enumerate() method for generic async array

Question

Hi I try to create enumerate() method for my custom async array, but stil have compilation error. My Custom class looks like that:

class AsyncArray<T> {

private var array = [T]()
private var queue = DispatchQueue(label: "async_queue", attributes: .concurrent)

    func enumerated() -> EnumeratedSequence<[T]> {
        var value = EnumeratedSequence<[T]>() // compilation error Cannot invoke initializer for type 'EnumeratedSequence<[T]>' with no arguments
        queue.sync {
            value = self.array.enumerated()
        }
        return value
    }
}

how can I implement enumerated() method to my custom class?

Answer 1

You're assigning to value only to immediately overwrite it in the sync call. There's no need.

DispatchQueue.sync lets you directly return a result:

import Dispatch

class AsyncArray<T> {
    private var array = [T]()
    private var queue = DispatchQueue(label: "async_queue", attributes: .concurrent)

    func enumerated() -> EnumeratedSequence<[T]> {
        return queue.sync {
            return self.array.enumerated()
        }
    }
}

For reference: If you needed to assign to a captured variable, as you tried to do (which would be necessary an async call, for example), it would look something like this:

import Dispatch

class AsyncArray<T> {
    private var array = [T]()
    private var queue = DispatchQueue(label: "async_queue", attributes: .concurrent)

    func enumerated() -> EnumeratedSequence<[T]> {
        var result: EnumeratedSequence<[T]>?
        return someAsyncCall {
            result = self.array.enumerated()
        }
        waitForAsyncCallToFinish()
        return result!
    }
}