Writing New Tests

A layout test is simply a web page. The layout test machinery renders the web page, and then dumps the internal representation, the render tree, with details of the layout. This lets engineers working on the project know if they do anything that changes the layout. Once we get a test rendering properly, we check it in as part of the layout test suite.

The following are some guidelines to follow when writing new layout tests:

  1. The test should be the smallest possible code fragment that tests the feature.
  2. The test should fit on one page (unless of course it’s testing scrolling).
  3. The test should clearly describe what feature it is testing.
  4. The test should clearly describe the expected result. It should be possible through visual inspection alone to determine that the test has failed.

An example of a layout test that follows these guidelines is fast/events/event-creation.html.

A layout test should work both in the browser itself, and in the layout test tool. The layout test tool provides an additional object on the window object called the layout test controller with some methods that control test output. One you should know about is the testRunner.dumpAsText method. Calling this from JavaScript within a test arranges for the output to be written out as plain text rather than as a render tree dump. This is useful for tests that are testing something other than layout. The event creation test mentioned above is a good example of how to do this and when it makes sense.

Some tests require pixel-level comparisons. For these tests, you must generate expected output for a specific machine type, operating system, and color profile. When you add such a test, you can generate new expected output automatically using the run-webkit-tests --pixel command. This will automatically configure the color profile, and place the resulting rendered image (and checksum) in the appropriate platform directory for checkin.

The Ahem font is useful in testing text layout, since its glyphs’ metrics are well known, and it is always available to tests running in the layout test tool. However, if you want to view tests that use Ahem with your own browser, you should the download and install the Ahem and AhemExtra font files on your system. To install the font on OS X, open it in the Font Book application, and click on the Install Font button to install it.

The CSS working group has an excellent document on test writing guidelines for CSS tests. This wiki article has more information on writing good tests and the DumpRenderTree tool.