Cancellation handling in Swift - Mini Project: Build & Apply

Cancellation Handling in Swift Async Tasks

📖 Scenario: Imagine you are building a simple Swift app that fetches user data from the internet. Sometimes, the user might want to cancel the fetch if it takes too long or they change their mind.

🎯 Goal: You will create a Swift async function that simulates fetching data, add a cancellation check, and then handle the cancellation properly.

📋 What You'll Learn

Create an async function called fetchUserData() that simulates a delay

Create a Task to run fetchUserData()

Add a cancellation check inside fetchUserData() using Task.isCancelled

Cancel the Task before it finishes

Print messages to show if the task completed or was cancelled

💡 Why This Matters

🌍 Real World

Handling cancellation is important in apps where users may want to stop long-running tasks like downloads or data fetching to save time and resources.

💼 Career

Understanding cancellation in async tasks is essential for Swift developers building responsive and user-friendly iOS or macOS apps.

1

Create the async function to simulate data fetching

Write an async function called fetchUserData() that waits for 5 seconds using Task.sleep and then prints "User data fetched".

Swift

# Write the async function fetchUserData() with a 5-second delay
# Your code here

Need a hint?

Use Task.sleep(nanoseconds: 5_000_000_000) to wait 5 seconds inside the async function.

2

Create a Task to run the async function

Create a Task called userTask that runs fetchUserData() asynchronously.

Swift

import Foundation

func fetchUserData() async {
    try? await Task.sleep(nanoseconds: 5_000_000_000)
    print("User data fetched")
}

// Create userTask to run fetchUserData()
// Your code here

Need a hint?

Use let userTask = Task { await fetchUserData() } to start the async task.

3

Add cancellation check inside the async function

Modify fetchUserData() to check Task.isCancelled after sleeping. If cancelled, print "Fetch cancelled" and return early.

Swift

import Foundation

func fetchUserData() async {
    try? await Task.sleep(nanoseconds: 5_000_000_000)
    // Check for cancellation here
    // Your code here
    print("User data fetched")
}

let userTask = Task {
    await fetchUserData()
}

Need a hint?

Use if Task.isCancelled { print("Fetch cancelled"); return } after the sleep.

4

Cancel the task and print the result

Call userTask.cancel() immediately after creating the task. Then wait 6 seconds using Task.sleep to allow the task to finish or cancel.

Swift

import Foundation

func fetchUserData() async {
    try? await Task.sleep(nanoseconds: 5_000_000_000)
    if Task.isCancelled {
        print("Fetch cancelled")
        return
    }
    print("User data fetched")
}

let userTask = Task {
    await fetchUserData()
}

// Cancel the task and wait 6 seconds
// Your code here

Need a hint?

Use userTask.cancel() to cancel and then Task.sleep(nanoseconds: 6_000_000_000) to wait.