Writing system tests can be sometimes tricky because we want to assert a reality that’s not yet rendered. Ad-hoc sleeping is something that can mostly address the problem, but it’s not the most elegant solution and should be avoided if possible.
The problem
When we write a test with Capybara, we might request an action that takes a bit longer to manifest.
Imagine a form submit that updates a record:
page.fill_in :data, with: "foo bar"
page.click_on "Update"
# test the expectation
assert page.has_content?("Success")
In this case, Capybara has us covered. has_content? will wait.
What about if we want to assert the opposite?
# ...
assert_not page.has_content?("Failure")
This code won’t work as expected, unfortunately. A natural quick fix could be using sleep:
# ...
sleep 0.5
assert_not page.has_content?("Failure")
Similarly, the same issue is true for other assertions:
assert_not page.has_css?(".flash")
The fix
Fix is mostly easy. It’s important to use the Capybara alternatives for asserting the negative:
# will wait
assert page.has_no_content?("Success")
# will wait
assert page.has_no_css?(".flash")
In a similar fashion, we should rely on has_no_field?, has_no_link?, has_no_table?, and other Capybara negative assertions.
We can also adjust the maximum waiting time with default_max_wait_time:
Capybara.default_max_wait_time = ENV.fetch("MAX_WAIT_TIME_IN_SECONDS", 2).to_i
If we need to increase this number just for one test, there is using_wait_time that takes the number of seconds:
# in test
using_wait_time(5) do
assert page.has_no_css?(".flash")
end
If we need to use sleep with something else than Capybara helpers, we can write a helper sleeping up to a predefined wait time:
class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
...
def wait_until(time: Capybara.default_max_wait_time)
Timeout.timeout(time) do
until value = yield
sleep(0.1)
end
value
end
end
# in test
wait_until(time: 2.5) do
page.page_path == current_path
end
Conclusion
We cannot really avoid sleeping in Rails system tests, but we should let Capybara do the heavy lifting and skip polluting our tests with random sleep calls. Also, sleep is a tool. If you open a Capybara test suite, you’ll find ad-hoc sleep calls.
Get Test Driving Rails while it's in prerelease.
