Why async/await simplifies concurrent code in iOS Swift - Build It to Prove It

Build: AsyncAwaitDemo

This screen shows how async/await makes running tasks one after another easier to read and understand compared to old callback style.

Target UI

-------------------------
| Async/Await Demo      |
|-----------------------|
| [Start Tasks]         |
|                       |
| Result:               |
|                       |
|                       |
-------------------------

Add a button labeled 'Start Tasks' that triggers two asynchronous tasks sequentially.

Each task waits 2 seconds and returns a string result.

Display the combined result of both tasks in the Result area after both complete.

Use async/await syntax to run tasks sequentially.

Show a loading indicator while tasks run.

Starter Code

iOS Swift

import SwiftUI

struct AsyncAwaitDemo: View {
    @State private var resultText = ""
    @State private var isLoading = false

    var body: some View {
        VStack(spacing: 20) {
            Text("Async/Await Demo")
                .font(.title)

            Button("Start Tasks") {
                // TODO: Call async function to run tasks
            }
            .padding()
            .disabled(isLoading)

            if isLoading {
                ProgressView()
            }

            Text("Result: \n" + resultText)
                .padding()
                .multilineTextAlignment(.center)

            Spacer()
        }
        .padding()
    }

    // TODO: Add async functions and task runner here
}

struct AsyncAwaitDemo_Previews: PreviewProvider {
    static var previews: some View {
        AsyncAwaitDemo()
    }
}

Task 1

Task 2

Task 3

Task 4

Solution

iOS Swift

import SwiftUI

struct AsyncAwaitDemo: View {
    @State private var resultText = ""
    @State private var isLoading = false

    var body: some View {
        VStack(spacing: 20) {
            Text("Async/Await Demo")
                .font(.title)

            Button("Start Tasks") {
                Task {
                    await runTasksSequentially()
                }
            }
            .padding()
            .disabled(isLoading)

            if isLoading {
                ProgressView()
            }

            Text("Result: \n" + resultText)
                .padding()
                .multilineTextAlignment(.center)

            Spacer()
        }
        .padding()
    }

    func taskOne() async -> String {
        try? await Task.sleep(nanoseconds: 2_000_000_000) // 2 seconds
        return "Task One Done"
    }

    func taskTwo() async -> String {
        try? await Task.sleep(nanoseconds: 2_000_000_000) // 2 seconds
        return "Task Two Done"
    }

    func runTasksSequentially() async {
        isLoading = true
        let firstResult = await taskOne()
        let secondResult = await taskTwo()
        resultText = "\(firstResult) and \(secondResult)"
        isLoading = false
    }
}

struct AsyncAwaitDemo_Previews: PreviewProvider {
    static var previews: some View {
        AsyncAwaitDemo()
    }
}

This example uses Swift's async/await to run two tasks one after another. Each task waits 2 seconds and returns a string. The runTasksSequentially function awaits each task in order, making the code easy to read like normal sequential code.

The button triggers the async function using Task { await ... }. While tasks run, a loading spinner shows and the button disables to prevent multiple taps.

This approach is simpler and cleaner than nested callbacks or completion handlers. It reads like normal code but runs asynchronously without blocking the UI.

Final Result

Completed Screen

-------------------------
| Async/Await Demo      |
|-----------------------|
| [Start Tasks]         |
|                       |
| Loading...            |
|                       |
| Result:               |
| Task One Done and     |
| Task Two Done         |
-------------------------

User taps 'Start Tasks' button.

Loading spinner appears and button disables.

After 4 seconds total, result text updates to 'Task One Done and Task Two Done'.

Loading spinner disappears and button re-enables.

Stretch Goal

Add a third async task that runs concurrently with the second task and combine all results.

💡 Hint

Use async let to start tasks concurrently and await their results together.

Practice

1. What is the main benefit of using async/await in Swift for concurrent tasks?

easy

A. It replaces all synchronous code with asynchronous code.

B. It automatically speeds up the app without any code changes.

C. It prevents any errors from happening in the code.

D. It makes asynchronous code easier to read and write.

Solution

1. Step 1: Understand async/await purpose

   Async/await helps write asynchronous code that looks like normal, simple code.

2. Step 2: Compare benefits

   It does not automatically speed up or replace all code, nor prevent all errors, but it improves readability and flow.

3. Final Answer:

   It makes asynchronous code easier to read and write. -> Option D

4. Quick Check:

   Async/await improves code readability = C [OK]

Hint: Async/await makes async code look like normal code [OK]

Common Mistakes:

• Thinking async/await automatically speeds up code
• Believing async/await removes all errors
• Confusing async/await with synchronous code

2. Which of the following is the correct syntax to declare an asynchronous function in Swift?

easy

A. async func fetchData() throws -> String

B. func fetchData() async throws -> String

C. func async fetchData() throws -> String

D. func fetchData() throws async -> String

Solution

1. Step 1: Recall async function syntax

   In Swift, the async keyword comes after the function parentheses.

2. Step 2: Check option syntax

   func fetchData() async throws -> String correctly places async after parentheses and before throws.

3. Final Answer:

   func fetchData() async throws -> String -> Option B

4. Quick Check:

   Async keyword after parentheses = A [OK]

Hint: Remember async goes right after function parentheses [OK]

Common Mistakes:

• Placing async before func keyword
• Putting async after throws keyword
• Mixing order of async and throws

3. What will be printed by this Swift code using async/await?

func fetchNumber() async -> Int {
  return 5
}

Task {
  let number = await fetchNumber()
  print(number)
}

medium

A. 5

B. Optional(5)

C. Task

D. No output

Solution

1. Step 1: Understand async function return

   The function fetchNumber() returns an Int value 5 asynchronously.

2. Step 2: Await and print result

   The await keyword waits for the result, then print(number) outputs 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Await fetch returns 5 printed = D [OK]

Hint: Await unwraps async result before printing [OK]

Common Mistakes:

• Expecting Optional(5) instead of 5
• Thinking Task prints itself
• Assuming no output without await

4. Identify the error in this Swift async/await code:

func loadData() async -> String {
  return "Data"
}

let result = await loadData()
print(result)

medium

A. Cannot use await outside an async context

B. Missing return type in function

C. Function should not be async

D. Await keyword is misspelled

Solution

1. Step 1: Check await usage context

   In Swift, await must be called inside an async function or async context.

2. Step 2: Analyze code structure

   The code calls await loadData() at top-level, which is not allowed.

3. Final Answer:

   Cannot use await outside an async context -> Option A

4. Quick Check:

   Await requires async context = A [OK]

Hint: Await only works inside async functions [OK]

Common Mistakes:

• Calling await in synchronous code
• Ignoring async context requirement
• Assuming await works anywhere

5. You want to fetch user info and user posts concurrently using async/await in Swift. Which code correctly runs both tasks at the same time and waits for both results?

hard

A. let user = await fetchUser()
let posts = await fetchPosts()

B. fetchUser()
fetchPosts()
print("Done")

C. async let user = fetchUser()
async let posts = fetchPosts()
let userInfo = await user
let userPosts = await posts

D. Task {
let user = fetchUser()
let posts = fetchPosts()
print(user, posts)
}

Solution

1. Step 1: Understand concurrent async calls

   Using async let starts tasks concurrently without waiting immediately.

2. Step 2: Await both results

   Awaiting user and posts later waits for both to finish, running in parallel.

3. Final Answer:

   async let user = fetchUser() async let posts = fetchPosts() let userInfo = await user let userPosts = await posts -> Option C

4. Quick Check:

   async let runs tasks concurrently = B [OK]

Hint: Use async let to run tasks in parallel [OK]

Common Mistakes:

• Awaiting tasks one after another (sequential)
• Not awaiting async let variables
• Calling async functions without await