Modern Concurrency: Getting Started
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Part 1: Tasks
In the course materials, locate the starter playground and open it.
print("Doing some work on a task")
let sum = (1...100000).reduce(0, +)
print("1 + 2 + 3 ... 100000 = \(sum)")
print("Doing some work on the main actor")
Doing some work on a task
1 + 2 + 3 ... 100000 = 5000050000
Doing some work on the main actor
🟩Task {🟥
print("Doing some work on a task")
let sum = (1...100000).reduce(0, +)
print("1 + 2 + 3 ... 100000 = \(sum)")
🟩}🟥 // typing this re-indents the 3 lines
print("Doing some work on the main actor")
Doing some work on a task
Doing some work on the main actor
1 + 2 + 3 ... 100000 = 5000050000
Check if running on main actor
Comment out all this code, then scroll down to see some code that helps you check if code is running on the main actor:
let specificKey = DispatchSpecificKey<String>()
DispatchQueue.main.setSpecific(key: specificKey, value: "main")
if DispatchQueue.getSpecific(key: specificKey) == "main" {
print("\nPlayground runs on main actor")
}
Task {
print("\nDoing some work on a task")
if DispatchQueue.getSpecific(key: specificKey) == "main" {
print("Task runs on main actor")
} else {
print("Task doesn't run on main actor")
}
}
print("Doing some work on the main actor")
Playground runs on main actor
Doing some work on a task
Task doesn't run on main actor
Task {
print("\nDoing some work on an 🟩unnamed🟥 task")
let sum = (1...100000).reduce(0, +)
print("🟩Unnamed task done: 🟥1 + 2 + 3 ... 100000 = \(sum)")
}
print("Doing some work on the main actor")
🟩print("Doing more work on the main actor")🟥
// This task runs after previous task finishes
🟩let task = Task {
print("\nDoing some work on a named task")
let sum = (1...100000).reduce(0, +)
print("🟩Named🟥 task done: 1 + 2 + 3 ... 100000 = \(sum)")
}
print("Doing yet 🟩more🟥 work on the main actor")
🟥
Doing some work on an unnamed task
Doing some work on the main actor
Unnamed task done: 1 + 2 + 3 ... 100000 = 5000050000
Doing more work on the main actor
Doing some work on a named task
Doing yet more work on the main actor
Named task done: 1 + 2 + 3 ... 100000 = 5000050000
Part 2: async/await
Scroll down to the section titled Start here. You might’ve seen the sleep(_:) function before.
sleep(1)
print("wake up")
print("Hello")
Task.sleep(until: .now + .seconds(1), clock: .continuous)
// I'm using Xcode 14, which has this new Task.sleep method, so I can use seconds instead of nanoseconds.
print("Goodbye")
🟩Task {🟥
print("\nHello")
Task.sleep(until: .now + .seconds(1), clock: .continuous)
print("Goodbye")
🟩}🟥
Task {
print("\nHello")
🟩try🟥 Task.sleep(until: .now + .seconds(1), clock: .continuous)
print("Goodbye")
}
Task {
print("\nHello")
try 🟩await🟥 Task.sleep(until: .now + .seconds(1), clock: .continuous)
print("Goodbye")
}
Asynchronous functions
When you define an asynchronous function that can throw errors, you reverse the order of the keywords.
func helloPauseGoodbye() async throws {
}
func helloPauseGoodbye() async throws {
🟩
print("Hello function") // add " function"
try await Task.sleep(until: .now + .seconds(1), clock: .continuous)
print("Goodbye function") // add " function"
🟥
}
Task {
try await helloPauseGoodbye()
}
URLSession
Scroll up to the section titled URLSession. You’ll download and decode a list of learning domains from raywenderlich.com.
// all annotated public
struct Domains: Decodable {
let data: [Domain]
}
struct Domain: Decodable {
let attributes: Attributes
}
struct Attributes: Decodable {
let name: String
let description: String
let level: String
}
func fetchDomains() -> [Domain] {
let url = URL(string: "https://api.raywenderlich.com/api/domains")!
// and add a dummy return so the compiler doesn't complain
return []
}
Add async call: async throws / try await
Now, before the return statement, fetch the data:
func fetchDomains() -> [Domain] {
let url = URL(string: "https://api.raywenderlich.com/api/domains")!
🟩let (data, _) = try await URLSession.shared.data(from: url)🟥
// data(from:) is grayed out because the compiler doesn't think it should be used here
// select it anyway
return []
}
func fetchDomains() 🟩async 🟥 -> [Domain] {
func fetchDomains() async 🟩throws 🟥 -> [Domain] {
Handle data, response
Normally, you would first verify the server response, but skip that and just decode the data. Replace the dummy return [] line:
return try JSONDecoder().decode(Domains.self, from: data).data
Call async method from Task
fetchDomains() is an async function, so you need to call it in a Task:
Task {
let domains = try await fetchDomains()
}
Task {
🟩
do {
let domains = try await fetchDomains()
} catch {
print(error)
}
🟥
}
Task {
do {
let domains = try await fetchDomains()
🟩
for domain in domains {
let attr = domain.attributes
print("\(attr.name): \(attr.description) - \(attr.level)")
}
🟥
} catch {
print(error)
}
}
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