I have a method in my Kotlin app that looks like this:

coroutineScope{
      val aFetcher = async { a.fetch()}
      val bFetcher = async { b.fetch()}
      val cFetcher = async { c.fetch()}
      val dFetcher = async { d.fetch()}

      Merged(a.await(),b.await(),c.await(),d.await())
}      

The problem I am having is that I can't find a way to make one request depend on the other. In my case, I need for cFetcher to wait until bFetcher ended it's work before starting.

What's the right way to do that in Kotlin?

CodePudding user response：

Just make the part that should be synchronous, synchronous.

coroutineScope{
      val aFetcher = async { a.fetch() }
      val dFetcher = async { d.fetch() }
      val bResult = b.fetch()
      val cFetcher = async { c.fetch(bResult) }

      Merged(aFetcher.await(), bResult, cFetcher.await(), dFetcher.await())
}      

If you have more than one of these dependencies and want to run them in parallel you could do something like this I suppose:

coroutineScope{
      val aFetcher = async { a.fetch() }
      val dFetcher = async { d.fetch() }
      val bAndCFetcher = async {
        val bResult = b.fetch()
        bResult to c.fetch(bResult) 
      }

      Merged(aFetcher.await(), bAndCFetcher.await().first, bAndCFetcher.await().second, dFetcher.await())
}