# 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).