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

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

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

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

Hint

Use print(messages) to show the array.

Practice

1. What is the main purpose of declaring a @resultBuilder in Swift?

easy

A. To handle errors automatically in functions

B. To create a new data type like a class or struct

C. To optimize memory usage during runtime

D. To define how multiple code parts combine into a single result

Solution

1. Step 1: Understand the role of @resultBuilder

   A @resultBuilder is used to customize how multiple code blocks combine into one result, making code cleaner.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated concepts like error handling, data types, and optimization, which are not the purpose of result builders.

3. Final Answer:

   To define how multiple code parts combine into a single result -> Option D

4. Quick Check:

   Result builder = combine code parts [OK]

Hint: Result builders combine code parts into one result [OK]

Common Mistakes:

• Confusing result builders with classes or structs
• Thinking result builders handle errors
• Assuming result builders optimize memory

2. Which of the following is the correct way to start declaring a custom result builder in Swift?

easy

A. @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } }

B. @resultBuilder class MyBuilder { func buildBlock() {} }

C. class MyBuilder { func buildBlock() -> Int { 0 } }

D. struct MyBuilder { static func buildBlock() {} }

Solution

1. Step 1: Identify correct syntax for result builder

   A custom result builder must be declared with @resultBuilder attribute on a struct, and have static methods like buildBlock returning a value.

2. Step 2: Check each option

   @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } } correctly uses @resultBuilder, declares a struct, and has a static buildBlock method returning a String. The other options miss the attribute, use class instead of struct, or have non-static buildBlock.

3. Final Answer:

   @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } } -> Option A

4. Quick Check:

   Result builder = @resultBuilder + struct + static methods [OK]

Hint: Use @resultBuilder on a struct with static buildBlock method [OK]

Common Mistakes:

• Using class instead of struct for result builder
• Forgetting the @resultBuilder attribute
• Making buildBlock non-static

3. Given this custom result builder and usage, what is the output?

 @resultBuilder
struct IntSumBuilder {
    static func buildBlock(_ numbers: Int...) -> Int {
        numbers.reduce(0, +)
    }
}

func sum(@IntSumBuilder _ content: () -> Int) -> Int {
    content()
}

let result = sum {
    5
    10
    3
}
print(result)

medium

A. 5103

B. 18

C. Error: buildBlock requires array

D. 0

Solution

1. Step 1: Understand buildBlock behavior

   The buildBlock method takes multiple Int values and sums them using reduce(0, +).

2. Step 2: Evaluate the sum call

   The closure passed to sum returns three Int values: 5, 10, and 3. The builder sums them: 5 + 10 + 3 = 18.

3. Final Answer:

   18 -> Option B

4. Quick Check:

   Sum of 5, 10, 3 = 18 [OK]

Hint: buildBlock sums all Int inputs passed in closure [OK]

Common Mistakes:

• Thinking values concatenate as strings
• Expecting buildBlock to receive an array instead of variadic
• Confusing output with zero or error

4. Identify the error in this custom result builder declaration:

 @resultBuilder
struct TextBuilder {
    func buildBlock(_ components: String...) -> String {
        components.joined(separator: ", ")
    }
}

medium

A. buildBlock must be static

B. buildBlock must return Int

C. struct cannot be used with @resultBuilder

D. buildBlock cannot take variadic parameters

Solution

1. Step 1: Check method requirements in result builder

   Methods like buildBlock inside a @resultBuilder must be declared as static so they can be called without an instance.

2. Step 2: Analyze the given code

   The buildBlock method is missing the static keyword, which causes a compile error.

3. Final Answer:

   buildBlock must be static -> Option A

4. Quick Check:

   buildBlock method = static [OK]

Hint: buildBlock must always be static in result builders [OK]

Common Mistakes:

• Omitting static keyword on buildBlock
• Thinking buildBlock must return Int always
• Believing struct can't be used with @resultBuilder

5. You want to create a custom result builder that builds a dictionary from key-value pairs written inside a closure. Which of the following buildBlock implementations correctly combines multiple (String, Int) tuples into a single dictionary?

hard

A. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { pairs.map { [$0.0: $0.1] } }

B. static func buildBlock(_ pairs: [(String, Int)]) -> [String: Int] { pairs.reduce([:]) { $0 + $1 } }

C. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) }

D. static func buildBlock(_ pairs: (String, Int)...) -> [(String, Int)] { pairs }

Solution

1. Step 1: Understand the goal

   The builder should combine multiple key-value pairs into a single dictionary.

2. Step 2: Analyze each option

   static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) } uses Dictionary(uniqueKeysWithValues:) which correctly creates a dictionary from tuples. static func buildBlock(_ pairs: [(String, Int)]) -> [String: Int] { pairs.reduce([:]) { $0 + $1 } } tries to reduce with + which is invalid for dictionaries. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { pairs.map { [$0.0: $0.1] } } maps to an array of dictionaries, not a single dictionary. static func buildBlock(_ pairs: (String, Int)...) -> [(String, Int)] { pairs } returns an array of tuples, not a dictionary.

3. Final Answer:

   static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) } -> Option C

4. Quick Check:

   Use Dictionary(uniqueKeysWithValues:) to combine tuples [OK]

Hint: Use Dictionary(uniqueKeysWithValues:) to build dictionary from tuples [OK]

Common Mistakes:

• Returning array of dictionaries instead of one dictionary
• Using invalid + operator to combine dictionaries
• Returning array of tuples instead of dictionary