-
Notifications
You must be signed in to change notification settings - Fork 3
/
Copy pathswift-codable-decodingencoding-with-context.html
363 lines (308 loc) · 17.9 KB
/
swift-codable-decodingencoding-with-context.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
<!DOCTYPE html>
<html lang="en">
<head>
<script src="https://use.fontawesome.com/afd448ce82.js"></script>
<!-- Meta Tag -->
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<!-- SEO -->
<meta name="author" content="Bruno Rocha">
<meta name="keywords" content="Software, Engineering, Blog, Posts, iOS, Xcode, Swift, Articles, Tutorials, OBJ-C, Objective-C, Apple">
<meta name="description" content="The Codable protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like Unbox, Codable has the advantage of being powered by the compiler. One of my favorite features in Unbox was to give a context to the decoding operation. Let's take a look at how we can achieve the same with Codable and how I use to power a type-erased Decodable type.">
<meta name="title" content="Swift Codable: Decoding / Encoding With Context">
<meta name="url" content="https://swiftrocks.com/swift-codable-decodingencoding-with-context">
<meta name="image" content="https://swiftrocks.com/images/thumbs/thumb.jpg?4">
<meta name="copyright" content="Bruno Rocha">
<meta name="robots" content="index,follow">
<meta property="og:title" content="Swift Codable: Decoding / Encoding With Context"/>
<meta property="og:image" content="https://swiftrocks.com/images/thumbs/thumb.jpg?4"/>
<meta property="og:description" content="The Codable protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like Unbox, Codable has the advantage of being powered by the compiler. One of my favorite features in Unbox was to give a context to the decoding operation. Let's take a look at how we can achieve the same with Codable and how I use to power a type-erased Decodable type."/>
<meta property="og:type" content="website"/>
<meta property="og:url" content="https://swiftrocks.com/swift-codable-decodingencoding-with-context"/>
<meta name="twitter:card" content="summary_large_image"/>
<meta name="twitter:image" content="https://swiftrocks.com/images/thumbs/thumb.jpg?4"/>
<meta name="twitter:image:alt" content="Page Thumbnail"/>
<meta name="twitter:title" content="Swift Codable: Decoding / Encoding With Context"/>
<meta name="twitter:description" content="The Codable protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like Unbox, Codable has the advantage of being powered by the compiler. One of my favorite features in Unbox was to give a context to the decoding operation. Let's take a look at how we can achieve the same with Codable and how I use to power a type-erased Decodable type."/>
<meta name="twitter:site" content="@rockbruno_"/>
<!-- Favicon -->
<link rel="icon" type="image/png" href="images/favicon/iconsmall2.png" sizes="32x32" />
<link rel="apple-touch-icon" href="images/favicon/iconsmall2.png">
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Source+Sans+3:ital,wght@0,200..900;1,200..900&display=swap" rel="stylesheet">
<!-- Bootstrap CSS Plugins -->
<link rel="stylesheet" type="text/css" href="css/bootstrap.css">
<!-- Prism CSS Stylesheet -->
<link rel="stylesheet" type="text/css" href="css/prism4.css">
<!-- Main CSS Stylesheet -->
<link rel="stylesheet" type="text/css" href="css/style48.css">
<link rel="stylesheet" type="text/css" href="css/sponsor4.css">
<!-- HTML5 shiv and Respond.js support IE8 or Older for HTML5 elements and media queries -->
<!--[if lt IE 9]>
<script src="https://oss.maxcdn.com/html5shiv/3.7.3/html5shiv.min.js"></script>
<script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
<![endif]-->
<script type="application/ld+json">
{
"@context": "https://schema.org",
"@type": "BlogPosting",
"mainEntityOfPage": {
"@type": "WebPage",
"@id": "https://swiftrocks.com/swift-codable-decodingencoding-with-context"
},
"image": [
"https://swiftrocks.com/images/thumbs/thumb.jpg"
],
"datePublished": "2020-02-05T07:25:00+00:00",
"dateModified": "2020-04-12T14:00:00+02:00",
"author": {
"@type": "Person",
"name": "Bruno Rocha"
},
"publisher": {
"@type": "Organization",
"name": "SwiftRocks",
"logo": {
"@type": "ImageObject",
"url": "https://swiftrocks.com/images/thumbs/thumb.jpg"
}
},
"headline": "Swift Codable: Decoding / Encoding With Context",
"abstract": "The Codable protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like Unbox, Codable has the advantage of being powered by the compiler. One of my favorite features in Unbox was to give a context to the decoding operation. Let's take a look at how we can achieve the same with Codable and how I use to power a type-erased Decodable type."
}
</script>
</head>
<body>
<div id="main">
<!-- Blog Header -->
<!-- Blog Post (Right Sidebar) Start -->
<div class="container">
<div class="col-xs-12">
<div class="page-body">
<div class="row">
<div><a href="https://swiftrocks.com">
<img id="logo" class="logo" alt="SwiftRocks" src="images/bg/logo2light.png">
</a>
<div class="menu-large">
<div class="menu-arrow-right"></div>
<div class="menu-header menu-header-large">
<div class="menu-item">
<a href="blog">blog</a>
</div>
<div class="menu-item">
<a href="about">about</a>
</div>
<div class="menu-item">
<a href="talks">talks</a>
</div>
<div class="menu-item">
<a href="projects">projects</a>
</div>
<div class="menu-item">
<a href="software-engineering-book-recommendations">book recs</a>
</div>
<div class="menu-item">
<a href="games">game recs</a>
</div>
<div class="menu-arrow-right-2"></div>
</div>
</div>
<div class="menu-small">
<div class="menu-arrow-right"></div>
<div class="menu-header menu-header-small-1">
<div class="menu-item">
<a href="blog">blog</a>
</div>
<div class="menu-item">
<a href="about">about</a>
</div>
<div class="menu-item">
<a href="talks">talks</a>
</div>
<div class="menu-item">
<a href="projects">projects</a>
</div>
<div class="menu-arrow-right-2"></div>
</div>
<div class="menu-arrow-right"></div>
<div class="menu-header menu-header-small-2">
<div class="menu-item">
<a href="software-engineering-book-recommendations">book recs</a>
</div>
<div class="menu-item">
<a href="games">game recs</a>
</div>
<div class="menu-arrow-right-2"></div>
</div>
</div>
</div>
<div class="content-page" id="WRITEIT_DYNAMIC_CONTENT">
<!--WRITEIT_POST_NAME=Swift Codable: Decoding / Encoding With Context-->
<!--WRITEIT_POST_HTML_NAME=swift-codable-decodingencoding-with-context-->
<!--WRITEIT_POST_SITEMAP_DATE_LAST_MOD=2020-04-12T14:00:00+02:00-->
<!--WRITEIT_POST_SITEMAP_DATE=2020-02-05T07:25:00+00:00-->
<!--Add here the additional properties that you want each page to possess.-->
<!--These properties can be used to change content in the template page or in the page itself as shown here.-->
<!--Properties must start with 'WRITEIT_POST'.-->
<!--Writeit provides and injects WRITEIT_POST_NAME and WRITEIT_POST_HTML_NAME by default.-->
<!--WRITEIT_POST_SHORT_DESCRIPTION=The Codable protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like Unbox, Codable has the advantage of being powered by the compiler. One of my favorite features in Unbox was to give a context to the decoding operation. Let's take a look at how we can achieve the same with Codable and how I use to power a type-erased Decodable type.-->
<title>Swift Codable: Decoding / Encoding With Context</title>
<div class="blog-post">
<div class="post-title-index">
<h1>Swift Codable: Decoding / Encoding With Context</h1>
</div>
<div class="post-info">
<div class="post-info-text">
Published on 05 Feb 2020
</div>
</div>
<p>The <code>Codable</code> protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like <code>Unbox</code>, <code>Codable</code> has the advantage of being powered by the compiler.</p>
<div class="sponsor-article-ad-auto hidden"></div>
<p>One of my favorite features in <code>Unbox</code> was to give a <b>context</b> to the decoding operation. Let's take a look at how we can achieve the same with <code>Codable</code>.</p>
<h2>Example 1: Giving different values to the same property</h2>
<p>Decoding with "context" refers to the ability to change how types are decoded depending on what's going on in your app. Let's say that our app is sending a request asking for information regarding a header that should be displayed on the screen, with our app locally storing an additional <code>textColor</code> property into the model:</p>
<pre><code>struct HeaderInformation: Decodable {
let title: String
let imageUrl: String
var textColor: UIColor {
return .black
}
}</code></pre>
<p>Now, imagine the following situation: What if we want <code>textColor</code> to be <b>different</b> depending on which screen such header is being presented? For example, displaying headers on the app's home could have a green text, while headers at the profile screen could be blue.</p>
<p>There are multiple ways to achieve this, but let's focus on <code>Codable</code>. <code>Codable</code> allows you to provide context to a <code>JSONDecoder</code>/<code>JSONEncoder</code> through their <code>userInfo</code> property:</p>
<pre><code>/// Contextual user-provided information for use during decoding.
open var userInfo: [CodingUserInfoKey : Any]</code></pre>
<p>The <code>CodingUserInfoKey</code> is a <code>RawRepresentable</code> string enum, so you can add pretty much anything to <code>userInfo</code>. When this is done, the same <code>userInfo</code> values will be accessible as part of the inner <code>Decoder</code>/<code>Encoder</code> instance:</p>
<pre><code>struct HeaderInformation: Decodable {
let title: String
let imageUrl: String
let textColor: UIColor
static var textColorUserInfoKey: CodingUserInfoKey {
return CodingUserInfoKey(rawValue: "textColor")!
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
title = try container.decode(String.self, forKey: .title)
imageUrl = try container.decode(String.self, forKey: .imageUrl)
textColor = decoder.userInfo[Self.textColorUserInfoKey] as? UIColor ?? .black
}
}</code></pre>
<p>To make this work, simply fill the <code>userInfo</code> dictionary before decoding the data.</p>
<pre><code>let decoder = JSONDecoder()
decoder.userInfo[HeaderInformation.textColorUserInfoKey] = UIColor.red
let headerInfo = decoder.decode(HeaderInformation.self, from: data)</code></pre>
<p><code>CodingUserInfoKey</code> requires force-unwrapping because it is <code>RawRepresentable</code>, but doing so will never result in a crash. To hide it, I like to abstract it inside an extension that takes a regular <code>String</code> instead:</p>
<pre><code>extension Decoder {
func getContext(forKey key: String) -> Any? {
let infoKey = CodingUserInfoKey(rawValue: key)!
return userInfo[infoKey]
}
}
extension JSONDecoder {
func set(context: Any?, forKey key: String) {
let infoKey = CodingUserInfoKey(rawValue: key)!
userInfo[infoKey] = context
}
}</code></pre>
<p>We can now use the type's own <code>CodingKeys</code> as the userInfo key, resulting in the following improvements:</p>
<pre><code>let key = HeaderInformation.CodingKeys.textColor.stringValue
decoder.set(context: UIColor.red, forKey: key)
//
textColor = decoder.getContext(forKey: CodingKeys.textColor.stringValue)</code></pre>
<h2>Example 2: Powering modularized type-erased structs</h2>
<p>One cool example of how I use <code>Codable</code> contexts is how <code>AnyRoute</code> works in <a href="https://github.com/rockbruno/RouterService">the RouterService library:</a></p>
<pre><code>/// A type-erased container for a `Route`, used for route decoding purposes.
public struct AnyRoute {
public let value: Route
public let routeString: String
}</code></pre>
<p>In short, we have the following environment:</p>
<p>- Apps can define their own "routes", which are structs used to navigate between screens.</p>
<p>- These routes are registered into a <code>RouterService</code> instance, which receives said routes and pushes the related view controllers.</p>
<p>- Routes can be decoded from a specific string format, allowing your backend to dictate which screen the app should navigate to.</p>
<p>The latter is done through <code>AnyRoute</code> -- an erased <code>Route</code> type that knows how to decode such string format into the actual <code>Route</code> type defined by the app. The problem is: <b>because AnyRoute is defined inside the RouterService library, it has no access to the app's Routes. How can it decode the correct Route type?</b></p>
<p>This can be achieved by using <code>Codable</code>'s context features. Because <code>Routes</code> have to be registered in the main <code>RouterService</code> instance, we can inject it into the decoding operation and have it determine which <code>Route</code> should be decoded:</p>
<pre><code>extension AnyRoute: Decodable {
static var contextUserInfoKey: CodingUserInfoKey {
// swiftlint:disable:next force_unwrapping
return CodingUserInfoKey(rawValue: "routerservice_anyroute_context")!
}
public init(from decoder: Decoder) throws {
let ctx = decoder.userInfo[AnyRoute.contextUserInfoKey]
guard let context = ctx as? RouterServiceAnyRouteDecodingProtocol else {
preconditionFailure("TRIED TO DECODE ANYROUTE WITHOUT A CONTEXT!")
}
let data = try context.decodeAnyRoute(fromDecoder: decoder)
self.value = data.0
self.routeString = data.1
}
}
public protocol RouterServiceAnyRouteDecodingProtocol {
func decodeAnyRoute(fromDecoder decoder: Decoder) throws -> (Route, String)
}</code></pre>
<div class="sponsor-article-ad-auto hidden"></div>
<p>Assuming that <code>RouterService</code> was injected into the <code>JSONDecoder</code>, the decoding operation will be deferred to it:</p>
<pre><code>extension RouterService: RouterServiceAnyRouteDecodingProtocol {
public func decodeAnyRoute(fromDecoder decoder: Decoder) throws -> (Route, String) {
let container = try decoder.singleValueContainer()
let identifier = try container.decode(String.self)
guard let routeString = RouteString(fromString: identifier) else {
throw RouteDecodingError.failedToParseRouteString
}
guard let routeType = registeredRoutes[routeString.scheme]?.0 else {
throw RouteDecodingError.unregisteredRoute
}
do {
let value = try routeType.decode(JSONDecoder(), routeString.parameterData)
return (value, routeString.originalString)
} catch {
throw error
}
}
public enum RouteDecodingError: Swift.Error {
case unregisteredRoute
case failedToParseRouteString
}
}</code></pre>
<p>This decentralized behavior is especially important for modular apps (which is <code>RouterService</code>'s use case), as different targets can reference and decode <code>AnyRoutes</code> without having access to the app's real <code>Routes</code>.</p>
</div></div>
<div class="blog-post footer-main">
<div class="footer-logos">
<a href="https://swiftrocks.com/rss.xml"><i class="fa fa-rss"></i></a>
<a href="https://twitter.com/rockbruno_"><i class="fa fa-twitter"></i></a>
<a href="https://github.com/rockbruno"><i class="fa fa-github"></i></a>
</div>
<div class="footer-text">
© 2025 Bruno Rocha
</div>
<div class="footer-text">
<p><a href="https://swiftrocks.com">Home</a> / <a href="blog">See all posts</a></p>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<!-- Blog Post (Right Sidebar) End -->
</div>
</div>
</div>
<!-- All Javascript Plugins -->
<script type="text/javascript" src="js/jquery.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl+NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP+JcXn/tWtIaxVXM" crossorigin="anonymous"></script>
<script type="text/javascript" src="js/prism4.js"></script>
<!-- Main Javascript File -->
<script type="text/javascript" src="js/scripts30.js"></script>
<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-H8KZTWSQ1R"></script>
<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'G-H8KZTWSQ1R');
</script>
</body>
</html>