Custom result builder declaration in Swift - Mini Project: Build & Apply

Custom Result Builder Declaration in Swift

📖 Scenario: Imagine you want to create a simple way to build a list of messages using a special syntax in Swift. This will help you write cleaner code when combining multiple messages.

🎯 Goal: You will create a custom result builder called MessageBuilder that combines multiple String messages into one array. Then, you will use it to build a list of messages easily.

📋 What You'll Learn

Create a custom result builder named MessageBuilder

Implement the required static function buildBlock that takes multiple String parameters and returns an array of String

Create a function buildMessages that uses the @MessageBuilder attribute

Use the buildMessages function to combine messages

Print the combined messages array

💡 Why This Matters

🌍 Real World

Custom result builders help write clean and readable code when combining multiple pieces of data, such as building UI views, SQL queries, or configuration lists.

💼 Career

Understanding result builders is useful for Swift developers working on SwiftUI, DSLs, or any code that benefits from declarative syntax.

1

Create the MessageBuilder result builder

Create a custom result builder called MessageBuilder using @resultBuilder. Inside it, write a static function buildBlock that takes a variable number of String parameters named components and returns an array of String containing those components.

Swift

# Create the @resultBuilder struct MessageBuilder with static func buildBlock
# Your code here

Need a hint?

Use @resultBuilder before the struct declaration. The buildBlock function should accept String... and return [String].

2

Create the buildMessages function using @MessageBuilder

Create a function called buildMessages that uses the @MessageBuilder attribute. This function should return an array of String. Inside the function, write multiple String messages on separate lines: "Hello", "Welcome", and "Goodbye".

Swift

@resultBuilder
struct MessageBuilder {
    static func buildBlock(_ components: String...) -> [String] {
        return components
    }
}

// Create buildMessages function with @MessageBuilder
// Your code here

Need a hint?

Use @MessageBuilder before the function. Inside the function, just write the strings on separate lines without commas.

3

Call buildMessages and store the result

Call the function buildMessages() and store its result in a variable called messages.

Swift

@resultBuilder
struct MessageBuilder {
    static func buildBlock(_ components: String...) -> [String] {
        return components
    }
}

@MessageBuilder
func buildMessages() -> [String] {
    "Hello"
    "Welcome"
    "Goodbye"
}

// Call buildMessages and store in messages
// Your code here

Need a hint?

Simply assign the call buildMessages() to messages.

4

Print the combined messages array

Print the variable messages to display the combined array of messages.

Swift

@resultBuilder
struct MessageBuilder {
    static func buildBlock(_ components: String...) -> [String] {
        return components
    }
}

@MessageBuilder
func buildMessages() -> [String] {
    "Hello"
    "Welcome"
    "Goodbye"
}

let messages = buildMessages()

// Print messages
// Your code here

Need a hint?

Use print(messages) to show the array.