Initialization of Blink runtime features in content layer

This document outlines how to initialize your Blink runtime features in the Chromium content layer, more specifically in content/child/runtime_features.cc. To learn more on how to set up features in blink, see Runtime Enabled Features.

Step 1: Do you need a custom Blink feature enabler function?

If you simply need to enable/disable the Blink feature you can simply use WebRuntimeFeatures::EnableFeatureFromString().

However, if there are side effects (e.g. you need to disable other features if this feature is also disabled), you should declare a custom enabler function in

• third_party/blink/public/platform/web_runtime_features.h
• third_party/blink/public/platform/web_runtime_features.cc

Step 2: Determine how your feature is initialized.

1) Depends on OS-specific Macros:

Add your code for controlling the Blink feature in SetRuntimeFeatureDefaultsForPlatform() using the appropriate OS macros.

2) Depends on the status of a base::Feature:

Add your code to the function SetRuntimeFeaturesFromChromiumFeatures().

If your Blink feature has a custom enabler function, add a new entry to blinkFeatureToBaseFeatureMapping. For example, a new entry like this:

{wf::EnableNewFeatureX, features::kNewFeatureX, kEnableOnly},

will call wf::EnableNewFeatureX to enable it only if features::kNewFeatureX is enabled.

If your Blink feature does not have a custom enabler function, you need to add the entry to runtimeFeatureNameToChromiumFeatureMapping. For example, a new entry like this:

{"NewFeatureY", features::kNewFeatureY, kUseFeatureState},

will call wf::EnableFeatureFromString with your feature name to set it to whichever state your features::kNewFeatureY is in.

For more detailed explanation on the options you have, read the comment in enum RuntimeFeatureEnableOptions.

3) Set by a command line switch to enable or disable:

Add your code to the function SetRuntimeFeaturesFromCommandLine().

If your Blink feature has a custom enabler function, add a new entry to switchToFeatureMapping. For example, a new entry like this:

{wrf::EnableNewFeatureX, switches::kNewFeatureX, false},

will call wf::EnableNewFeatureX to disable it only if that switches::kNewFeatureX exists on the command line.

4) Controlled by parameters from a field trial:

Add your code to the function SetRuntimeFeaturesFromFieldTrialParams().

5) Combination of the previous options or not covered:

For example, you Blink feature could be controlled by both a base::Feature and a command line switch. In this case, your custom logic should live here in SetCustomizedRuntimeFeaturesFromCombinedArgs().