# appendInterpolation(_:formatter:) Appends an optionally-formatted instance of an Objective-C subclass to a string interpolation. ``` mutating func appendInterpolation(_ subject: Subject, formatter: Formatter? = nil) where Subject : NSObject ``` ## Parameters `subject` An to append. `formatter` A formatter to convert `subject` to a string representation. ## Discussion Don’t call this method directly; it’s used by the compiler when interpreting string interpolations. The following example shows how to use a value and a to create a [`LocalizedStringKey`](/documentation/SwiftUI/LocalizedStringKey) that uses the formatter style when generating the measurement’s string representation. Rather than calling `appendInterpolation(_:formatter)` directly, the code gets the formatting behavior implicitly by using the `\()` string interpolation syntax. ``` let siResistance = Measurement(value: 640, unit: UnitElectricResistance.ohms) let formatter = MeasurementFormatter() formatter.unitStyle = .long let key = LocalizedStringKey ("Resistance: \(siResistance, formatter: formatter)") let text1 = Text(key) // Text contains "Resistance: 640 ohms" ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)