Function Builders

You can only type certain syntax when creating a view:

This is because SwiftUI uses something called a function builder to define an attribute struct called @ViewBuilder.

public protocol View {
    associatedtype Body : View
    @ViewBuilder var body: Self.Body { get }
}

That can convert a collection of views like this.

VStack {
    Text("Hi")
    Text("Hi")
}

Into a tuple of views that SwiftUI can use to render. It is really syntactical sugar that works like this:

An example

To make all this work we need to define a struct marked with @_functionBuilder (renamed @resultBuilder) and include a static method called buildBlock.

@resultBuilder
struct StringCharacterCounterBuilder {
  static func buildBlock(_ strings: String...) -> [Int] {
    return strings.map { $0.count }
  }
}

Then to use it we create a class with an initializer that takes an argument marked with our function builder:

import UIKit

@resultBuilder
struct StringCharacterCounterBuilder {
    static func buildBlock(_ strings: String...) -> [Int] {
        return strings.map { $0.count }
    }
}

class CharacterCounter {
    let counterArray: [Int]

    init(@StringCharacterCounterBuilder _ content: () -> [Int]) {
        counterArray = content()
    }

    func showCounts() {
        counterArray.forEach { print($0) }
    }
}

let characterCounts = CharacterCounter {
    "Andy"
    "Ibanez"
    "Collects Pullip"
}

characterCounts.showCounts()

Links that help

• Article1
• Article2