Modern Concurrency: Getting Started

Oct 18 2022 · Swift 5.5, iOS 15, Xcode 13.4

Part 2: Asynchronous Sequences

09. Your Second Asynchronous App

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Previous episode: 08. Introduction Next episode: 10. Concurrency With async let

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For this part of the course, you’ll work on this cloud file storage app. Your first job is to download a list of files from the server and display it in this view. It’s very similar to how you downloaded and displayed the list of stock symbols for LittleJohn.

Non-async code in starter

In SuperStorageModel, here’s availableFiles():

func availableFiles() async throws -> [DownloadFile] {
  guard let url = URL(string: "http://localhost:8080/files/list") else {
    throw "Could not create the URL."
  }
  return []

Add async call: async throws / try await

Now, to fetch the data, I added this line, before the return statement:

let (data, response) = try await URLSession.shared.data(from: url)
🟥return []

Handle data, response

Next, to check the server response and return the fetched data, I replaced the dummy return statement:

let (data, response) = try await URLSession.shared.data(from: url)
🟩
guard (response as? HTTPURLResponse)?.statusCode == 200 else {
  throw "The server responded with an error."
}

// Just for a change, I'll throw my own error if `decode` fails...
guard let list = try? JSONDecoder()
  .decode([DownloadFile].self, from: data) else {
  throw "The server response was not recognized."
}
return list

Call async method from SwiftUI view

Next, I updated the SwiftUI view to use this new async method.

// TODO: Call model.availableFiles()
🟩
.task {
  guard files.isEmpty else { return }  // first check if I already fetched the file list
  do {
    files = try await model.availableFiles()  // if not, call `availableFiles()`
  } catch {
    lastErrorMessage = error.localizedDescription
  }
}