XCTWaiter.wait的意外行为

Question

出于测试目的，我有以下测试功能：

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)")
}  

此测试失败
等待结果为.timedOut，日志为

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

虽然设置了var string，但我不明白为什么等待失败
但是，当我输出注释DispatchQueue.main.async时，即当我同步执行其块时，测试成功。然后日志为

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

据我所知，异步版本的测试函数也应该可以工作
怎么了？

Answer 1

它是谓词(以及捕获的语义)。您认为您正在将对string的引用交给谓词求值，但事实并非如此；你只是通过了nil，而nil不会很快神奇地成为非nil。为了减少混淆，请重写为

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