docs/initialize_blink_features.md

# 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][runtime_features]. To learn more on how to
set up features in blink, see
[Runtime Enabled Features][RuntimeEnabledFeatures].

## 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()][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][web_runtime_features.h]
- [third_party/blink/renderer/platform/exported/web_runtime_features.cc][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()][SetRuntimeFeatureDefaultsForPlatform]
using the appropriate OS macros.
### 2) Depends on the status of a base::Feature:
Add your code to the function
[SetRuntimeFeaturesFromChromiumFeatures()][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, kDefault},
```
will call `wf::EnableNewFeatureX` to enable it only if `features::kNewFeatureX`
is enabled, or to set it to the same status as `features::kNewFeatureX` if its
default status is overridden by any field trial or command line switch.

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, kDefault},
```
will call `wf::EnableFeatureFromString` with your feature name instead of
`wf::EnableNewFeatureX` in the same cases as above.

The following table summarizes the relationship between the default status of
the Chromium feature and the status of the blink feature, when `kDefault` is
specified, if **not overridden** by field trial or command line switches
(horizontal headers: blink feature status; vertical headers: chromium feature
default status):

| |No status|`status:"test"`|`status:"experimental"`|`status:"stable"`|
|---|---------|-----------------|--------------------------|-------------------|
|`FEATURE_DISABLED_BY_DEFAULT`|Disabled everywhere|Blink feature is enabled for tests, or everywhere with `--enable-blink-test-features` [1]|Blink feature is enabled for tests, or everywhere with `--enable-experimental-web-platform-features` [1]|Blink feature is enabled everywhere [2]|
|`FEATURE_ENABLED_BY_DEFAULT`|Enabled everywhere|Enabled everywhere|Enabled everywhere|Enabled everywhere|

\[1]: `base::FeatureList::IsEnabled(features::kNewFeatureX)` is still
false. These combinations are suitable for features there are fully implemented
at blink side. Otherwise normally the blink feature should not have a status so
that the Chromium feature can fully control the feature.

\[2]: This combination is counter-intuitive and should be avoided.

Field trial and command line switches can always override the Chromium feature
status and the blink feature status.

Besides `kDefault`, there are also other options for the relationship
between the Chromium feature and the blink feature. These other options should
only be used in rare cases when the default relationship doesn't work.

For more detailed explanation on the options you have, read the comment in enum
[RuntimeFeatureEnableOptions][EnableOptions].
### 3) Set by a command line switch to enable or disable:
Add your code to the function
[SetRuntimeFeaturesFromCommandLine()][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) 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()`][SetCustomizedRuntimeFeaturesFromCombinedArgs].


[EnableOptions]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc#135>
[runtime_features]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc>
[RuntimeEnabledFeatures]:
<https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/renderer/platform/RuntimeEnabledFeatures.md>
[web_runtime_features.h]:
<https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/public/platform/web_runtime_features.h>
[web_runtime_features.cc]:
<https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/renderer/platform/exported/web_runtime_features.cc>
[EnableFeatureFromString]:<https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/public/platform/web_runtime_features.h#56>
[SetRuntimeFeatureDefaultsForPlatform]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc#46>
[SetCustomizedRuntimeFeaturesFromCombinedArgs]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc#487>
[SetRuntimeFeaturesFromChromiumFeatures]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc#160>
[SetRuntimeFeaturesFromCommandLine]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc#390>
[SetRuntimeFeaturesFromFieldTrialParams]:<https://chromium.googlesource.com/chromium/src/+/HEAD/content/child/runtime_features.cc#448>