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Swift Apprentice: Beyond the Basics

Second Edition · iOS 18 · Swift 6 · Xcode 16.2

Before You Begin

Section 0: 6 chapters
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Section I: Beyond the Basics

Section 1: 13 chapters
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3. Result Builders
Written by Eli Ganim

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Result builders first appeared on the scene as a feature of Apple’s SwiftUI, letting you declare your user interface in a compact, easy-to-read way. It was since expanded as a general language feature that lets you build values by combining a sequence of expressions. Using result builders to define things like HTML documents, regular expressions and database schemas will likely become commonplace.

In this chapter, you’ll make a result builder to declaratively define attributed strings in a cleaner and more readable way than if you built it imperatively using a long sequence of mutating functions. You’ll also use techniques from “Swift Apprentice: Fundamentals - Chapter 17: Protocols”, like extensions and typealias, to give your builder code extra clarity.

Meet NSAttributedString

To demonstrate how result builders work, you’ll build a small project that uses NSAttributedString to show a fancy greet message. By the end of this chapter, you’ll create a string that looks like this:

NSAttributedString is a special object that holds a string and lets you add attributes, like color and font, to the whole string or only to part of it.

First, you’ll write some simple “regular” imperative code to generate the greeting. Later, you’ll convert that code to use a result builder.

Open Xcode, go to File ▸ New ▸ Playground…, choose Blank and name it ResultBuilders.

Enter this function into the playground:

func greet(name: String) -> NSAttributedString {
  let message = NSAttributedString(string: "Hello " + name)
  return message
}

Now, call the function by adding greet(name: "Daenerys") below it. Finally, run the playground and observe the result by clicking the Show Result button to the right:

Adding Color With an Attribute

Right now, you aren’t using any of the capabilities of NSAttributedString. You’ll change that by adding color to the greeting message using an attribute.

Adding Color to a Specific String

What if you wanted to change only the text color of the name of the person you’re greeting and not the word “Hello”? There are two ways to do that: using Range or combining two separate attributed strings. Here, you’ll use the second approach because it’s easier to understand.

Adding Another Attributed String

If you want to add another string to the mix — for example, one with a different font size — you need yet another attributed string. Add the following code before the return statement:

Creating a Result Builder

Start by creating a new enum called AttributedStringBuilder. To make it an actual result builder, you use the @resultBuilder annotation, which goes above the enum definition.

Building the Greeting String With the Result Builder

Now, you’ll use the result builder to construct the same greeting string you created earlier by creating a new method.

Improving Readability by Using Extensions and Type Aliases

Earlier in the chapter, you applied attributes like color and font size by creating a dictionary of attributes and then using that dictionary in the attributed string initializer. Now, you’ll use a fancier approach that makes the code more readable.

Adding Fonts and Color

Now, go back to greetBuilder, which you created earlier, and use some fonts and color! Replace it with this:

Using typealias

While the result builder code is pretty straightforward, too many NSMutableAttributedString are floating around. Fortunately, you can use typealias to make this code even shorter and more specific to your needs.

Using Conditional Logic

If you pass in an empty title, you’ll get a weird result that looks like this:

Using Complex Conditional Logic

Next, you’ll add one final touch: If the title is empty, you’ll make the greet building method append “No title” to the final result. Start by adding an else clause to the existing if statement:

Using Loops with Result Builders

If you’re familiar with Daenerys from the television show “Game of Thrones”, you know she has many titles: Mother of Dragons, Khaleesi, First of Her Name, Breaker of Chains and more. She insists on having all her titles next to her name, so you need support for multiple titles.

Supporting Multiple Data Types

The greeting string is getting long, so you’d like to be able to break each title into a new line. This feature should be simple. Add Text("\n") line right after Text(", ") so the function looks like this:

Key Points

Result builders have use beyond Apple’s SwiftUI. Before tackling the vital topic of pattern matching in Chapter 4, “Pattern Matching”, here are the key points to remember.

2. Custom Operators, Subscripts & Keypaths
4. Pattern Matching
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