Where is the String interpolation from "Structure your app for SwiftUI previews" documented?

Using Swift
1

(I tried to post in the Apple dev forums but they don't really work yet.)

I'd never seen this before:

Screen Shot 2020-06-26 at 11.00.46 PM
Screen Shot 2020-06-26 at 11.00.46 PM1542Ă—660 160 KB

How are you supposed to format it if you define the arguments locally?

Text(
  """
  \(
    { var nameComponents = PersonNameComponents()
      nameComponents.givenName = "Given"
      nameComponents.familyName = "Family"
      return nameComponents as NSPersonNameComponents
    } (),
    formatter: {
      let formatter = PersonNameComponentsFormatter()
      formatter.style = .long
      return formatter
    } ()
  )
  """
)
2

You mean ExpressibleByStringInterpolation? Most of the SwiftUI types use LocalizedStringKey which has its own StringInterpolation. You can consult StringInterpolationProtocol on how the transformation work.

TD;DR, LocalizedStringKey supports:

  • \(x as String)
  • \(x as T, specifier: y as String) where T is the curious _FormatSpecifiable :woman_shrugging:
  • \(x as Subject, formatter: y as Formatter?) where
    • Subject is NSObject, or
    • Subject is ReferenceConvertible.

And the new ones (beta):

  • \(x as Text)
  • \(x as Image)
  • \(x as ClosedRange<Date>)
  • \(x as DateInterval)
  • \(x as Date, style: y as Text.DateStyle)

You can also add your own if you'd like.

3 Likes
3

Thanks!

Can it be formatted without """ or a giant line with semicolons?

4

I'm pretty sure it works the same way with all string literals (Strings and Characters–String Literal). They only differ in how special characters, like \r\n\t, are handled*.

I don't think it'd work with semicolon, a; b would make them multi-statements, rather than an expression, which is what the interpolation needs (they're just normal functions).

* Since #"extended string#" also changes how \ is handled. You'd need to match the number of # in the beginning for \ to have effect, so you'd need \###(x) instead of regular \(x) if you use ###"..."###.

1 Like
5

It works. I don't know the reasoning. I just saw \() not working on one line so I added more "s (the above) and that works too. They just seem redundant because it's not a multi-line literal.

Text(
  "\( { var nameComponents = PersonNameComponents(); nameComponents.givenName = "Given"; nameComponents.familyName = "Family"; return nameComponents as NSPersonNameComponents } (), formatter: { let formatter = PersonNameComponentsFormatter(); formatter.style = .long; return formatter } () )"
)
6

Oh I see. That would work. What you’re essentially doing is creating a closure (bag of statements) and then call it, not too dissimilar to calling a function. So it’s an expression alright.

Because this:

{
  var nameComponents = PersonNameComponents()
  nameComponents.givenName = "Given"
  nameComponents.familyName = "Family"
  return nameComponents as NSPersonNameComponents
} ()

is just this:

// Defined somewhere else
func foo() -> NSPersonNameComponent {
  var nameComponents = PersonNameComponents()
  nameComponents.givenName = "Given"
  nameComponents.familyName = "Family"
  return nameComponents as NSPersonNameComponents
}

// simple function call
foo()

And since Swift can also use semicolon to separate statements (in addition to new line), this:

let formatter = PersonNameComponentsFormatter()
formatter.style = .long
return formatter

is the same as this

let formatter = PersonNameComponentsFormatter(); formatter.style = .long; return formatter
7

The specifier argument takes a printf-style format string. Example:

Text("\(Double.pi, specifier: "%.4f")")

produces the text "3.1416", and

Text("0x\(255, specifier: "%llX")")

produces "0xFF".

SwiftUI uses the _FormatSpecifiable protocol to make this interpolation available to the standard number types. The integer and floating-point types from the stdlib conform to it, plus CGFloat. (Float16 doesn't in Xcode 12 beta 1 – I don't know if that's an oversight.)