Settings, Experiments, and Features in Chromium DevTools

From a developers perspective the Chromium DevTools experience can be customized in three ways: via Settings>Preferences, Settings>Experiments, or by passing a command line flag to Chromium.

How to add experiments

If you want to launch a new feature in DevTools behind an experiment flag, you will need to do two things:

1. Make Chromium aware of the experiment and enable it to track users enabling/disabling the experiment.
2. Register the experiment in DevTools to have it added to the Settings UI.

Chromium

In Chromium, edit tools/metrics/histograms/enums.xml. Find the enum titled DevToolsExperiments (your best bet is to search for this text in your editor, as enums.xml is very large).

Go to the end of this enum, and add a new entry. Make sure that the value is increased by one from the previous entry. The label can be anything you like but make sure it is easily identifiable.

<int value="95" label="yourExperimentNameHere"/>

See an example Chromium CL here.

DevTools front-end

In DevTools, you need to register the experiment. This is done in front_end/entrypoints/main/MainImpl.ts and is done by calling Root.Runtime.experiments.register:

Root.Runtime.experiments.register(
  'yourExperimentNameHere',
  'User facing short description of experiment here',
  false);

The first argument is the experiment's label, and this must match the label you used in your Chromium CL.

The second argument is a short description of the experiment. This string will be shown to users.

Finally, the third argument marks the experiment as unstable. You should pass true if you want your experiment to be marked as unstable. This moves it into a separate part of the UI where users are warned that enabling the experiment may cause issues.

You may also pass in two additional arguments which can be used to link users to documentation and a way to provide feedback:

Root.Runtime.experiments.register(
  'jsProfilerTemporarilyEnable',
  'Enable JavaScript Profiler temporarily',
   /* unstable= */ false,
   /* documentation */ 'https://goo.gle/js-profiler-deprecation',
   /* feedback */ 'https://crbug.com/1354548'
);

You must also add the experiment to front_end/core/host/UserMetrics.ts. Add an entry to the DevToolsExperiments enum, using the same label and integer value that you used in the Chromium CL. You should also increase the MaxValue entry by one.

Once the experiment is registered, you can check if it is enabled and use this to run certain code blocks conditionally:

if(Root.Runtime.experiments.isEnabled('yourExperimentNameHere')) {
   // Experiment code here
}

How to add command line flags

go/chrome-devtools:command-line-config

In some situations it would be convenient (or is even necessary) to configure DevTools directly via the command line interface (CLI). This is particularly useful for features which have both a DevTools front-end and a Chromium back-end component, and allows configuration from a single source of truth. For live demos of features which are still under development, this can be very helpful as well. Presenters would have peace of mind, knowing that their feature is working correctly, as long as they are re-using the right command to launch Chromium.

Define a new base::Feature

Add a new base::Feature to DevTools' features.cc. This feature will automatically be available as a Chrome command line parameter:

// in browser_features.cc

BASE_FEATURE(kDevToolsNewFeature, "DevToolsNewFeature",
             base::FEATURE_DISABLED_BY_DEFAULT);

// Optionally add feature parameters
const base::FeatureParam<std::string> kDevToolsNewFeatureStringParam{
    &kDevToolsNewFeature, "string_param", /*default_value=*/""};
const base::FeatureParam<double> kDevToolsNewFeatureDoubleParam{
    &kDevToolsNewFeature, "double_param", /*default_value=*/0};

Start Chrome via command line including flags:

out/Default/chrome --enable-features=DevToolsNewFeature

You can even pass additional feature parameters:

out/Default/chrome --enable-features="DevToolsNewFeature:string_param/foo/double_param/0.5"

Add the new feature to the host configuration being sent to DevTools

Add the new feature to DevToolsUIBindings::GetHostConfig (link to code, example CL).

In DevTools, use the new property added to HostConfig

• Update the type definition in Runtime.ts.
• Update the dummy value in InspectorFrontendHost.ts.
• For tests, update the stub in EnvironmentHelpers.ts.
• Access the host config via Common.Settings.Settings.instance().getHostConfig().

Please refer to this example CL.