Kotlin coroutine concurrency and cached value

Question

I'm currently trying to create a caching layer around a web request. So far I've written:

class Repository(private val webServices: WebServices) {
  private var cachedItems: List<Item>? = null

  suspend fun getItems(): List<Item> {
    cachedItems?.let { return it }

    val items = withContext(Dispatchers.IO) { webServices.getItems() }
    cachedItems = items
    return items
  }
}

My concern is what will happen when getItems() is invoked by two callers simultaneously. Ideally, I'd only want one web request to occur. What's the recommended approach for dealing with this issue when using coroutines?

Answer 1

Here's a simple solution.

class Repository(private val webServices: WebServices) {
  private val cachedItems = async(Dispatchers.IO, start = CoroutineStart.LAZY) {
    webServices.getItems()
  }

  suspend fun getItems(): List<Item> {
    return cachedItems.await()
  }
}