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As it stands, neither Double.round, NumberFormatter, nor NSFormat have anything to do with SwiftUI. The first is part of the Swift standard library, and the latter two are part of the Foundation framework that's been around for about 30 years now.
As well as FedyaninT's suggestion of the specifier: argument within LocalizedStringKey's string interpolation implementation, NumberFormatter would also work when configured appropriately. Note that Double.round() is a mathematical function; it's used to round floating-point values to the closest integer value, and thus doesn't have anything to do with what you need.
NumberFormatter has a vast array of options, and will let you format integer and real values in almost every way defined as part of the Unicode and CLDR standard (the only exception I'm aware of is 'shortened' formats, e.g. "10 M" for "10000000"). To round a value to two decimal places you need to set a numberStyle of .decimal and set both .minimumFractionDigits and .maximumFractionDigits to '2'.
From memory, this is what you'd need:
static let formatter: NumberFormatter = {
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.minimumFractionDigits = 2
formatter.maximumFractionDigits = 2
return formatter
}()
...
let value = 123.456
Text("Your value is: \(value, formatter: formatter)") // "Your value is: 123.45"
Alternatively, if you're trimming to two decimal places in order to print a value in some currency, I would advise using NumberFormatter's .currency style instead:
static let formatter: NumberFormatter = {
let formatter = NumberFormatter()
formatter.numberStyle = .currency
return formatter
}()
...
let value = 123.456
Text("Your value is: \(value, formatter: formatter)")
// "Your value is: $123.45" (or 123.45 €, if you're using the FR_fr locale)
