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Selenium integration

Setting up Selenium tests normally requires manual work to obtain the Selenium server JAR file and a driver program for your browser, launch the server, and connect to the server with your client library. It can be error-prone to make sure the versions are compatible and everything runs smoothly.

The sandwich-webdriver extension streamlines this by automatically downloading the latest compatible binary files and introducing the contexts you need to use the webdriver package within Sandwich. Here's how easy it is to get started:

import Test.Sandwich
import Test.Sandwich.WebDriver
import Test.WebDriver
spec :: TopSpec
spec = introduceWebDriver (defaultWdOptions "/tmp/tools") $ do
it "opens Google and searches" $ withSession1 $ do
openPage ""
search <- findElem (ByCSS "input[title='Search']")
click search
sendKeys "asdf\n" search
main :: IO ()
main = runSandwich defaultOptions spec

To see a demo, try running stack run demo-webdriver in the Sandwich repo.

Browser sessions#

You can start a Selenium session using the withSession function. It accepts a string key representing the name of the session. Each time a new session name is seen, it will be created if it doesn't already exist.

The library provides withSession1/withSession2 as convenience functions for withSession "browser1"/withSession "browser2", but you can use your own keys if you need.

For example, the code below opens two windows with a different site in each.

spec :: TopSpec
spec = introduceWebDriver (defaultWdOptions "/tmp/tools") $ do
describe "two browser sessions" $ do
it "opens Google" $ withSession1 $ openPage ""
it "opens Yahoo" $ withSession2 $ openPage ""

Window positioning#

You can use the functions in Test.Sandwich.WebDriver.Windows to arrange browser windows on the screen. This is useful when you want to watch two browsers simultaneously accessing a collaborative app.

The code below extends the previous example with window positioning. You can find this in the webdriver-positioning demo.

positioning :: TopSpec
positioning = introduceWebDriver (defaultWdOptions "/tmp/tools") $ do
describe "two windows side by side" $ do
it "opens Google" $ withSession1 $ do
openPage ""
it "opens Yahoo" $ withSession2 $ do
openPage ""

Launching browsers in the background#

This package makes it easy to run Selenium tests in the background, using either Xvfb or the headless mode of your browser.


Many browsers now have the ability to natively run in headless mode. For example, passing these modified WdOptions to introduceWebDriver will run using headless Firefox.

wdOptions = (defaultWdOptions "/tmp/tools") {
capabilities = firefoxCapabilities
, runMode = RunHeadless defaultHeadlessConfig

Alternatively, if you use Sandwich's runSandwichWithCommandLineArgs in conjunction with introduceWebDriverOptions, you can enable headless mode by passing --headless.


Xvfb can be used to run your browser on a separate, "virtual" X11 display, different from the one connected to your monitor. This was more useful before headless browser modes existed, but it's still important because it gives you the ability to record video. When a Selenium test is running on an Xvfb display, you can use ffmpeg to record videos of the test runs for later examination.

Xvfb mode can be configured manually just like headless mode.

wdOptions = (defaultWdOptions "/tmp/tools") {
capabilities = chromeCapabilities
, runMode = RunInXvfb XvfbConfig

Or, if you use Sandwich's runSandwichWithCommandLineArgs in conjunction with introduceWebDriverOptions, you can enable Xvfb mode by passing --xvfb.


Xvfb and ffmpeg must be installed in the test environment to use these features.

Recording videos#

As discussed above, recording video doesn't work in headless (--headless) mode. It requires normal (--current) or Xvfb (--xvfb) mode.


Using the methods in Test.Sandwich.WebDriver.Video, you can wrap arbitrary sections of a test in video recording. The example below can be found in the webdriver-video demo.

manualVideo :: TopSpec
manualVideo = introduceWebDriver (defaultWdOptions "/tmp/tools") $ do
describe "video recording" $ do
it "opens Google" $ withSession1 $ do
openPage ""
Just dir <- getCurrentFolder
let path = dir </> "video" -- No extension needed
bracket (startBrowserVideoRecording path defaultVideoSettings) endVideoRecording $ \_ -> do
search <- findElem (ByCSS [i|input[title="Search"]|])
click search
sendKeys "Haskell Sandwich" search
findElem (ByCSS [i|input[type="submit"]|]) >>= click

You can also wrap video around multiple tests by using the around node, or around individual tests using aroundEach, etc.

With command line options#

If you use Sandwich's runSandwichWithCommandLineArgs in conjunction with introduceWebDriverOptions, then you can take advantage of the built-in command line arguments --individual-videos and --error-videos. The former will record videos of every individual test and store them in the corresponding folder on disk. The latter will do the same, but will delete the videos if the test ran successfully, so you only end up with error videos.

You can try this out by running:

stack run demo-webdriver -- --individual-videos


Because every Sandwich test tree has an associated directory in the filesystem, it's easy to capture screenshots during a test.

Just dir <- getCurrentFolder
screenshot >>= liftIO . BL.writeFile (dir </> "screenshot.png")

Custom Selenium and driver binaries#

By default, the WebDriver machinery will obtain the Selenium JAR and driver binaries from the web automatically. However, you can also manually configure which ones are used by passing a disk path.

data WdOptions = WdOptions {
, seleniumToUse = UseSeleniumAt "/path/to/selenium.jar"
, chromeDriverToUse = UseChromeDriverAt "/path/to/chromedriver"
, geckoDriverToUse = UseGeckoDriverAt "/path/to/geckodriver"

Alternatively, you can pass DownloadSeleniumFrom with a URL to download, and similarly for the other options. Please see the Haddocks for more details.

Running tests in parallel with a webdriver pool#

If you have a lot of test files, you probably want to run them in parallel so the test suite finishes faster. However, you probably don't want to run them all in parallel, since running that many browser sessions could bog down your CI machine. A good solution is to use Data.Pool to introduce a fixed-size pool of reusable WebDriver contexts.

You can follow along with this example in the webdriver-pool demo.

The first thing we need to do is come up with a label and introduce a pool of the desired size.

webDriverPool = Label :: Label "webDriverPool" (Pool WebDriver)
type HasWebDriverPool context = HasLabel context "webDriverPool" (Pool WebDriver)
introduceWebDriverPool poolSize wdOptions' =
introduce "Introduce webdriver pool" webDriverPool alloc cleanup
alloc = do
wdOptions <- addCommandLineOptionsToWdOptions
<$> (getCommandLineOptions @())
<*> pure wdOptions'
runRoot <- fromMaybe "/tmp" <$> getRunRoot
liftIO $ createPool
(allocateWebDriver' runRoot wdOptions) cleanupWebDriver' 1 30 poolSize
cleanup = liftIO . destroyAllResources

There's some plumbing here because we want to pipe the command line options through. The important part is at the end, where we make a pool that knows how to allocate and deallocate WebDrivers using the lower-level allocateWebDriver' and cleanupWebDriver' functions.

Next, we make another introduce node to claim a WebDriver from the pool.

claimWebdriver spec = introduceWith' (
defaultNodeOptions {nodeOptionsRecordTime=False, nodeOptionsCreateFolder=False}
) "Claim webdriver" webdriver wrappedAction spec
wrappedAction action = do
pool <- getContext webDriverPool
withResource pool $ \sess ->
(void $ action sess) `finally` closeAllSessions sess

There's also some plumbing here where we tweak the node options of this to play better with test timing. This code obtains the pool context, then claims a WebDriver to pass to its sub-nodes. It also cleans up at the end by calling closeAllSessions.

Having written these functions, we can finally write our tests. The following will run all the tests, up to four at a time, re-using the WebDrivers among them.

tests :: TopSpecWithOptions
tests =
introduceWebDriverPool 4 (defaultWdOptions "/tmp/tools") $
parallel $
replicateM_ 20 $
claimWebdriver $
it "opens Google" $ withSession1 $
openPage ""

Of course, in real use you probably want to introduce different tests to run in parallel. You can use Test Discovery to automatically generate them.