Async and await: suspension is not blocking

A report may need data from an operation that finishes later. An async function can suspend while waiting; await marks a possible suspension point. This allows other work to progress, but it does not promise a new thread, a particular scheduling order or parallel execution.

An async call returns its result when the awaited operation completes. The example uses an immediate async function to isolate the syntax, not a network request. Real fetching also needs errors, cancellation and appropriate isolation for shared state.

Save this complete program as main.swift and run it with the Swift toolchain. Use a toolchain supporting the demonstrated async top-level code.

Swift · complete program

func loadScore() async -> Int { 80 }
print("Before request")
let score = await loadScore()
print("Score:", score)
print("After result")

Expected output

Before request
Score: 80
After result

Follow the execution

The caller prints Before request, awaits the function’s result, then prints the score and final message. The await establishes the dependency before score is used. There is no timed delay or network activity in this teaching function.

Try this program in the Swift compiler. Paste the complete source above.

Your task: Does adding async guarantee loadScore executes on a separate thread?

Swift · complete program

func loadScore() async -> Int { 80 }
print("Before request")
let score = await loadScore()
print("Score:", score)
print("After result")

Worked answer

No. Async describes the ability to suspend under Swift’s concurrency model, not a promise of a dedicated thread. Isolation and execution rules depend on the declaration, language mode and context.

Use throws with async when an operation can fail. A suspension point can allow other operations to run; never assume shared mutable state stays unchanged across an await simply because nearby source lines look sequential.

No. Async describes the ability to suspend under Swift’s concurrency model, not a promise of a dedicated thread. Isolation and execution rules depend on the declaration, language mode and context. Use throws with async when an operation can fail.

Further reading: official Swift language guide.