How to benchmark app launch time using XCTOSSignpostMetric.applicationLaunch

Swift version: 5.6

Paul Hudson @twostraws October 23rd 2019

Optimizing your application’s launch time helps ensure users spend more time using your app and less time staring at your launch screen, and helpfully Xcode comes with a built-in way to help us measure and monitor that time: XCTOSSignpostMetric.applicationLaunch.

To use this, start a new measure() block, asking for that value as one of the metrics you want to track. Given that is focusing specifically on launch time, it’s probably a good idea to ask for that single metric. When you’re ready, call XCUIApplication().launch() to launch your app, like this:

func testLaunchPerformance() {
    measure(metrics: [XCTOSSignpostMetric.applicationLaunch]) {
        XCUIApplication().launch()
    }
}

Like a regular measure() block, this causes your test to be run multiple times to ensure a fair spread of readings. When it finishes, you’ll be able to set the baseline for your launch time, to make sure changes made in the future don’t dramatically affect it.

Although Apple doesn’t specify a precise number of seconds that we should aim for with app open times, they do say “a low number of seconds”. Based on my experience, anything more than three seconds on the newest device will feel slow, particularly because it’s likely to be five or more seconds on older devices.

Tip: If you create a new UI Test target you’ll get an application launch test included in the template.