Holtdroid WebView

Holtdroid WebView is a small Android app for testing web pages inside a WebView. It is made for Holtdroid sessions but works on any phone or emulator on its own.

It opens a page in the system WebView (or GeckoView), lets you change how the WebView is set up, and keeps remote debugging on, so every page shows up in Holtdroid’s DevTools tab and in chrome://inspect. Settings are grouped into presets on the start screen, and everything — presets, single settings and actions like resize or freeze — can be driven from adb while the page is open.

Install

Releases are on GitHub:

File	What
holtdroid-webview-<version>.apk	WebView and Custom Tabs, about 2 MB, any ABI
holtdroid-webview-gecko-<version>-x86_64.apk	plus GeckoView, for emulators (Holtdroid sessions)
holtdroid-webview-gecko-<version>-arm64-v8a.apk	plus GeckoView, for phones

Both builds use the package name io.github.holtdroid.webview, so installing one replaces the other. In Holtdroid, upload the APK or publish it into a source (for example with the GitHub sync) and launch it like any other app; through ADB adb install -r holtdroid-webview-<version>.apk works too.

The start screen

• Open a page: type an address or a search and pick the preset to open it with. Recent addresses sit under the field.
• Engines: the active WebView provider with its version. Tap it to see the installed providers and copy the command that switches between them.
• Presets: tap a card to open its start address (or the address you typed) with its settings. The menu edits, duplicates, reorders, deletes a preset or makes it the default for links and adb. Tags on the card summarise what the preset changes; the risky ones are red.
• The top bar switches the theme (system, light, dark) and opens app settings: Material You colours, external control on/off, restoring the built-in presets, clearing every engine’s data.
• Documentation opens this page.

The built-in presets:

Preset	What it does
Browse	A plain browser window: toolbar, strict TLS, DevTools on
Unlocked	TLS errors ignored, mixed content, file access, popups, permissions granted, Eruda, autoplay, screen kept on
Kiosk	Full screen without browser UI, navigation locked to the start host, Back opens the controls, no zoom, popups or long press
App shell	Edge to edge like an app’s own WebView, with a floating handle
Small phone	A 360 × 640 viewport framed in the middle of the screen
Desktop site	Desktop user agent, overview viewport, landscape
Fresh start	Wipes cookies, storage, cache and history on every launch, no HTTP cache
Proxy	All traffic through 127.0.0.1:8080 with TLS errors ignored

The Proxy preset fits Holtdroid’s connected computers: run mitmproxy or Charles on port 8080 of your computer, publish that port on the Computers page and watch the traffic. User-installed CA certificates are trusted too, so a strict preset also works once the proxy’s CA is installed on the device.

The page

In Standard and Immersive mode a toolbar shows back, the address (tap to edit), the keyboard button, reload and the controls button. Edge-to-edge and Kiosk modes hide it; the floating handle (draggable) opens the controls instead. Back goes back in history and then to the start screen, or opens the controls when Back opens controls is on.

The control panel holds:

• Freeze: pauses timers, animations and rendering of the page and blocks input until you resume. Useful to inspect a transient state in DevTools.
• Hard reload: clears the HTTP cache and reloads.
• Desktop: toggles the desktop user agent and overview mode.
• Dark: switches the app theme; pages see it through prefers-color-scheme.
• Console: messages the page logged, and a prompt that evaluates JavaScript in the page.
• Eruda: injects the Eruda in-page DevTools. Eruda is bundled into the APK and injected the way DevTools runs scripts, so it works offline and on pages with a strict Content Security Policy (only its icon font may be blocked there).
• Keyboard: shows or hides the on-screen keyboard for the focused field (see below).
• Rotate and Settings: swap the frame’s width and height; edit every setting of the open page, apply it, or save it as a preset.
• Display: the display mode and the toolbar position.
• Viewport: frame presets from 320 × 568 to 1280 × 800, and the scale. The badge under a framed page shows its size in CSS pixels and its scale.
• Clear data: cache, cookies, storage or everything.
• DevTools: remote debugging on/off and the adb forward command for the page’s DevTools socket.
• Engine: WebView, GeckoView or a Custom Tab.

On-screen keyboard

Android hides the on-screen keyboard while a hardware keyboard is attached. Holtdroid’s emulators report one (hw.keyboard=yes), so tapping a field there brings up no keyboard; you type through the session’s keyboard instead. To get the on-screen one, focus the field and press the keyboard button. If the system still hides it, the app shows two fixes:

# show the on-screen keyboard next to a hardware one, for every app
adb shell settings put secure show_ime_with_hard_keyboard 1

# or let the app switch that setting on by itself when you press the button
adb shell pm grant io.github.holtdroid.webview android.permission.WRITE_SECURE_SETTINGS

The same setting is Use on-screen keyboard in the system’s physical keyboard settings, which the dialog opens.

Engines

• WebView is Android’s system WebView. Android uses one provider for every app, and only the system can change it: run adb shell cmd webviewupdate set-webview-implementation <package> (the Engines dialog copies it for each installed provider), or use Developer options → WebView implementation. Holtdroid’s device card switches it too.
• GeckoView is Firefox’s engine, built into the -gecko APK. Settings marked “WebView only” don’t apply to it; the console can’t evaluate scripts. Debug it from Firefox on a computer: about:debugging → USB devices.
• Custom Tab opens the page in the default browser’s Custom Tab, which shares that browser’s cookies. None of the app’s settings apply there.

Settings

Every setting has a key. The editor shows it under the description, and the same key works in adb commands and in the holtdroid-webview:// link. Booleans take true/false (also 1/0, on/off, yes/no), choices are case-insensitive, numbers are plain integers.

Key	Values	Default	What
url	address	https://www.google.com/	Start address
engine	webview gecko custom-tabs	webview	Engine
display	standard immersive edge kiosk	standard	Display mode
toolbar	top bottom	top	Toolbar position
handle	boolean	true	Floating handle when the toolbar is hidden
lockBack	boolean	false	Back opens the controls instead of leaving
orientation	auto portrait landscape locked	auto	Orientation lock (locked keeps the current one)
keepScreenOn	boolean	false	Keep the screen on
width, height	dp, 0 = fill	0	Viewport frame
scale	percent, 0 = fit	0	Frame scale
javaScript	boolean	true	JavaScript
domStorage	boolean	true	localStorage and sessionStorage
zoom	boolean	true	Pinch to zoom
zoomControls	boolean	false	On-screen zoom buttons
wideViewport	boolean	true	Honour <meta name=viewport>
overviewMode	boolean	false	Fit the page width to the screen
textZoom	25–400	100	Text size, percent
darkening	boolean	false	Algorithmic darkening of light pages in the dark theme
mediaAutoplay	boolean	false	Play media without a gesture
blockImages	boolean	false	Don’t load images
offline	boolean	false	Block every network load
cacheMode	default no-cache cache-first cache-only	default	HTTP cache
userAgent	default desktop custom	default	User agent
customUserAgent	text		Used with userAgent=custom
userAgentSuffix	text		Appended to the user agent
headers	Name: value lines		Extra headers on loads you start (not on subresources)
proxy	host:port or scheme://host:port		Proxy for all WebView traffic
proxyBypass	hosts, comma separated		Hosts that skip the proxy
ssl	strict ask ignore	strict	TLS certificate errors
mixedContent	never compatible always	compatible	http resources on https pages
safeBrowsing	boolean	true	Safe Browsing
cookies	boolean	true	Cookies
thirdPartyCookies	boolean	true	Third-party cookies
permissions	ask grant deny	ask	Camera, microphone and location requests
allowedHosts	hosts, comma separated		Pages may only navigate to these hosts and their subdomains
externalApps	boolean	true	Follow tel:, mailto:, intent:, market: links
popups	boolean	false	window.open and target=_blank (opened in the same view)
jsDialogs	boolean	true	alert, confirm, prompt, beforeunload
longPress	boolean	true	Text selection and context menu
fileAccess	boolean	false	file:// URLs
universalFileAccess	boolean	false	file:// pages may read anything
contentAccess	boolean	true	content:// URLs
clearOnStart	boolean	false	Wipe all data whenever the page opens
debugging	boolean	true	Remote debugging
eruda	boolean	false	Inject the bundled Eruda
console	boolean	true	Keep console messages
userScript	JavaScript		Injected at document start into every frame
hardwareAcceleration	boolean	true	Off renders in software
offscreenPreRaster	boolean	false	Draw tiles outside the viewport ahead of time

allowedHosts limits navigation the page starts; addresses you load yourself (the address bar, cmd=load) always open.

Control from adb

The app listens to two entry points:

• am start opens a page, starting the app if needed.
• am broadcast acts on the page that is open now. The app must be running. The broadcast prints the result: result=-1 means success, result=1 an error, and data carries the message or the requested value.

Every command is also logged under the HoltdroidWebView tag (adb logcat -s HoltdroidWebView).

P=io.github.holtdroid.webview

# open with the default preset
adb shell am start -n $P/.MainActivity -d https://example.com

# open with a preset (id or name) and overrides
adb shell am start -n $P/.MainActivity --es preset unlocked --es url https://example.com \
  --es display kiosk --ez keepScreenOn true

# change settings of the open page
adb shell am broadcast -a $P.CONTROL -p $P --es ssl ignore --es userAgent desktop

# resize the viewport, then back to full size
adb shell am broadcast -a $P.CONTROL -p $P --es cmd resize --es size 390x844
adb shell am broadcast -a $P.CONTROL -p $P --es cmd resize

# run JavaScript and print the result (quote the whole command for the device shell)
adb shell "am broadcast -a $P.CONTROL -p $P --es cmd eval --es js 'document.title'"

--es, --ei and --ez all work; values are converted to the setting’s type. Commands go in cmd; without one, am start opens a page and am broadcast applies the settings to the open page.

cmd	Arguments	What
open	preset, url, any setting	Open a page with a preset (default preset if omitted) and overrides
set	any setting, url	Change settings of the open page; url also loads it
load	url	Load an address in the open page
resize	width, height or size=WxH, scale	Frame the viewport; no arguments returns to full size
rotate		Swap width and height
freeze, unfreeze, toggle-freeze		Pause or resume the page
reload	hard=true	Reload, optionally clearing the HTTP cache first
back, forward, stop		Navigation
home		Close the page and return to the start screen
clear	what=cache|cookies|storage|history|all	Clear data (default all)
eval	js	Evaluate JavaScript; prints the JSON result
console	lines (default 50)	Print the latest console messages
theme	theme=system|light|dark	Switch the app theme
keyboard	show=true|false	Show, hide or toggle the on-screen keyboard for the focused field
state		Print the screen, page, viewport and every setting as JSON
presets		Print preset ids and names

Links do the same: holtdroid-webview://<cmd>?<key>=<value>&… opens the app and runs the command, for example holtdroid-webview://open?preset=kiosk&url=https%3A%2F%2Fexample.com. The app is also offered as a browser for http and https links, which then open with the default preset.

Any app on the device can send these broadcasts and links. Turn External control off in app settings to accept only plain url links.

DevTools

With debugging on, the WebView exposes the DevTools socket webview_devtools_remote_<pid>. In a Holtdroid session it appears in the DevTools target list like any other app’s WebView (see WebView DevTools). On your own computer:

adb forward tcp:9222 localabstract:webview_devtools_remote_$(adb shell pidof io.github.holtdroid.webview)

then open chrome://inspect, or http://localhost:9222/json. The control panel copies the exact command.