Getting all values for [EventLoopFuture] before proceeding

Question

I have an array of values I map to multiple promises that give me each a EventLoopFuture. So I end up with a method that has a variable-size [EventLoopFuture], and I need all the responses to succeed before I can continue. If one or more of them returns an error, I need to perform the error scenario.

How can I await the entire [EventLoopFuture] to complete before continuing with either the succeed path or error path?

Answer 1

EventLoopFuture's got a reduce(into: ...) method that can be used quite well for that purpose (and other tasks where you want to accumulate multiple values):

let futureOfStrings: EventLoopFuture<[String]> =
    EventLoopFuture<String>.reduce(into: Array<String>(),
                                   futures: myArrayFutureStrings,
                                   on: someEventLoop,
                                   { array, nextValue in array.append(nextValue) })   

To specifically turn [EventLoopFuture<Something>] into EventLoopFuture<[Something]> you can also use the shorter whenAllSucceed

let futureOfStrings: EventLoopFuture<[String]> =
    EventLoopFuture<String>.whenAllSucceed(myStringFutures, on: someEventLoop)