DispatchQueue.main.async not calling escaping closure

Question

I'm trying to understand concurrency and can't figure out why this is the escaping closure is not being called.

var closure: (() -> ()) = {
    defer { print("inner") }
}

func outer(asyncClosure: @escaping (() -> ())) {
    defer { print("outer") }

    DispatchQueue.main.async { asyncClosure() }
}

outer(asyncClosure: closure)

// output is "outer" only

Why is my escaping closure not being called? It's an async function on the main queue and is not blocking anything.

There's probably something wrong with your XCode or you're testing this code in `XCUnitTest`. This code works just fine (just tested!). In `XCUnitTest`s for `@escaping` closures you have to tell the test class to wait for sometime. (search for `expectations` in Unit Tests) — farzadshbfn, Oct 08 '17 at 13:51

score 2 · Accepted Answer · answered Oct 08 '17 at 13:53

I'm guessing you are running this in the Playground? The playground may "finished" running at the closing brace of outer() and the async block that comes after is not run.

Try:

import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

DispatchQueue.main.async not calling escaping closure

1 Answers1