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Hitches

  • Scrolling animation sometimes appears to jitter, those jitters in Xcode are called hitches

  • A hitch is a frame that is displayed on the screen later than expected.

  • iPhones and iPads have screens with 60Hz refresh rate, iPad Pros go up to 120Hz. 60Hz means that each frame should stay on screen for 16.67ms, 120Hz means 8.33ms

  • VSYNC is what usually manages swapping frames on screen, we see a hitch when a frame misses its expected VSYNC

Hitch Metrics

  • Hitch time: how many ms a frame is late
  • Hitch ratio: Hitch time in ms per second for a given duration of hitches for a given duration (for a test, scroll event or transition)
  • Hitch time isn’t accurate as the desired screen refresh rate isn’t always 60 or 120fps, it can be perfectly fine for VSYNC to swipe zero frames for an idle screen
  • This is why we use Hitch ratio, it’s consistent across different cases and tests

User impact targets for hitch ratio:

You should always aim to have <= 5ms/s hitch ratio

Measuring Hitches

  • It’s possible to measure hitches in development and production environment (production for iOS 14 only) using different tools:

  • XCTest Metrics is used to measure hitches in development environment, while MetricKit and Xcode Organizer can show you measures from the customers devices directly

  • XCTest Metrics has many tests you can use to measure different parts of your application, the one we’re concerned with here is XCTOSSignPostMetric

  • XCTOSSignPostMetric used to measure animation duration in previous versions of Xcode, new in Xcode 12, this metric will provide 5 additional information:

    • Total count of hitches
    • Total duration of hitches
    • Hitch time ratio
    • Frame rate
    • Frame count

  • To collect these metrics, you should write a test case that collects os_signpost intervals with the new

  • In Xcode 11, this produced a non-animation os_signpost intervals:

os_signpost(.begin, log: logHandle, name: "performInterval")
os_signpost(.end, log: logHandle, name: "performInterval")
  • Now in Xcode 12, you can use the .animationBegin instead to specify the extra metrics related to animation:
os_signpost(.animationBegin, log: logHandle, name: "performAnimationInterval")
os_signpost(.end, log: logHandle, name: "performAnimationInterval")
  • You can also use one of the predefined UIKit instrumented intervals for testing around navigation transitions and scrolling, these are sub-metrics provided on the XCTOSSignpostMetric class.
extension XCTOSSignpostMetric {
	 open class var navigationTransitionMetric: XCTMetric { get }
	 open class var customNavigationTransitionMetric: XCTMetric { get }
	 open class var scrollDecelerationMetric: XCTMetric { get }
	 open class var scrollDraggingMetric: XCTMetric { get }
}
  • Here is a sample test case that launches the app, taps on ”Meal Planner" and measures the scrolling animation in a .fast velocity:
// Measure scrolling animation performance using a Performance XCTest
func testScrollingAnimationPerformance() throws {
	app.launch()
	app.staticTexts["Meal Planner"].tap()
	let foodCollection = app.collectionViews.firstMatch

	measure(metrics: [XCTOSSignpostMetric.scrollDecelerationMetric]) {
	foodCollection.swipeUp(velocity: .fast)
	}
}
  • To avoid swiping between different content in the 5 iterations of the measure block, we can reset the application state during measurements:
func testScrollingAnimationPerformance() throws { 
	app.launch()
	app.staticTexts["Meal Planner"].tap()
	let foodCollection = app.collectionViews.firstMatch

	let measureOptions = XCTMeasureOptions()
	measureOptions.invocationOptions = [.manuallyStop]

	measure(metrics: [XCTOSSignpostMetric.scrollDecelerationMetric],
		options: measureOptions) {
	foodCollection.swipeUp(velocity: .fast)
	stopMeasuring()
	foodCollection.swipeDown(velocity: .fast)
	}
}

Tips for creating performance tests:

  • Create a separate test scheme for your Performance XCTest
  • Use Release Build Configuration and uncheck Debug executable
  • Switch off Automatic Screenshots and Code Coverage
  • Turn off all diagnostic options, checkers, sanitizers and memory management