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The Chromium Chronicle #21: Chrome OS End-to-end UI Automation

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Episode 21: by Brendan Hansknecht in Mountain View, CA (May, 2021)
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Historically, automating the Chrome OS UI in end-to-end (E2E) tests across the entire fleet of devices has been difficult. In Tast, a new UI library has been created that uses the Chrome a11y (accessibility) tree to control the Chrome OS UI. This library enables developers to easily create E2E tests against any visible UI surface (Chrome OS desktop UI, native apps, web apps, Chrome Browser UI).

The library has a simple, chainable way to describe how to find a UI element. For example, the Downloads folder in the Files App could be defined as:

filesWindow := nodewith.NameStartingWith("Files")
downloadsButton := nodewith.Name("Downloads")

Once you have defined a node finder, it is possible to interact with the node in many ways. From simple clicks to waiting for focus, the UI library gives stable access to many operations. For example, to right click the Downloads folder and left click the copy button, you could write:

ui := uiauto.New(tconn)
if err := uiauto.Combine("copy downloads",
)(ctx); err != nil { /* do error handling */ }

There are existing wrappers around common UI areas (settings, launcher, files app, etc).

The uiauto.Run function used above takes a list of actions. In this context, an action is just a func(context.Context) error. With such a simple API, other kinds of actions can be mixed in with UI actions. For example, using the keyboard is as easy as:

if err := uiauto.Combine("do some random stuff",
ui.FocusAndWait( /* some text field */ ),
kb.TypeAction("Hello, world!"),
func(ctx context.Context) error {
// My custom action!
)(ctx); err != nil { /* do error handling */ }

For a more in depth guide, see Tast Codelab: Chrome UI Automation

When writing these tests, it is very useful to dump the Chrome a11y tree for debugging. Simply add the following code to do so:

defer faillog.DumpUITreeOnError(ctx, s.OutDir(), s.HasError, tconn)
s.Fatal("I would like to see the ui tree here")

The a11y tree will now be stored with other tast logs as faillog/ui_tree.txt

If you have any questions contact the tast-users group.

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