@resultBuilder attribute in Swift - Step-by-Step Execution

Concept Flow - @resultBuilder attribute

Start Function Call

↓

Collect All Statements in Closure

↓

Call buildBlock with All Components

↓

buildBlock Joins Components

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Return Final Result

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Function Returns Built Result

The @resultBuilder attribute collects all statements in a closure and passes them as arguments to buildBlock in a single call, which combines them into one result.

Execution Sample

Swift

import Foundation

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

func makeSentence(@StringBuilder content: () -> String) -> String {
    "Sentence: " + content()
}

let sentence = makeSentence {
    "Hello"
    "world"
    "from"
    "@resultBuilder"
}
print(sentence)

This code uses @resultBuilder to pass all strings from the closure to buildBlock, which joins them into one string.

Execution Table

Step	Closure Statements	buildBlock Input	buildBlock Output	Combined Result
1	"Hello" "world" "from" "@resultBuilder"	["Hello", "world", "from", "@resultBuilder"]	"Hello, world, from, @resultBuilder"	"Hello, world, from, @resultBuilder"
2	Return from makeSentence	N/A	"Sentence: Hello, world, from, @resultBuilder"	"Sentence: Hello, world, from, @resultBuilder"

💡 All closure statements collected at once and passed to buildBlock, which combines them before the function returns.

Variable Tracker

Variable	Before Closure	After Collection	After buildBlock	Final
components	[]	["Hello", "world", "from", "@resultBuilder"]	["Hello", "world", "from", "@resultBuilder"]	["Hello", "world", "from", "@resultBuilder"]
content() result	""	""	"Hello, world, from, @resultBuilder"	"Hello, world, from, @resultBuilder"
sentence	""	N/A	N/A	"Sentence: Hello, world, from, @resultBuilder"

Key Moments - 3 Insights

How does @resultBuilder handle multiple statements in the closure?

@resultBuilder collects all statements and passes them directly to buildBlock as variadic arguments in one call, as shown in execution_table step 1.

What does the buildBlock function receive and return?

buildBlock receives an array of all String components from the closure and returns a single joined String.

Why is the output a single prefixed string?

makeSentence wraps the single result from the builder closure with 'Sentence: ', as in execution_table step 2.

Visual Quiz - 3 Questions

In the execution_table step 1, what is passed to buildBlock?

A ["Hello"]

B ["Hello", "world", "from", "@resultBuilder"]

C ["Hello", "world"]

D []

💡 Hint

Check the buildBlock Input column at step 1; all statements are collected at once.

How many times is buildBlock called for this closure?

A 4

B 2

C 1

D 0

💡 Hint

Review the execution_table; there's one call with all components.

What happens without a buildBlock method in the result builder?

A Compilation error; required to combine statements.

B Returns only the last statement.

C Returns an array automatically.

D No change; Swift joins strings.

💡 Hint

@resultBuilder relies on buildBlock (and others) to define how to combine.

Concept Snapshot

@resultBuilder attribute lets you write multiple statements in a closure
and combines them into one result using builder methods like buildBlock.
You define a struct/class with static buildBlock methods.
The closure's statements become inputs to buildBlock.
buildBlock returns the final combined value.
Used to create DSL-like syntax in Swift.

Full Transcript

The @resultBuilder attribute in Swift allows a function to accept a closure with multiple statements that get collected and passed to buildBlock in one call to produce a single result. In the example, all four strings are passed to buildBlock, which joins them. The execution table shows the single collection and combination step. The variable tracker tracks components from collection to final sentence. Key moments explain the all-at-once collection and buildBlock's role. Visual quizzes test grasp of the process. This creates clean, declarative syntax.