Issues with custom string interpolation in SwiftUI Text Views

Using Swift
,
1

I created a custom string interpolation function that allows me to pass in a string format specifier. I've noticed that, for whatever reason it does not work inside of a SwiftUI Text view unless I use the init(verbatim:) initializer. Consider this code:

import SwiftUI

struct ContentView: View {
    
    let price = 5.99
    var body: some View {
        Text("The price is $\(price, format: "%.2f")")
    }
}

extension String.StringInterpolation {

    mutating func appendInterpolation(_ value: Double, format: String) {
        appendInterpolation(String(format: format, value))
    }

}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

I get the error
Incorrect argument label in call (have '_:format:', expected '_:specifier:')
It seems that there's some other interpolation function using '_:specifier:', but I want to use my own custom function.

If I change
Text("The price is $\(price, format: "%.2f")")
to
Text(verbatim: "The price is $\(price, format: "%.2f")")
then it works fine. Why?

How can I implement the interpolation function in such a way that it works inside Text views?

2

Text.init uses LocalizedStringKey, so you need to extend the interpolation there.

Also, I’d try to avoid using a common structure like \(double, format: format) in case SwiftUI decides to add it unless it’s intentional, maybe add some prefix?

3

@Lantua Is there anyway to extend both String and LocalizedStringKey using a protocol extension? I'd prefer not to duplicate code.

1 Like
4

If this is all you want, LocalizedStringKey can do it already, just do:

Text("The price is $\(price, specifier: "%.2f")")

5

@young But what if I want to make custom interpolation functions that work with both LocalizedStringKey and normal strings? How do I avoid writing a separate interpolation function for each?

6

The Text("abcd string literal") initializor is meant for localization as it do lookup of Localizable.strings. You might be able to extend it, but it worn't do lookup of your extension, I don't think.

If you just want your own interpolation on String, use:

Text(verbatim: "The price is $(price, format: "%.2f")")

This one takes a String so your custom interpolation should just work, but then you cannot localize it.

7

I don't care about lookups, but I also don't want to have to type the extra parameter verbatim every time I want to do string interpolations. I've discovered how to write a extension for localized strings and for regular strings, but I would like to know if I can write just a single function in a protocol extension so that I don't have to duplicate code. Is this possible? Currently, I have to do this. Notice that the functions are exactly the same.

public extension LocalizedStringKey.StringInterpolation {
    mutating func appendInterpolation(_ value: Double, format: String) {
        appendInterpolation(String(format: format, value))
    }
}

public extension String.StringInterpolation {
    mutating func appendInterpolation(_ value: Double, format: String) {
        appendInterpolation(String(format: format, value))
    }
}
8

In this case, then it makes no sense to extend LocalizedStringKey.StringInterpolation

Text(_:)
Text(verbatim:)

are two different functions, you have to type it out so the compiler can tell what you want. It can't read you mind.

9

Making a new protocol with the correct required method and extension, and then extending LocalizedStringKeys/String.StringInterpolation with this new protocol seems to work for me:

protocol Formattable {
    mutating func appendInterpolation(_: String)
}

extension Formattable {
    mutating func appendInterpolation(_ value: Double, format: String) {
        appendInterpolation(String(format: format, value))
    }
}

extension LocalizedStringKey.StringInterpolation: Formattable {}
extension String.StringInterpolation: Formattable {}
2 Likes
10

I should've been more clear about this. What I meant was that I don't want to have to type Text(verbatim: "abc") everytime I want to do a string interpolation. I want to be able to just type Text("abc") and be able to use the same string interpolation functions as if I had just used a normal string.

@odmir has provided the exact answer that I'm looking for. Thanks Odmir!

Don't forget about how powerful protocol extensions are.

1 Like