TransactionsRepository.kt

class TransactionsRepository(
  private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {

  private val transactionsCache = mutableMapOf<User, List<Transaction>()

  private suspend fun addTransaction(user: User, transaction: Transaction) =
    // CAREFUL! Access to the cache is not protected.
    // Concurrency bugs can happen: threads can see stale data
    // and race conditions may occur.
    withContext(defaultDispatcher) {
      if (transactionsCache.contains(user)) {
        val oldList = transactionsCache[user]
        val newList = oldList!!.toMutableList()
        newList.add(transaction)
        transactionsCache.put(user, newList)
      } else {
        transactionsCache.put(user, listOf(transaction))
      }
    }
}