Swift 4.2: How the weirdest error message led to reimagining implicitly unwrapped optionals

In this tutorial, I’ll show you how researching and learning about the current programming language you’re using (i.e., Swift) can lead you to new discoveries, a better understanding of the language, and becoming a better problem solver. Recently, I ran into a Swift error that at first had me scratching my head. The error had to do with implicitly unwrapped optionals. My debugging and contemporaneous research led me to the little-known fact that, in Swift 4.2, a proposal “to remove the ImplicitlyUnwrappedOptional type from the Swift type system and replace it with an IUO attribute on declarations” was accepted and implemented. Despite the fact that I followed and covered language changes leading to Swift 4.2, I missed this seemingly innocuous but nonetheless prescient modification.

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Swift 4.2 improvements? Member/dot syntax for subscripts. Trying it out in a protocol-oriented, generic linked list.

The code shown herein will only compile and link in Xcode 10 beta and run on iOS 12 beta and/or OS X 10.14 beta.

While working on a Swift protocol-oriented and generic linked list, I got to thinking about Apple’s “improvements” to version 4.2 of their flagship language. Since a linked list is a list, I thought, “Why not add a subscript to my linked list to facilitate finding specific nodes in my list?” I did that in Swift 4.1 and got what most developers would’ve expected, e.g., used linkedList["node4"] to get the node in the list associated with the keyword “node4.” With Swift 4.2, I can use the controversial new @dynamicMemberLookup language attribute and implement dot/member notation, like linkedList.node4 to get that same node in the list associated with “node4.” Big improvement, huh? Well, maybe. We’ll talk about how this new and improved subscript is more than just about syntactic sugar, but that the “driving motivation for this feature is to improve interoperability with inherently dynamic languages like Python, Javascript, Ruby and others.” Note that all code shown in this tutorial was written in two Xcode 10 beta playgrounds.

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Tutorial: delegates and delegation in Swift 4

The original article – Understanding Delegates and Delegation in Swift 4 was published on appcoda.com.

[Download Xcode 9 project with full Swift 4 source from GitHub.]


I’m going to talk about “delegates” and “delegation.” I’ll lead you through a simple example of implementing the delegation design pattern in Swift 4, with full source code. My intent here is to show you how delegation works without getting bogged down in some crazy complex example. To help you become the best of the best, I’m going to introduce you to one of the greatest design tools to aid in object-oriented software development, UML. I’ll show you a UML diagram that I drew up to design and document the implementation of the delegation design pattern used in the sample app we’ll build together. Download the Xcode 9 project with full Swift 4 source from GitHub to follow along.

(Note: compare this post’s approach of using delegation with my next post’s approach of using NSNotificationCenterto accomplish the same goal.)

I’ll show you how to build a user interface (UI) helper, a class that downloads a file at a specified URL. Most importantly, I’ll show you how, through delegation, a UIViewController subclass can be notified by the helper that an image file has finished downloading, and then the view controller can display the image on screen. For the sake of simplicity and clarity, we’ll pretend that Swift has minimal support for downloading a file from a URL. We’ll manually wire up the notification that the file has finished downloading using the delegation design pattern. Here’s the app we’ll build:

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Writing expressive, meaningful, and readable code in Swift 4

Let’s talk about creating understandable and maintainable code in Swift 4, code that can be easily read by other programmers, can be readily debugged, can be reused, and can be enhanced and extended in the future. We’re going to limit today’s discussion to: 1) how best to come up with good names for functions and how best to name their arguments, parameters, and argument labels, and 2) how to use meaningful and descriptive variable (var) names — and constant (let) and enumeration (enum) names, etc. The goal is to create code that reads as to close to English as possible.

You may believe that this topic is overly simplistic or pedantic, but sometimes it’s the “little things” in life that really matter — especially when you have applications that are made up of millions of lines of code, made up of “little things” like function definitions and function calls.

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Xcode tidbits: Open as Hex, Open As Source Code, (Git) line endings, and text encoding

With this article, I’m starting a series about all the goodies — useful tools — that can be found in Xcode. Some of these tidbits are tools everyone knows about while others are barely documented to undocumented. For example, how many of you know that you can view, inspect, and debug all your Auto Layout constraints live during app execution using the “Debug View Hierarchy” Xcode feature? I discussed that feature in detail in this article, “Troubleshooting Auto Layout using Xcode’s Debug View Hierarchy.” Today, we’ll discover two editors that ship with Xcode, the “Open As > Hex” and “Open As > Source Code” editors, both only available by right-clicking on files in the “Project Navigator” to reveal a contextual menu.

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Creating a new Git/GitHub repository for your Xcode project — a detailed tutorial

Let’s talk about source/version control, why it’s so important, and how you can easily put all your iOS code under source/version control management (SCM). I’m going to show you the manual steps involved in putting your code into a Git SCM “repository” (repo) so you fully understand how source/version control works. Jump straight to the tutorial if you’re already familiar with the concept of source control. I can’t explain everything about SCM in one blog post, but I’ll get you started and provide many online resources for you to reference. Why am I using Git? Like it or not, Git has become the de facto standard in SCM systems, mainly because it “is a free and open source [and] distributed version control system.” I don’t buy into the “Git is easy to learn” argument. I find Git to be overly complicated, cryptic, and generally requiring more steps to accomplish source control tasks than say centralized SCM systems like TFS or Subversion/SVN. Git does have some advantages over other SCM products, and it even becomes quite efficacious once you pay your dues learning how to use it properly.

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Oh, my – App Transport Security has blocked a cleartext HTTP (https://) resource load since it is insecure

App Transport Security (ATS) is enabled by default for apps linked against the iOS 9.0 or OS X v10.11 SDKs or later, as indicated by the default Boolean value of NO for the NSAllowsArbitraryLoads key. This key is at the root level of the NSAppTransportSecurity dictionary.

With ATS enabled, HTTP connections must use HTTPS (RFC 2818). Attempts to connect using insecure HTTP fail. ATS employs the Transport Layer Security (TLS) protocol version 1.2 (RFC 5246). For background on secure Internet connections, read HTTPS Server Trust Evaluation.

Apple, Information Property List Key Reference

With the advent of iOS 9, Apple decided that developers should avoid accessing insecure, unencrypted clear text HTTP (https://) resources on the Internet. Today I’ll show you how to access HTTP sites/services in your apps. I’ll explain the special hoops that Apple wants you to jump through just to use HTTP — and help you keep your app from being rejected.

For Apple to assume that anything HTTP is dangerous is a bit overboard as there are legitimate reasons to access a resource via clear text, like downloading an image (clear binary). Grabbing an image won’t reveal information about users’ private lives. A web/REST service that consumes someone’s name and Social Security number is a different story — that info must be encrypted.

Fortunately, Apple has made some accommodations in allowing continued use of HTTP as long as you provide “justification” when submitting your apps.

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Xcode secrets: save time with context-sensitive help and documentation

How many times have you been looking at Swift or Objective-C code in Xcode and can’t remember what a framework method, argument, constant, etc. means? Did you know that help — full documentation — is just a keystroke away? Did you also know that you can add the same type of pop-up, context-sensitive help to your own code? Here’s how. Let’s say you’re looking at the following NSString class method and can’t remember exactly what the call does, what parameters it takes, and what is its return value:

Highlight the method name, parameter, even the enum, and then press the following key combination on your keyboard:

[command] + [control] + [shift] + [?]

      or, using key symbols:

⌘ ⌃ ⇧ ?

Here’s what you’ll see: a context-sensitive help/documentation popup. Note that I added the red lines to highlight content. The red highlighting is not what Xcode provides (click to enlarge):

Figure 1: Xcode framework context sensitive help and documentation.

So you immediately get information about the method, parameter, enum, even constant’s:

  • Declaration (formal language signature);
  • Description (textual explanation of the entity’s purpose);
  • Parameters (a full list of names and definitions);
  • Returns (the value returned by a method/function, if applicable);
  • Availability (what version of iOS that the entity became available in — and sometimes in what version it was deprecated);
  • Declared In (the framework which contains the entity); and,
  • More (generally, the formal definition of the entity with explanations and other links).

Check out what I get if I click on the “More Type Method Reference” link as shown in Figure 1 (click to enlarge):

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