UIView.animateWithDuration:animations:completion: 在 XCTest 中被取消

Question

I'm trying to unit test an extension on UIView , which calls animateWithDuration:animations:completion: .

Unfortunately all animations in the test case gets canceled immediately and therefore the isFinished parameter of the completion block is always false .

Does anyone have an idea how to get animations working in a test case?

This is my playground code:

import UIKit
import XCTest

extension UIView {
    func fadeOut(duration: TimeInterval, completion: ((Bool) -> Void)?) {
        UIView.animate(withDuration: duration,
                       animations: {
                            self.alpha = 0.0
                       },
                       completion: { isFinished in
                            self.isHidden = isFinished

                            completion?(isFinished)
                       })
    }
}

class UIViewTests: XCTestCase {

    func testFadeView() {
        // Given
        let expectation = self.expectation(description: "Expect completion handler to be called.")

        let view = UIView()
        view.alpha = 1.0

        // When
        print("Date before", Date())
        view.fadeOut(duration: 1.0) { (isFinished) in
            print("Date completed", Date())
            print("isFinished", isFinished)

            expectation.fulfill()
        }

        // Then
        wait(for: [expectation], timeout: 1.1)

        XCTAssertTrue(view.isHidden) // <- this assertion always fails, cause `isFinished` inside the completion handler is always `false`.
        XCTAssertEqual(view.alpha, 0.0, accuracy: CGFloat.ulpOfOne)
    }
}

UIViewTests.defaultTestSuite.run()

The assertion XCTAssertTrue(view.isHidden) always fails. Furthermore the log statements output:

Date before 2019-05-20 23:30:35 +0000
Date completed 2019-05-20 23:30:35 +0000

So basically the animation is killed immediately.

Answer 1

For the animation to complete with the correct flag, the view needs to in a UIWindow that is visible.

let window = UIWindow()
window.addSubview(view)
window.isHidden = false

With this, your test succeeds. But UIKit won't clean up the window without giving the run loop an extra kick at the very end of the test. So add

func tearDown() {
    super.tearDown()
    RunLoop.current.run(until: Date())
}

Then the window (and anything in it) will be deallocated.

Now that it's working, you can save time by reducing your duration. I got the test down to 23ms by using duration: 0.001