Chrome Devtools Kotlin

An asynchronous coroutine-based multiplatform Kotlin client for the Chrome DevTools Protocol.

Protocol version & Code generation

Part of this client is generated based on information from the "latest" (a.k.a "tip-of-tree") JSON descriptors found in the ChromeDevTools/devtools-protocol repository. All the domains' commands and events defined in these protocol descriptors are therefore available in chrome-devtools-kotlin, as well as their doc and deprecated/experimental status.

The protocol definitions are automatically updated daily, but releases of chrome-devtools-kotlin are still manual. If you're missing some APIs or updates, don't hesitate to open an issue to request a new release with updated protocol.

You can find the protocol version used by chrome-devtools-kotlin after the - in the version number. For instance, version 1.3.0-861504 of chrome-devtools-kotlin was built using version 861504 of the Chrome DevTools Protocol (this is technically the Chromium revision, but it gives the tip-of-tree version number of the protocol).

Concepts

Domains

The Chrome Devtools Protocol defines domains that expose some commands and events. They are subsets of the protocol's API. You can find the list and documentation of all domains in the protocol's web page.

This library defines a type for each domain (e.g. PageDomain, StorageDomain...), which exposes:

• a suspend method for each command, accepting a request type and returning a response type, respectively containing the input and output parameters defined in the protocol for that command.
• additional suspend functions for each command with a more convenient signature.
• a method for each type of event, returning a Flow of this particular event type
• an events() method, returning a Flow of all events of the domain

The domains usually also expose an enable() command which is required to enable events. Without calling it, you will receive no events in the Flow subscriptions.

Targets

Clients can interact with different parts of Chrome such as pages (tabs), service workers, and extensions. These parts are called targets. The browser itself is also a target.

Each type of target supports a different subset of the domains defined in the protocol.

Sessions

The protocol requires you to attach to a target in order to interact with it. Attaching to a target opens a target session of the relevant type, such as BrowserSession or PageSession.

When connecting to Chrome, a browser target is automatically attached, thus you obtain a BrowserSession. You can then use this session to attach to other targets (child targets), such as pages.

Each of the supported domains are defined as properties of the session type, which provides a type-safe way to know if the attached target supports a given domain. For instance, PageSession.dom gives access to the DOM domain in this page session, which allows to issue commands and listen to DOM events.

Note: The supported set of domains for each target type is not clearly defined by the protocol, so I have to regularly extract this information from Chromium's source code itself and update my own extra definition file: target_types.json.

Because of this, there might be some missing domains on some session types at some point in time that require manual adjustment. If this is the case, use the PageSession.unsafe() method on the session object to get full access to all domains (also, please open an issue so I can fix the missing domain).

Quick Start

Add the dependency

This library is available on Maven Central.

It requires a Ktor engine to work, so make sure to add one that supports the web sockets feature (check the compatibility table). For example, you can pick the CIO engine by adding this engine to your dependencies next to chrome-devtools-kotlin:

dependencies {
    implementation("org.hildan.chrome:chrome-devtools-kotlin:$version")
    implementation("io.ktor:ktor-client-cio:$ktorVersion")
}

Get a browser running

This library doesn't provide a way to start a browser programmatically. It assumes a browser was started externally and exposes a debugger server.

For instance, you can start a Headless Chrome browser with the following docker command, which exposes the debugger server at http://localhost:9222:

docker container run -d -p 9222:9222 zenika/alpine-chrome --no-sandbox --remote-debugging-address=0.0.0.0 --remote-debugging-port=9222 about:blank

Connecting to the browser

The starting point of this library is the ChromeDPClient, which is created using the "remote-debugging" URL that was passed to Chrome. You can then open a webSocket() to the browser debugger, which automatically attaches to the browser target and starts a "browser session":

val client = ChromeDPClient("http://localhost:9222")
val browserSession: BrowserSession = client.webSocket()

When you're done with the session, don't forget to close() it in order to close the underlying web socket. You can also use the familiar use { ... } extension, which automatically closes the session in every code path leaving the lambda (even in case of exception).

Connecting to targets

The BrowserSession is attached to a browser target, but we're usually interested in more useful targets, such as pages. Once you have your browser session, you can use it to create child page targets and attach to them. Here is an example to create a new page target and attach to it:

ChromeDPClient("http://localhost:9222").webSocket().use { browserSession ->
    browserSession.newPage().use { pageSession ->
        // goto() navigates the current page to the URL and awaits the 'load' event by default
        pageSession.goto("http://example.com")

        // This page session has access to many useful protocol domains (e.g. dom, page...)
        val doc = pageSession.dom.getDocument().root
        val base64Img = pageSession.page.captureScreenshot {
            format = ScreenshotFormat.jpeg
            quality = 80
        }
    } // automatically closes the pageSession
} // automatically closes the browserSession

High level extensions

In addition to the generated domain commands and events, some extensions are provided for higher-level functionality. Here are some of them:

• DOMDomain.findNodeBySelector(selector: String): NodeId?: finds a node using a selector query
• PageSession.clickOnElement(selector: String, clickDurationMillis, mouseButton): finds a node using a selector query and simulates a click on it
• PageSession.goto(url: String): navigates to a URL and also waits for the next load event, or other events
• PageDomain.captureScreenshotToFile(outputFile: Path, request: CaptureScreenshotRequest = ...) (JVM-only)
• Runtime.evaluateJs(js: String): T?: evaluates JS and returns the result (uses Kotlinx serialization to deserialize JS results)

Examples

Example usage of Runtime.evaluateJs(js: String):

@Serializable
data class Person(val firstName: String, val lastName: String)

val evaluatedInt = pageSession.runtime.evaluateJs<Int>("42")
assertEquals(42, evaluatedInt)

val evaluatedPerson = pageSession.runtime.evaluateJs<Person>("""eval({firstName: "Bob", lastName: "Lee Swagger"})""")
assertEquals(Person("Bob", "Lee Swagger"), evaluatedPerson)

Note that the deserialization here uses Kotlinx Serialization, which requires annotating the deserialized classes with @Serializable and using the corresponding compiler plugin.

Troubleshooting

Host header is specified and is not an IP address or localhost

Sometimes this error also appears in the form of an HTTP 500.

Chrome doesn't accept a Host header that is not an IP nor localhost, but in some environments it might be hard to provide this (e.g. docker services in a docker swarm, communicating using service names).

To work around this problem, simply set overrideHostHeader to true when creating ChromeDPClient. This overrides the Host header to "localhost" in the HTTP requests to the Chrome debugger to make it happy, and also replaces the host in subsequent web socket URLs (returned by Chrome) by the initial host provided in remoteDebugUrl. This is necessary because Chrome uses the Host header to build these URLs, and it would be incorrect to keep this.

License

This project is distributed under the MIT license.

Alternatives

If you're looking for Chrome Devtools Protocol clients in Kotlin, I have only found one other so far, chrome-reactive-kotlin. This is the reactive equivalent of this project. The main differences are the following:

• it only supports the JVM platform
• it uses a reactive API (as opposed to coroutines and suspend functions)
• it doesn't distinguish target types, and thus doesn't restrict the available domains at compile time (it's the equivalent of always using unsafe() in chrome-devtools-kotlin)

I needed a coroutine-based API instead and more type-safety, which is how this chrome-devtools-kotlin project was born.

You can find alternative Chrome DevTools libraries in other languages in this awesome list.

Credits

Special thanks to @wendigo and his project chrome-reactive-kotlin which inspired the creation of this project.