Swift Consequence Builders: Getting Began

Including @resultBuilder in Swift 5.4 was necessary, however you might need missed it. It’s the key engine behind the simple syntax you employ to explain a view’s structure: @ViewBuilder. Should you’ve ever questioned whether or not you possibly can create customized syntax like that in your tasks, the reply is sure! Even higher, you’ll be amazed at how simple it’s.

On this tutorial, you’ll study:

  • Swift syntax for making a consequence builder
  • Suggestions for planning your consequence builder
  • Tips on how to use a consequence builder to create a mini-language

Observe: This beginner-level tutorial assumes you’re snug constructing an iOS app utilizing Xcode and Swift, conversant in the Swift sort system and have a superb understanding of SwiftUI.

Getting Began

Obtain the starter undertaking by clicking the Obtain Supplies button on the prime or backside of this tutorial. Open the starter undertaking.

Introducing Decoder Ring

Agent: Your mission, must you select to simply accept it, is to finish the Decoder Ring app. Though you could have top-secret code consultants at your disposal to design one of the best ciphers, they would favor to not spend a lot time implementing them in Swift. Are you able to design a Area Particular Language that permits them to focus on cipher implementation and never be bothered with that Swift intricacies? After all, you may!

Observe: A Area Particular Language (DSL) is a programming language particularly tailor-made for a specific function (or area). This stands in distinction to a general-purpose language like Swift, which can be utilized for numerous software program functions.

Should you construct and run Decoder Ring, one can find a easy app with a single display screen.

The highest area is a textual content entry area the place an agent can sort a message to be enciphered, which is then displayed within the backside area. By switching the mode from Encode to Decode, the agent can as a substitute paste an enciphered message into the highest area to be deciphered within the backside area. Presently, the app lacks enciphering/deciphering performance.

It’s time to get cracking!

Making Your First Consequence Builder

To grasp how consequence builders operate, it’s finest to dive proper in. Create a file named CipherBuilder.swift. Add the next code:

// 1
// 2
enum CipherBuilder {
  // 3
  static func buildBlock(_ elements: String...) -> String {
      .joined(separator: " ")
      .replacingOccurrences(of: "e", with: "🥚")
  1. You begin with the @resultBuilder attribute, used to specify that the next definition is a consequence builder. @resultBuilder can annotate any sort that permits a static methodology.
  2. You’ve used an enum as a result of CipherBuilder doesn’t must have situations created. As a substitute, it solely comprises static strategies.
  3. You implement a static buildBlock(_:) operate. That is the one requirement for a consequence builder. Your operate takes any variety of String arguments and returns a String containing all of the arguments joined with an area and all situations of the letter e changed with the egg emoji: 🥚.

The company’s eggheads have known as this the Egg Cipher. Subsequent, it’s good to use your new consequence builder someplace within the app. Open ContentView.swift and add the next on the finish of the file:

// 1
// 2
func buildEggCipherMessage() -> String {
  // 3
  "A secret report inside the guild."
  "4 planets have come to our consideration"
  "relating to a plot that might jeopardize spice <a href="https://yourselfhood.com/revolutionizing-manufacturing-unleashing-the-power-of-cloud-mrp-and-cloud-manufacturing-software/">manufacturing</a>."
  1. Now, you should utilize CipherBuilder to annotate your code. You specify that buildEggCipherMessage() is a consequence builder applied in CipherBuilder.
  2. Your methodology returns a String, matching the return sort of your consequence builder.
  3. Inside your methodology, you listing a number of strings matching the anticipated argument sort String... in your consequence builder.

To indicate the output within the view physique, add a modifier to the tip of the ZStack:

.onAppear {
  secret = buildEggCipherMessage()

This code calls your consequence builder and set the output label to the returned worth. Construct and run to see the consequence.

The egg cipher results

As anticipated, the three strings are joined, and every occasion of “e” is changed with an egg.

Understanding Consequence Builders

It’s value exploring what’s happening right here. You’re merely itemizing strings within the physique of buildEggCipherMessage(). There aren’t any commas, and it’s not an array. So how does it work?

The compiler rewrites the physique of your buildEggCipherMessage() in keeping with the principles you’ve outlined in CipherBuilder. So when Xcode compiles this code:

  "A secret report inside the guild."
  "4 planets have come to our consideration"
  "relating to a plot that might jeapardize spice manufacturing."

You may think about it turns into one thing like this:

return CipherBuilder.buildBlock(
  "A secret report inside the guild.",
  "4 planets have come to our consideration",
  "relating to a plot that might jeapardize spice manufacturing."

As you broaden your information of consequence builders, imagining what the compiler interprets your code to will allow you to perceive what’s occurring. As you’ll see, every kind of programming logic will be supported utilizing consequence builders, together with loops and if-else statements. It’s all rewritten auto-magically to name your consequence builder’s foundational static operate.

When was the idea of a consequence builder first launched?
Consequence builders have been in Swift since 5.1 below completely different guises. With the arrival of SwiftUI, earlier than consequence builders had been formally a part of the Swift language, they existed as a proposed characteristic known as @_functionBuilder. This was the primary implementation from Apple that powered the @ViewBuilder syntax of SwiftUI. Initially, the anticipated official identify was @functionBuilder. Nonetheless, after revising the proposal (SE-0289), that identify turned @resultBuilder. Bear in mind that you just would possibly discover references to @functionBuilder and even @_functionBuilder in blogs and different sources.

Planning Your Cipher Builder

Now, the Egg Cipher isn’t precisely uncrackable. Again to the drafting board!

Any efficient cipher may have steps, or cipher guidelines, to carry out. Every rule applies an operation on the textual content and supplies a brand new consequence. Taking the key message as plain textual content, the cipher performs every rule sequentially till it yields the ultimate enciphered textual content.

A planning diagram showing two rules

In your cipher, every rule will take a String enter, modify it not directly and output a String consequence that’s handed to the next rule. Ultimately, the final rule will output the ultimate textual content. The deciphering course of would be the similar besides in reverse. Your CipherBuilder might want to assist any variety of guidelines and, ideally, share guidelines throughout cipher definitions so you may check completely different combos of ciphers.

As you’ll see, the quantity of code it’s good to implement the consequence builder is kind of small. Most of your time goes towards planning the kinds you’ll want on your DSL to make sense and be sensible.

Defining a Cipher Rule

First, it’s good to outline what a cipher rule is. Create a file known as CipherRule.swift and add:

protocol CipherRule {
  func encipher(_ worth: String) -> String
  func decipher(_ worth: String) -> String

There might be a number of rule sorts, so that you’ve properly opted for a protocol. Each encipher(_:) and decipher(_:) take a String and output a String. When enciphering a message, the plain textual content passes by way of every rule’s encipher(_:) operate to supply the cipher textual content; when deciphering, the cipher textual content passes by way of every rule’s decipher(_:) operate to supply the plain textual content.

Open CipherBuilder.swift. Replace buildBlock(_:) to make use of CipherRule as its sort.

static func buildBlock(_ elements: CipherRule...) -> CipherRule {

As a result of your agent coaching has raised your powers of commentary properly above common, you’ll have seen an issue: How can a various variety of CipherRule arguments be output as a single CipherRule? Can an array of CipherRule parts even be a CipherRule, you ask? Wonderful concept; make it so!

Add the next extension under the CipherRule protocol:

// 1
extension Array: CipherRule the place Factor == CipherRule {
  // 2
  func encipher(_ worth: String) -> String {
    // 3
    scale back(worth) { encipheredMessage, secret in

  func decipher(_ worth: String) -> String {
  // 4
    reversed().scale back(worth) { decipheredMessage, secret in
  1. You lengthen Array by implementing CipherRule when the Factor can be a CipherRule.
  2. You fulfill the CipherRule definition by implementing encipher(_:) and decipher(_:).
  3. You utilize scale back(_:_:) to cross the cumulative worth by way of every component, returning the results of encipher(_:).
  4. You reverse the order and use scale back(_:_:) once more, this time calling decipher(_:).

This code is the core of any cipher in Decoder Ring and implements the plan within the earlier diagram.

Don’t worry in regards to the compiler error, you’ll resolve it within the Constructing a Cipher part.

Writing the Guidelines

It’s time to write down your first rule: The LetterSubstitution rule. This rule will take a string and substitute every letter with one other letter based mostly on an offset worth. For instance, if the offset was three, then the letter “a” is changed by “d”, “b” is changed by “e”, “c” with “f” and so forth…

Create a file known as LetterSubstitution.swift and add:

struct LetterSubstitution: CipherRule {
  let letters: [String]
  let offset: Int

  // 1
  init(offset: Int) {
    self.letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".map(String.init)
    self.offset = max(1, min(offset, 25))
  // 2
  func swapLetters(_ worth: String, offset: Int) -> String {
    // 3
    let plainText = worth.map(String.init)
    // 4
    return plainText.scale back("") { message, letter in
      if let index = letters.firstIndex(of: letter.uppercased()) {
        let cipherOffset = (index + offset) % 26
        let cipherIndex = cipherOffset < 0 ? 26 
          + cipherOffset : cipherOffset
        let cipherLetter = letters[cipherIndex]
        return message + cipherLetter
      } else {
        return message + letter
  1. Your initializer creates an array of all of the upper-case letters and checks that the offset is between 1 and 25.
  2. You implement the core logic of the rule in swapLetters(_:offset:).
  3. You create an array of all of the letters within the message and assign it to the plainText variable.
  4. You loop by way of every letter in plainText and construct a consequence utilizing the suitable substitute letter decided by the offset. After all, you are cautious to verify that the offset of the substitute is legitimate.

Subsequent, you have to add the CipherRule capabilities wanted to satisfy the protocol. Add the next above swapLetters(_:offset:):

func encipher(_ worth: String) -> String {
  swapLetters(worth, offset: offset)

func decipher(_ worth: String) -> String {
  swapLetters(worth, offset: -offset)

Each required capabilities name swapLetters(_:offset:). Discover that decipher(_:) passes within the unfavourable offset to reverse the enciphered letters.

That is your first rule. Effectively accomplished, Agent.

Constructing a Cipher

Now, it is time to put your CipherBuilder to the check. The eggheads at HQ have an concept for one thing they name the Tremendous-secret-non-egg-related-so-really-uncrackable Cipher. That is fairly the mouthful, so how about simply making a file known as SuperSecretCipher.swift and including the next:

struct SuperSecretCipher {
  let offset: Int

  var cipherRule: CipherRule {
    LetterSubstitution(offset: offset)

SuperSecretCipher has an Int property for the letter offset plus a particular property: cipherRule. cipherRule is particular since you’ve added the @CipherBuilder annotation, similar to you probably did for buildEggCipherMessage(). This implies cipherRule is now a consequence builder. Contained in the physique of the consequence builder, you employ your new LetterSubstitution rule and the offset worth.

Open ContentView.swift. Take away onAppear(carry out:) and buildEggCipherMessage().

Substitute the physique of processMessage(_:) with the next:

let cipher = SuperSecretCipher(offset: 7)
change secretMode {
case .encode:
  return cipher.cipherRule.encipher(worth)
case .decode:
  return cipher.cipherRule.decipher(worth)

processMessage(_:) known as at any time when the message textual content adjustments or the change is toggled. SuperSecretCipher has an offset of 7, however that is configurable and finally as much as the eggheads. If the mode is .encipher, it calls encipher(_:) on cipherRule. In any other case, it calls decipher(_:).

Construct and run to see the results of all of your exhausting work.

The app running your first cipher

Keep in mind to strive the decipher mode.

The app deciphering a secret code

Increasing Syntax Assist

These eggheads from HQ have reviewed your work and requested adjustments (in fact, they’ve). They’ve requested you enable them to specify what number of instances to carry out the substitution, so it is “doubly, no Triply, no QUADRUPLY uncrackable”. Possibly they’ve cracked below the pressure! :]

Hop to it, Agent. You could be questioning, given your considerate implementation…is it even that arduous?

Open SuperSecretCipher.swift. Add the next property to SuperSecretCipher:

let cycles: Int

Substitute `cipherRule` with the next:

Now, that is the place issues begin to get much more attention-grabbing. Replace the physique of cipherBuilder like so:

for _ in 1...cycles {
  LetterSubstitution(offset: offset)

Open ContentView.swift. In ContentView, replace processMessage(_:) with the brand new argument. Substitute:

let cipher = SuperSecretCipher(offset: 7)


let cipher = SuperSecretCipher(offset: 7, cycles: 3)

Should you construct, you see a brand new error:

Xcode build error with a fix button

Not an issue. Open CipherBuilder.swift.

Should you’re feeling fortunate, strive that Repair button. In any other case, add the next methodology to CipherBuilder:

static func buildArray(_ elements: [CipherRule]) -> CipherRule {

That is one other a type of particular static capabilities you may add to any consequence builder. Since you’ve deliberate and ensured that any array of CipherRules can be a CipherRule, your implementation of this methodology is to easily return elements. Effectively accomplished, you!

Construct and run. Your app ought to triple-encipher the message:

The app triple-enciphering


Understanding Consequence Builder Loops

How does that loop work? Add a breakpoint inside each consequence builder capabilities (by clicking the road numbers). Construct and run.

Adding Xcode breakpoints in your result builder

While you sort a letter, you may see every step. Every time execution stops, click on the proceed button to leap to the following breakpoint till it is completed.

Debugging your result builder

You will discover that the compiler hits the buildBlock thrice, the buildArray as soon as, after which the buildBlock one final time. You may think about the compiler creating one thing like this:

// 1
let rule1: CipherRule = CipherBuilder.buildBlock(
  LetterSubstitution(offset: 7)
let rule2: CipherRule = CipherBuilder.buildBlock(
  LetterSubstitution(offset: 7)
let rule3: CipherRule = CipherBuilder.buildBlock(
  LetterSubstitution(offset: 7)
// 2
let rule4: CipherRule = CipherBuilder.buildArray(
  [rule1, rule2, rule3]
  1. That is the place you loop thrice. The consequence builder calls buildBlock(_:) every time to output a single rule. On this case, the rule is an occasion of LetterSubstitution.
  2. The consequence builder assembles these three guidelines right into a single array and calls buildArray(_:). As soon as once more, the result’s output as a single rule.
  3. Lastly, the consequence builder calls buildBlock(_:) once more to return that rule because the consequence.

You will by no means see this code wherever, however imagining what’s occurring internally whenever you plan a consequence builder is useful. It is all within the planning and your use of CipherRule as the first sort that is paid off handsomely. Good work, Agent.

Including Assist for Optionally available Values

Okay…so now these eggheads are scrambling to supply an excellent stronger cipher. They really feel it is unwise to permit official terminology to be output within the cipher textual content. In order that they wish to optionally provide a dictionary of official phrases and an obfuscated alternative. Like swapping “brains” for “Swiss cheese”, you muse.

It is time for an additional CipherRule!

Create a file known as ReplaceVocabulary.swift and add:

struct ReplaceVocabulary: CipherRule {
  // 1
  let phrases: [(original: String, replacement: String)]

  func encipher(_ worth: String) -> String {
    // 2
    phrases.scale back(worth) { encipheredMessage, time period in
        of: time period.unique, 
        with: time period.alternative, 
        choices: .caseInsensitive

  func decipher(_ worth: String) -> String {
    // 3
    phrases.scale back(worth) { decipheredMessage, time period in
        of: time period.alternative, 
        with: time period.unique, 
        choices: .caseInsensitive
  1. phrases is an array of tuples with two Strings every, matching the unique time period with its alternative.
  2. In encipher(_:), you loop by way of the array and carry out the replacements in a case-insensitive method.
  3. decipher(_:) does the identical however swaps all of the replacements with originals.

Open SuperSecretCipher.swift. Add this property to let the eggheads management the optionality:

let useVocabularyReplacement: Bool

It is a easy Bool that you just now want to make use of in cipherRule. Add the next earlier than the cycles loop:

if useVocabularyReplacement {
  ReplaceVocabulary(phrases: [
    ("MESSAGE", "MESS"),

The concept is that, for a message reminiscent of “the doomsday protocol is initiated”, your cipher will first substitute it with “the BLUEBERRY PIE LEMON GELATO is initiated” earlier than the letter substitution happens. It will certainly confound enemy spies!

Should you construct and run the app, you see a well-known construct error:

Another Xcode build error with a fix button

This time, open CipherBuilder.swift. Add the next methodology to CipherBuilder:

static func buildOptional(_ element: CipherRule?) -> CipherRule {
  element ?? []

That is how consequence builders deal with optionality, reminiscent of an if assertion. This one calls buildOptional(_:) with a CipherRule or nil, relying on the situation.

How can the fallback worth for CipherRule be []? That is the place you make the most of the Swift sort system. Since you prolonged Array to be a CipherRule when the component sort is CipherRule, you may return an empty array when element is nil. You may broaden that operate physique to specific these sorts explicitly:

let fallback: [CipherRule] = .init(arrayLiteral: [])
return element ?? fallback

However you are within the enterprise of permitting the compiler to simply do its factor. :]

In your consequence builder’s design, that vacant array is not going to have an effect on the consequence, which is exactly what you are in search of within the if useVocabularyReplacement expression. Fairly good, Agent. That is the kind of on-your-feet considering that’ll get HQ’s consideration…and perhaps that promotion?

Open ContentView.swift. Replace cipher inside processMessage(_:) to soak up the brand new useVocabularyReplacement parameter:

let cipher = SuperSecretCipher(
  offset: 7,
  cycles: 3,
  useVocabularyReplacement: true

Construct and run to see how your SuperSecretCipher performs.

Your final rule is working in the app

Excellent! The eggheads are lastly happy, and your presence is required at HQ. On to the following mission, Agent, and do not forget that consequence builders are at your disposal.

The place to Go From Right here?

You’ve got solely begun to discover the chances of consequence builders. You could find details about further capabilities within the documentation:

For inspiration, you would possibly wish to try Superior consequence builders, a set of consequence builders you could find on GitHub.

Should you’re in search of an additional problem, strive implementing assist for if { ... } else { ... } statements and different consequence builder logic. Or try this listing of historic ciphers at Sensible Cryptography and decide one to type a brand new CipherRule. You will discover a few acquainted entries in that listing. :]

I hope you loved this tutorial on consequence builders. You probably have any questions or feedback, please be part of the discussion board dialogue under.

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