Where is the screenshot file from Xcode UI Test?

Screenshots are stored inside your Derived Data folder. You can find this by going to Xcode, Window > Projects and click the little arrow next to your derived data path for the project.

From your project derived data folder (should be the name of your folder with a load of letters after it), go to Logs/Test.

Inside the Test folder will be test summary .plist files with UUIDs in their names (one per test run) and a folder called Attachments - the screenshots are in here.

If any of these folders aren't there, make sure you've run the tests first.

To work out which screenshot you want, you'll need to dig through the *_TestSummaries.plist file. Open it in Xcode and follow the test summary down to the SubActivity you're interested in. Some SubActivities don't have screenshots, and will have the HasScreenshotData key set to NO.

When you find a SubActivity with screenshot data, match the UUID for that SubActivity with the filename of the screenshot in the Attachments folder.

My screenshots were located at: /Users/{username}/Library/Developer/Xcode/DerivedData/{project_name}-tywebfjsswmghapfjfbzhuazjf/Logs/Test/Attachments.

In the .plist, I followed this path to find details of the screenshot data for an event: TestableSummaries > Item 0 > Tests > Item 0 > Subtests > Item 0 > Subtests > Item 0 > ActivitySummaries > Item 1 > SubActivities > Item 0.

Xcode screenshot file from UI Test

To make screenshot you can call the next method

func makeScreenShot() {
    let screenshot = XCUIScreen.main.screenshot()
    let fullScreenshotAttachment = XCTAttachment(screenshot: screenshot)
    fullScreenshotAttachment.lifetime = .keepAlways

    add(fullScreenshotAttachment)
}

You can find screenshots in Xcode -> Report navigator -> select your test

Or go to

DerivedData -> ProjectName... -> Logs -> Test -> find .xcresult -> Show Package Contents -> Attachments

#For example
/Users/<mac_name>/Library/Developer/Xcode/DerivedData/<project_name>-<...>/Logs/Test/Test-<target_name>-<...>.xcresult

[Xcode run tests]

In Xcode 11, the contents of an .xcresults file changed to no longer include an Attachments folder that's browsable in Finder. You can still download individual attachments from the Test Report as shown in yoAlex5's answer, but to get the attachments without using the Xcode GUI, you have to use Apple's xcresulttool or a third party tool that leverages xcresulttool like xcparse. See this blog post for more information: https://www.chargepoint.com/engineering/xcparse/