Unexpected behavior of XCTWaiter.wait-CodePudding

For testing purposes, I have the following test function:

func test_wait() {
    var string: String?
    DispatchQueue.main.async {
        string = "set"
        print("string set")
    }
    let notNilPredicate = NSPredicate(format: "self != nil")
    let notNilExpectation = expectation(for: notNilPredicate, evaluatedWith: string)
    print("start waiting")
    let waitResult = XCTWaiter.wait(for: [notNilExpectation], timeout: 5)
    XCTAssert(waitResult == .completed, "wait for notNilExpectation failed with result \(waitResult)")
}

This test fails.
The wait result is .timedOut, and the log is

start waiting
string set
… : XCTAssertTrue failed - wait for notNilExpectation failed with result XCTWaiterResult(rawValue: 2)
Interrupting test

I do not understand why the wait fails although var string is set.
However, the test succeeds when I out comment DispatchQueue.main.async, i.e. when I execute its block synchronously. The log is then

string set
start waiting
Test Case '-[ShopEasyTests.CoreDataCloudKitContainerTest test_wait]' passed (1.087 seconds).

To my understanding, the async version of the test function should also work.
What is wrong?

CodePudding user response：

It's the predicate (and the semantics of capture). You think you are handing a reference to string into your predicate evaluation, but you aren't; you are just passing nil, and nil is not going to magically be not-nil any time soon. To confuse yourself less, rewrite as

let notNilPredicate = NSPredicate {_,_ in string != nil }
let notNilExpectation = expectation(for: notNilPredicate, evaluatedWith: nil)