servo/tests/wpt/web-platform-tests/web-animations

Web Animations Test Suite

Specification: https://drafts.csswg.org/web-animations/

Guidelines for writing tests

• Try to follow the spec outline where possible.

  For example, if you want to test setting the start time, you might be tempted to put all the tests in:

  /web-animations/interfaces/Animation/startTime.html

  However, in the spec most of the logic is in the “Set the animation start time“ procedure in the “Timing model” section.

  Instead, try something like:

  • /web-animations/timing-model/animations/set-the-animation-start-time.html
    Tests all the branches and inputs to the procedure as defined in the spec (using the Animation.startTime API).
  • /web-animations/interfaces/Animation/startTime.html
    Tests API-layer specific issues like mapping unresolved values to null, etc.

  On that note, two levels of subdirectories is enough even if the spec has deeper nesting.

  Note that most of the existing tests in the suite don't do this well yet. That's the direction we're heading, however.

• Test the spec.

  • If the spec defines a timing calculation that is directly reflected in the iteration progress (i.e. anim.effect.getComputedTiming().progress), test that instead of calling getComputedStyle(elem).marginLeft.

  • Likewise, don't add needless tests for anim.playbackState. The playback state is a calculated value based on other values. It's rarely necessary to test directly unless you need, for example, to check that a pending task is scheduled (which isn't observable elsewhere other than waiting for the corresponding promise to complete).

• Try to keep tests as simple and focused as possible.

  e.g.

  test(t => {
    const animation = createDiv(t).animate(null);
    assert_class_string(animation, 'Animation', 'Returned object is an Animation');
  }, 'Element.animate() creates an Animation object');
  

  test(t => {
    assert_throws({ name: 'TypeError' }, () => {
      createDiv(t).animate(null, -1);
    });
  }, 'Setting a negative duration throws a TypeError');
  

  promise_test(t => {
    const animation = createDiv(t).animate(null, 100 * MS_PER_SEC);
    return animation.ready.then(() => {
      assert_greater_than(animation.startTime, 0, 'startTime when running');
    });
  }, 'startTime is resolved when running');
  

  If you're generating complex test loops and factoring out utility functions that affect the logic of the test (other than, say, simple assertion utility functions), you're probably doing it wrong.

  It should be possible to understand exactly what the test is doing at a glance without having to scroll up and down the test file and refer to other files.

  See Justin Searls' presentation, “How to stop hating your tests” for some tips on making your tests simpler.

• Assume tests will run on under-performing hardware where the time between animation frames might run into 10s of seconds. As a result, animations that are expected to still be running during the test should be at least 100s in length.

• Avoid using GLOBAL_CONSTS that make the test harder to read. It's fine to repeat the the same parameter values like 100 * MS_PER_SEC over and over again since it makes it easy to read and debug a test in isolation. Remember, even if we do need to make all tests take, say 200s each, text editors are very good at search and replace.

• Use the assert_times_equal assertion for comparing times returned from the API. This asserts that the time values are equal using a tolerance based on the precision recommended in the spec. This tolerance is applied to both of the values being compared. That is, it effectively allows double the epsilon that is used when comparing with an absolute value.

  For comparing a time value returned from the API to an absolute value, use assert_time_equals_literal. This tests that the actual value is equal to the expected value within the precision recommended in the spec.

  Both assert_times_equal and assert_time_equals_literal are defined in a way that implementations can override them to meet their own precision requirements.

• There are quite a few bad tests in the repository. We're learning as we go. Don't just copy them blindly—please fix them!