Implementing Currency Converter in Mobile Applications
A currency converter seems straightforward until you need to ensure exchange rates stay current, handle rounding correctly, and prevent UI crashes when users enter decimal values across different locales.
Exchange Rate Sources
Choosing an API depends on update frequency and currency coverage requirements:
| API | Free Plan | Update Frequency | Features |
|---|---|---|---|
| ExchangeRate-API | 1500 requests/month | Daily | Simple REST, 170+ currencies |
| Open Exchange Rates | 1000 requests/month | Hourly | Historical data |
| Fixer.io | 100 requests/month | Hourly | EUR as base currency |
| CBR XML | Free | Daily | Official rates for RUB |
| NBB | Free | Daily | Official rates for BYN |
For most applications, daily updates suffice. The Central Bank of Russia provides XML via https://www.cbr.ru/scripts/XML_daily.asp, parsed through XMLParser on iOS or XmlPullParser on Android.
Key Implementation Details
Caching. Store rates locally using Core Data or Room for offline functionality with the latest known values. Display a timestamp showing the last update so users understand data currency.
Arithmetic. Never use Float or Double for financial calculations. On iOS, use NSDecimalNumber or Decimal; on Android, use BigDecimal. Rounding differences in pennies become noticeable for large currency conversions.
Number Input. Users enter "1.5" or "1,5" depending on device locale. While NumberFormatter (iOS) and NumberFormat (Android) handle this, explicitly set locale and maximumFractionDigits. Without this, the decimal comma on German locale gets interpreted as a thousands separator.
Real-time Updates. When users enter amounts, recalculate on each character change using debounce(300ms) to avoid API spam during rapid input.
Timeline: 3-5 days including API integration, caching, and UI implementation.