Several tasks inside a DispatchGroup. Will they run in order?

Question

In the following code, is it safe to append to an array? Is the order guaranteed to be maintained?

let processedData: [SomeType] = []
let dispatchGroup = DispatchGroup()
for _ in 0..<N {
    dispatchGroup.enter()
    startSomeAsyncTaskXYZ { (data, error) in
        // handle error and process data
        // add processed data to an array
        processedData.append(..)
        dispatchGroup.leave()
    }
}
dispatchGroup.notify(queue: .main) {
    // update UI
}

Answer 1

To stick with DispatchGroup while preserving the desired asynchronous nature and the expected ordering, make your array an array of optionals and populate it in whatever order the tasks complete:

var processedData: [SomeType?] = Array(repeating: nil, count: N)
let dispatchGroup = DispatchGroup()
for idx in 0..<N {
    dispatchGroup.enter()
    startSomeAsyncTaskXYZ { (data, error) in
        // Ensure we always .leave() after we're done
        // handling the completion of the task
        defer { dispatchGroup.leave() }

        guard let data = data,
              error == nil else {
            // TODO: Actual error handling
            return
        }

        // This needs to be .sync now (not .async) to ensure
        // the deferred dispatchGroup.leave() is not called
        // until *after* we've updated the array
        DispatchQueue.main.sync {
            processedData[idx] = SomeType(data: data)
        }
    }
}
dispatchGroup.notify(queue: .main) {
    // update UI
}