今后的iOS或macOS开发打下坚实的基础。"

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Decoding this into an array of Plane objects couldn’t be

simpler: take our call to decode(_:from:) from before, and

replace Plane.self with [Plane].self. This changes the

decoded type from a Plane object to an array of Plane objects.

Swift

let planes = try! decoder.decode([Plane].self, from: json)

[Plane] is syntactic shorthand for Array<Plane>, or an Array

with the generic constraint that its elements are Plane objects.

Why does this work? It’s thanks to conditional conformance —

another feature of Swift 4.

Swift

// swift/stdlib/public/core/Codable.swift.gyb

extension Array : Decodable where Element : Decodable {

// ...

}

Although top-level arrays are technically valid JSON, it’s

considered best practice to always have an object at the top

level instead. For example, the following JSON has an array

keyed off the top-level object at "planes":

JSON

{

"planes": [

{

"manufacturer": "Cessna",

"model": "172 Skyhawk",

"seats": 4

{

"manufacturer": "Piper",

"model": "PA-28 Cherokee",

"seats": 4

}

]

}

Chapter 1: First Flight

Like Array, the Dictionary type conditionally conforms to

Decodable if its associated KeyType and ValueType conform

to Decodable:

Swift

// swift/stdlib/public/core/Codable.swift.gyb

extension Dictionary : Decodable where Key : Decodable,

Value : Decodable {

// ...

}

Because of this, you can change the same call to

decode(_:from:) from before and pass [String:

Plane].self (shorthand for Dictionary<String, Plane>)

instead:

Swift

let keyedPlanes = try! decoder.decode([String: Plane].self,

from: json)

let planes = keyedPlanes["planes"]

Alternatively, you can create a new type that conforms to

Decodable and has a property whose name matches the JSON

key and has a value of type [Plane], and pass that to

decode(_:from:):

Swift

struct Fleet: Decodable {

var planes: [Plane]

}

let fleet = try! decoder.decode(Fleet.self, from: json)

let planes = fleet.planes

You typically do this when the structure of your payload is

semantically meaningful. If objects are nested arbitrarily, such

as to namespace keys, you should probably just use a standard

collection class.

Chapter 1: First Flight

Trying Our Best with Error Handling

Our rst couple go-arounds with Codable went well, but our

technique was a bit sloppy with those try! keywords. For this

last lap, let’s take a moment to show how the pros do it in real

applications.

Swift functions and initializers marked with the throws

keyword may generate an error instead of returning a value.

Both the Decodable initializer init(from:) and the

Encodable method encode(to:) are marked with the throws

keyword — which makes sense, because it may not always be

possible to complete the operation successfully. When

decoding a representation into an object, the data might be

corrupted, a key might be missing, there might be a type

mismatch, or a nonoptional value might be missing. It’s less

typical for there to be problems during encoding; it really

depends on the format.

Swift requires all errors to be handled one way or another. So

when you call an expression marked with the throws keyword,

you need to prex it with the try operator or either of its

variants.

Swift

do {

let plane = try decoder.decode(Plane.self, from: json)

} catch {

print("Error decoding JSON: \(error)")

}

For information about how to communicate errors to users, see the Human Interface

Guidelines.

You can use the optional-try operator (try?) to convert a

throwing expression into an optional expression. That is, the

expression returns nil if an error occurs, otherwise, it returns

Chapter 1: First Flight

the expression value in an optional. If you aren’t doing anything

meaningful in your error handling, this can be a useful

shorthand.

Swift

if let plane = try? decoder.decode(Plane.self, from: json) {

print(plane.model)

}

Alternatively, there’s the forced-try operator (try!), which

we’ve been using up until this point. Following Swift’s language

conventions, the trailing “bang”, or exclamation mark (!),

indicates unsafe behavior. If you use the forced-try operator

on an expression that results in an error, the program exits

immediately. Because sudden app termination makes for a

lousy user experience, you generally avoid try! in app code.

However, for the purposes of exploring a new concept — such as

in a Playground — failing fast can actually be to your advantage.

Thus concludes our rst ight with Codable. If you feel like

your head is spinning, don’t worry — it’ll get more comfortable

with practice.

Things get even more exciting in Chapter 2. We’ll apply what

we learned here to a more challenging payload, and learn

strategies for maximizing how much the compiler does for us.

Recap

•

Codable is a composite type consisting of the Decodable and

Encodable protocols.

•

Swift automatically synthesizes conformance for Decodable and

Encodable if a type adopts conformance in its declaration, and each of

its stored properties also conform to that protocol.

•

An Array or Dictionary conforms to Codable if its associated element

type is Codable-conforming.

Chapter 1: First Flight

“See, right here — I'm pretty sure that's a private API.”

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