CODEWITHSUNDEEP
swiftasynchronousswift-playgroundswift-concurrency
Patrick Bauers
Patrick Bauers

Reputation: 123

Swift Playground and async function

I try to learn something about asynchronous functions and I'm facing a nasty problem with Playground. I have a simple program to download some data, but unfortunately it crashes and all I get are these error messages:

error: execution stopped with unexpected state.
error: Execution was interrupted.
The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation.

The source looks like this:

import Foundation
import PlaygroundSupport

func fetchNews() async -> Data? {
    do {
        let url = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!
        let (data, _) = try await URLSession.shared.data(from: url)
        return data
    } catch {
        print("Failed to fetch data")
        return nil
    }
}

PlaygroundPage.current.needsIndefiniteExecution = true
if let data = await fetchNews() {
    print("Downloaded \(data.count) bytes")
} else {
    print("Download failed.")
}

I thought that the line PlaygroundPage.current.needsIndefiniteExecution = true would stop playground from exiting.

But I'm pretty sure I'm missing a very simple thing.

Upvotes: 0

Views: 154

Answers (2)

user3441734
user3441734

Reputation: 17534

Add

PlaygroundPage.current.finishExecution()

to the end of your Playground page. (Xcode 16)

Upvotes: 0

matt
matt

Reputation: 534885

You cannot just say await in the middle of nowhere; you have to be in an "async context", either an async function or a Task. Just wrap your call in a Task:

func fetchNews() async -> Data? {
    do {
        let url = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!
        let (data, _) = try await URLSession.shared.data(from: url)
        return data
    } catch {
        print("Failed to fetch data")
        return nil
    }
}

PlaygroundPage.current.needsIndefiniteExecution = true
Task {
    if let data = await fetchNews() {
        print("Downloaded \(data.count) bytes")
    } else {
        print("Download failed.")
    }
}

Result: Downloaded 83 bytes

Upvotes: 2

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