Modern Concurrency: Getting Started

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

Part 1: Asynchronous Code

03. Your First Asynchronous App

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Previous episode: 02. Getting Started Next episode: 04. Asynchronous Sequences

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Run the course server

Most of the projects in this course interact with a server. It’s included in the course materials.

LittleJohn app

For the rest of this part of the course, you’ll work on this simple ticker app. In this episode, you’ll fetch a list of stock symbols from the course server.

Fetch stock symbols

In your browser, reload the server page to check it’s still running.

Non-async code in starter

In LittleJohnModel, locate availableSymbols():

func availableSymbols() async throws -> [String] {
  guard let url = URL(string: "http://localhost:8080/littlejohn/symbols")
  else {
    throw "The URL could not be created."
  }
  return []
}

Add async call: async throws / try await

The async keyword indicates this function contains a suspension point, which you’ll add now… just before the return statement:

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

Handle data, response

Next, you need to verify the server response and return the fetched data. Replace the dummy return line with this code:

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

Call async method from SwiftUI view

Before you run the app, you need to update the SwiftUI view to use your new async method.

.padding(.horizontal)
// TODO: Call model.availableSymbols()
🟩
.onAppear {
  symbols = try await model.availableSymbols() 
}

Invalid conversion from 'async' function of type '() async throws -> ()' to synchronous function type '() -> Void'

.🟩task🟥 {
  symbols = try await model.availableSymbols()
}

.task {
  do {
    symbols = try await model.availableSymbols()
  } catch  {

  }
}

.task {
  🟩guard symbols.isEmpty else { return }🟥
  do {

} catch  {
  🟩lastErrorMessage = error.localizedDescription🟥
}

Test error handling

How about testing whether SymbolListView catches an error thrown by availableSymbols()?