A free currency converter API is easy to call from a mobile app. Doing it well takes three more things: the app must work offline with the last known rates, it must tell the user how old those rates are, and the API key must not be inside the app binary. This guide covers all three, with working code for iOS in Swift and Android in Kotlin.

The examples use the AllRatesToday rates endpoint, which returns mid-market rates for 160+ currencies, refreshed every 60 seconds.

The architecture: app, proxy, API

An iOS or Android app is a file the user downloads. Anything compiled into it, including an API key in Info.plist , BuildConfig or an obfuscated string, can be extracted with free tools in minutes.…