Flag UI

While WebView supports toggling arbitrary flags on debuggable devices, we also support toggling a curated set of experimental flags/features on production Android devices. We expose these features as part of WebView‘s on-device DevTools. This is similar to Chrome’s chrome://flags tool.

WebView flag UI

Launching WebView DevTools

The flag UI is part of “WebView DevTools,” an on-device tool that ships with device's WebView implementation. You can launch WebView DevTools by any of the following:

Launcher icon on pre-stable channels (preferred)

The best way to launch WebView DevTools is to download WebView Dev or WebView Canary. These channels will have a launcher icon which will launch WebView DevTools.

Note: the WebView DevTools icon does not appear by default on Android 7 through 9 (Nougat/Oreo/Pie). To enable the launcher icon, first change your WebView provider and then launch the same Chrome channel or any WebView app (ex. WebView shell browser, or open an email in Gmail).

Launch via adb

If you have adb installed, you can connect your Android device to launch DevTools:

adb shell am start -a "com.android.webview.SHOW_DEV_UI"

Launch via WebView Shell

Newer versions of WebView shell have a menu option to launch WebView DevTools. If your copy of WebView shell doesn't have this option, you may need to rebuild it yourself.

Using the flag UI

Once you've launched WebView DevTools, tap the “Flags” option in the bottom navgation bar. You can scroll through the list to find your desired feature/flag (ex. “highlight-all-webviews”), tap the dropdown (look for “Default”), and tap “Enabled” in the dialog popup. You can enable (or disable) as many flags as you need.

Kill and restart WebView apps so they pick up the new flags.

When you're done, open the notification tray and tap the WebView DevTools notification to go back to the flag UI. Tap “Reset all to default” and kill and restart WebView apps to go back to the default behavior.

Overriding variations/Field Trials

Like Chrome, WebView supports A/B experiments and feature rollouts through variations (AKA “field trials” or “Finch”). The flag UI can override the field trial config, either to enable an experimental feature to ensure your app works correctly, or to disable an experiment to determine if this is the root cause for a WebView behavior change breaking your app. Simply tap “Enabled” or “Disabled” in the UI; “Default” means WebView will pick up the random field trial experiment.

If you find an experiment is the root cause for app breakage, please file a bug, mention which experiment, and link to your app's Play Store page for our team to investigate.

Accelerating field trial config download

You can also use the flag UI to download new field trial configs (“seeds”) more quickly, to verify the next seed will fix app breakage. Enable all of the following:

finch-seed-expiration-age=0
finch-seed-min-update-period=0
finch-seed-min-download-period=0
finch-seed-ignore-pending-download

Restart your app, kill it, and restart it a second time. Your app should be running with the latest WebView variations seed.

Adding your flags and features to the UI

If you're intending to launch a feature in WebView or start a field trial (AKA Finch experiment), we highly encourage you to add to the list (ex. CL, CL).

Exposing your feature this way has several benefits:

This improves the manual test process. Testers can enable your feature with a button click instead of an adb command.
Typo-free: someone could mistype a flag or feature name in the commandline, but this UI ensures flag names are always spelled correctly.
Because this works on production Android devices, test team can validate your feature on devices from other OEMs.
You (and teammates) can dogfood your feature.
If users or third-party app developers report bugs, this UI is the only way they can toggle your feature to help root-cause the regression.