I often see code like this:
VStack {
Text("A")
Text("B")
}
From looking at the swift tutorials, I know that this is shorthand for specifying a closure as the last parameter to a function. However, afaik if the return keyword is not specified, a closure will implicitly return the result of the expression inside.
In this case, there are two Text objects. Are they returned as a tuple or a list? I know it works, but its unclear to me, what exactly is being returned from the closure.
The initial ability to omit the return statement was introduced in Swift Evolution 255 - This change only allowed for a single return value. In fact, the ability to return multiple values was considered in this proposal and discarded.
The structure you are asking about was originally introduced by Apple as a private implementation to support SwiftUI but has now been officially adopted in Swift Evolution 289 - Function Builders.
By referring to the introduction section of that document you can see that the values are, indeed, returned as a tuple.
// Original source code:
@TupleBuilder
func build() -> (Int, Int, Int) {
1
2
3
}
// This code is interpreted exactly as if it were this code:
func build() -> (Int, Int, Int) {
let _a = TupleBuilder.buildExpression(1)
let _b = TupleBuilder.buildExpression(2)
let _c = TupleBuilder.buildExpression(3)
return TupleBuilder.buildBlock(_a, _b, _c)
}
When we see this code:
VStack {
Text("A")
Text("B")
}
It is unclear what would be returned, unless we use VStack, then SwiftUI knows what we want, the code could be like this:
HStack {
Text("A")
Text("B")
}
Here also is unclear what we want, unless we say to SwiftUI that we want HStack, But if you use like this:
CustomView {
Text("A")
Text("B")
}
SwiftUI return it as VStack the inside content always! Also in all those cases that I said, Swift works with @ViewBuilder to make that happen. and they are tuple.
SwiftUI automatically returns the two Text’s in a type of TupleView. Swift’s resultBuilder is the magic, which was SE0289: https://github.com/apple/swift-evolution/blob/main/proposals/0289-result-builders.md
