android_webview/docs/commandline-flags.md

# Commandline flags

## Can I apply commandline flags on my device?

*** note
**Note:** this guide only applies to userdebug/eng devices and emulators. Most
users and app developers **do not** have debuggable devices, and therefore
cannot follow this guide.

If you need to toggle flags on production Android devices, you can use [WebView
DevTools](developer-ui.md).
***

You can check which Android image you have on your device with the following:

```sh
# If you don't have `adb` in your path, you can source this file to use
# the copy from chromium's Android SDK.
$ source build/android/envsetup.sh

# If this outputs "userdebug" or "eng" then you can apply flags following this
# guide. If it outputs "user" then you can only use WebView DevTools.
$ adb shell getprop ro.build.type
userdebug
```

If the above outputs "user," then you have two options:

* Try using [WebView DevTools](developer-ui.md) to toggle flags (also works on
  userdebug and eng devices)
* Reflash your device or create a debuggable emulator (see [device
  setup](device-setup.md))

## Applying flags

WebView reads flags from a specific file on the device as part of the startup
sequence. Therefore, it's important to always **kill the WebView-based app**
you're examining after modifying commandline flags, to ensure the flags are
picked up during the next app restart.

WebView always looks for the same file on the device
(`/data/local/tmp/webview-command-line`), regardless of which package is the
[the WebView provider](prerelease.md).

### Python script

The simplest way to set WebView flags is with the dedicated python script. This
works regardless of which package is the WebView provider:

```sh
# Overwrite flags (supports multiple)
build/android/adb_system_webview_command_line --highlight-all-webviews --force-enable-metrics-reporting
# Clear flags
build/android/adb_system_webview_command_line ""
# Print flags
build/android/adb_system_webview_command_line
```

### Generated Wrapper Script

If you have a locally compiled APK, you may instead set flags using the
Generated Wrapper Script like so:

```sh
autoninja -C out/Default system_webview_apk
# Overwrite flags (supports multiple)
out/Default/bin/system_webview_apk argv --args='--highlight-all-webviews --force-enable-metrics-reporting'
# Clear flags
out/Default/bin/system_webview_apk argv --args=''
# Print flags
out/Default/bin/system_webview_apk argv
```

*** note
**Note:** be careful if using a `monochrome_*` target, as the Generated Wrapper
Script writes to Chrome browser's flags file, and WebView **will not pick up
these flags**. If using Monochrome, you can set flags with the
`system_webview_*` Generated Wrapper Scripts, or use one of the other methods
in this doc.
***

### Manual

Or, you can use the `adb` in your `$PATH` like so:

```sh
FLAG_FILE=/data/local/tmp/webview-command-line
# Overwrite flags (supports multiple). The first token is ignored. We use '_'
# as a convenient placeholder, but any token is acceptable.
adb shell "echo '_ --highlight-all-webviews --force-enable-metrics-reporting' > ${FLAG_FILE}"
# Clear flags
adb shell "rm ${FLAG_FILE}"
# Print flags
adb shell "cat ${FLAG_FILE}"
```

## Verifying flags are applied

You can add the `--webview-verbose-logging` flag, which tells WebView to dump
its full commandline and variations state to logs during startup. You can filter
device logs with:

```shell
adb logcat | grep -iE 'Active field trial|WebViewCommandLine'
```

## Applying Features with flags

[`base::Feature`s](/base/feature_list.h) (or, "Features") are Chromium's
mechanism for toggling off-by-default code paths. While debugging flags are also
off-by-default, Features typically guard soon-to-launch product enhancements
until they're tested enough for field trials or public launch, at which point
the Feature is removed and the legacy code path is no longer supported and
removed from the codebase. On the other hand, debugging flags don't "launch," as
they're typically only helpful for debugging issues.

WebView supports the same syntax for toggling Features as the rest of chromium:
`--enable-features=feature1,feature2` and
`--disable-features=feature3,feature4`. You can apply `--enable-features` and
`--disable-features` like any other flags, per the steps above. Please consult
[`base/feature_list.h`](/base/feature_list.h) for details.

## Finding Features and flags

WebView supports toggling any flags/Features supported in any layer we
depend on (ex. content). For more details on Chromium's layer architecture, see
[this diagram](https://www.chromium.org/developers/content-module) (replace
"chrome" with "android\_webview"). Although we support toggling these flags, not
all flags will have an effect when toggled, nor do we guarantee WebView
functions correctly when the flag is toggled.

Some interesting flags and Features:

 * `--highlight-all-webviews`: highlight the entire contents of all WebViews, to
   quickly identify which app content is rendered by a WebView vs. native Views.
 * `--show-composited-layer-borders`: highlight rendering layers, to identify
   possible graphics issues.
 * `--force-enable-metrics-reporting`: enable UMA metrics reporting (does not
   override app opt-out)
 * `--finch-seed-expiration-age=0 --finch-seed-min-update-period=0 --finch-seed-min-download-period=0 --finch-seed-ignore-pending-download`: always request a new finch seed when an app starts

WebView also defines its own flags and Features:

 * C++ switches are defined in
   [`aw_switches.cc`](/android_webview/common/aw_switches.cc). We use
   [`java_cpp_strings`](/docs/android_accessing_cpp_switches_in_java.md) to
   automatically generate Java switch constants from the C++ switches (see
   [`AwSwitches.java`](https://source.chromium.org/chromium/chromium/src/+/main:out/android-Debug/gen/android_webview/common_java/generated_java/input_srcjars/org/chromium/android_webview/common/AwSwitches.java)).
 * C++ `base::Features` are defined in
   [`aw_features.cc`](/android_webview/common/aw_features.cc). We use
   [`java_cpp_features`](/docs/android_accessing_cpp_features_in_java.md) to
   automatically generate Java constants from the C++ Features (see
   [`AwFeatures.java`](https://source.chromium.org/chromium/chromium/src/+/main:out/android-Debug/gen/android_webview/common_java/generated_java/input_srcjars/org/chromium/android_webview/common/AwFeatures.java)).

## Implementation

See [CommandLineUtil.java](https://cs.chromium.org/chromium/src/android_webview/java/src/org/chromium/android_webview/common/CommandLineUtil.java).