我写了一个方法loadCountries()，它在调用时执行异步文件加载，然后通知它的输出工作完成。我试图在测试中测量这个方法的性能，但是我不知道如何在没有回调的情况下处理异步操作。
我发现有startMeasuring()和stopMeasuring()方法允许手动设置测试的入口和终点，我尝试在输出模拟中调用后者：

let output = InteractorOutputMock()
output.onDisplay = { _ in
    self.stopMeasuring()
}

let interactor = PremiumLocationsChooserInteractor()
interactor.output = output

measureMetrics([.wallClockTime], automaticallyStartMeasuring: false) {
    self.startMeasuring()
    interactor.loadCountries()
}

但是代码仍然在0秒内完成。我应该如何处理这个问题？
更新：
我也尝试过使用expectation，但是遇到了问题。我既不能在measure块内部调用它，也不能在measure块外部调用它。前者看起来如下所示，并导致代码在measure的第一次迭代中等待：

let outputCalledExpectation = XCTestExpectation(description: "Output   hasn't been called")
outputCalledExpectation.expectedFulfillmentCount = 10 // need to fullfill it 10 times since this is how much measure block is called
let output = InteractorOutputMock()
output.onDisplay = { _ in
    self.stopMeasuring() // called only once
    outputCalledExpectation.fulfill()
}

let interactor = PremiumLocationsChooserInteractor()
interactor.output = output

measureMetrics([.wallClockTime], automaticallyStartMeasuring: false) {
    startMeasuring()
    interactor.loadCountries()
    wait(for: [outputCalledExpectation], timeout: 5.0) // then stack here
}

如果我尝试将wait方法移出块，就会得到异常-stopMeasuring is only supported from a block passed to -measure...Block:

measureMetrics([.wallClockTime], automaticallyStartMeasuring: false) {
    startMeasuring()
    interactor.loadCountries()
}
wait(for: [outputCalledExpectation], timeout: 5.0) // get exception here