Commandline flags

Applying flags

Note: this requires either a userdebug or eng Android build (you can check with adb shell getprop ro.build.type). Flags cannot be enabled on production builds of Android.

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.

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:

# Overwrite flags (supports multiple)
build/android/adb_system_webview_command_line --show-composited-layer-borders --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:

autoninja -C out/Default system_webview_apk
# Overwrite flags (supports multiple)
out/Default/bin/system_webview_apk argv --args='--show-composited-layer-borders --force-enable-metrics-reporting'
# Clear flags
out/Default/bin/system_webview_apk argv --args=''
# Print flags
out/Default/bin/system_webview_apk argv
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:

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 '_ --show-composited-layer-borders --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 confirm you've applied commandline flags correctly by dumping the full state of the commandline flags with the WebView Log Verbosifier app and starting up a WebView app.

Applying Features with flags

WebView supports the same --enable-features=feature1,feature2 and --disable-features=feature3,feature4 syntax as the rest of Chromium. You can use these like any other flag. Please consult base/feature_list.h for details.

Interesting flags

WebView supports any flags supported in any layer we depend on (ex. content). Some interesting flags and Features:

  • --show-composited-layer-borders: highlight rendering layers, which is useful for identifying which content in the app is rendered by a WebView.
  • --force-enable-metrics-reporting: enable UMA metrics reporting (does not override app opt-out)

WebView also defines its own flags and Features:

Implementation

See CommandLineUtil.java.