margin-calendar/docs/mobile.md

Android and iOS

The same Rust core and the same React app, with a different shape of chrome and a different way of getting a token back from Google. Nothing is forked: the layout switches on a data-phone attribute and the OAuth flow switches on cfg(mobile).

Signing in, and what console work buys you

Both platforms sign in out of the box with the installed client the desktop build already uses. Android is finished at that point. iOS works, but signs the user in from scratch, and one OAuth client fixes it. That asymmetry is the whole of this section.

The flow that needs nothing

A phone uses the desktop client, secret and all, and catches Google's answer on the same loopback listener the desktop flow binds. Two facts make that work: a Desktop client may redirect to loopback on any port with nothing registered in advance, and Google's token endpoint validates the client id, the secret and the redirect URI without any way of knowing which operating system is asking. Client types are policy guidance rather than a protocol check.

What used to make this impossible on a phone was not the protocol. It was that opening the consent page in Safari or Chrome sends the user to another app, iOS suspends this process, and a suspended process is not accepting on its socket, so the redirect carrying the code arrives at nobody. The consent page now opens in front of the app instead, in the system's own browser component: SFSafariViewController on iOS, a Chrome Custom Tab on Android. This app stays foreground and its listener stays live. src-tauri/src/google/browser.rs is all of it.

Still the system browser, note, and never a WebView this app owns. Google blocks that outright with disallowed_useragent, and it deserves to be blocked, because a webview the app controls can read the password typed into it. A Safari sheet or a Custom Tab is a different process with the browser's own cookies and autofill and an address bar the app cannot forge.

Be clear about the trade. Reusing a Desktop client from a phone is off Google's stated guidance, which says to create an Android or iOS client per platform. It works because the protocol does not check, not because Google blesses it. If Google ever starts enforcing the guidance the symptom will be a rejected token exchange, and the fix is a per-platform client, which the code already supports in full.

Why Android needs nothing else

A Custom Tab is Chrome. It reads Chrome's cookie jar, so an account already signed in there is offered by name and there is no password to type. Loopback and shared session at once, for nothing.

Why iOS wants an ios client

SFSafariViewController has not shared cookies with Safari since iOS 11: every app gets its own storage. So the no-setup iOS flow puts a real Safari view in front of the user with an empty cookie jar behind it, and Google has no idea who they are. It signs in. It just asks for a full login every time the token store is emptied.

ASWebAuthenticationSession is the API Apple shipped for exactly this, and it is the only one that shares Safari's session, which is why iOS puts up its own "wants to use google.com to sign in" prompt before it opens. But it intercepts a custom scheme and nothing else, never an http loopback redirect, so on iOS a shared session and loopback are mutually exclusive. Adding an ios block is what buys the scheme, and with it the good version.

So auth.rs runs three flows, not two: loopback everywhere by default, ASWebAuthenticationSession plus the custom scheme when there is an ios client, and the external browser plus the deep link when there is an android one. load_credentials decides once, by whether the block is in the file.

An iOS client is about a minute of work: it wants the bundle identifier and nothing else, no fingerprint, no keystore. An Android client wants a SHA-1 per signing key and buys nothing, so there is no reason to make one unless Google forces the issue.

One thing observed rather than assumed, and worth knowing before anyone re-tests this on a simulator: a cookie set in the simulator's Safari showed up in neither surface, including ASWebAuthenticationSession after its sharing prompt was accepted. The prompt appears and the plumbing is right, so this looks like the simulator not backing the shared jar rather than the API. Judge the session sharing on a real device.

Creating the per-platform clients

Both mobile clients are public: no client secret exists, and PKCE is the only thing between an intercepted authorization code and a token. That is why the verifier is not optional anywhere in auth.rs.

In the Google Cloud console, on the same project that already has the Calendar API enabled:

1. Create an OAuth client of type Android. It wants the package name, which is studio.margin.calendar, and the SHA-1 fingerprint of the certificate that signs the build. For a debug build that is the shared debug keystore:

   keytool -list -v -keystore ~/.android/debug.keystore \
     -alias androiddebugkey -storepass android -keypass android
   

   A release build is signed with a different key and needs its fingerprint added too. An APK signed by a key Google has not been told about fails at consent, not at build.

2. Create an OAuth client of type iOS. It wants the bundle identifier, which is also studio.margin.calendar.

3. Put the client ids in google-credentials.json alongside the desktop one, matching google-credentials.example.json:

   {
     "installed": { "client_id": "...", "client_secret": "...", "...": "..." },
     "android": { "client_id": "YOUR_ANDROID_CLIENT_ID.apps.googleusercontent.com" },
     "ios": { "client_id": "YOUR_IOS_CLIENT_ID.apps.googleusercontent.com" }
   }
   

   The file is gitignored and embedded at build time. Adding a block for one platform leaves the other on loopback: the choice is per platform, not per file.

A refresh token belongs to the client that obtained it, so adding or removing a block invalidates whatever is already stored on that device. Disconnect and reconnect the account afterwards.

The redirect schemes

These matter only for the flow above. The loopback flow redirects to http://127.0.0.1:PORT and never touches a scheme.

Android redirects to studio.margin.calendar:/oauth2redirect. That is the package name, which is Google's documented form for an Android client, and being known at build time means it can sit in AndroidManifest.xml permanently rather than being pasted in per install.

iOS gets no such choice. Google requires the reversed client id, so the scheme is only knowable once your client id is: take the client id, drop the .apps.googleusercontent.com suffix, and prefix com.googleusercontent.apps..

Register it in src-tauri/tauri.conf.json, as a second entry in the deep link plugin's scheme list:

"deep-link": {
  "desktop": { "schemes": ["studio.margin.calendar"] },
  "mobile": [{ "scheme": ["studio.margin.calendar", "com.googleusercontent.apps.YOUR_ID"] }]
}

Not in Info.plist. Editing that by hand looks like it works and then silently stops working: the plugin's build script rewrites CFBundleURLTypes wholesale from this config on every build, and when the mobile array is empty it deletes the key outright. That one empty array is why the callback was dead on both platforms at first, so if a deep link ever stops arriving, look here before anywhere else.

The studio.margin.calendar scheme stays registered on both platforms whether or not anything uses it, because the OS is told about a scheme at install time and cannot be told about one later.

If the console shows you something different from either default, put it in the client's block in google-credentials.json as redirect_uri and it wins over both. Whatever you put there still has to have its scheme registered above, or the OS has no reason to hand the link to this app.

The consent browser, and taking it away again

Neither surface closes itself once the redirect has landed, so browser.rs dismisses both. What is on screen by then is the listener's own "you can close this" page, and leaving it up would look like a sign-in that hung while the token exchange quietly succeeded behind it.

On iOS that is a dismissViewControllerAnimated: on the main thread. SFSafariViewController has no objc2 binding, since objc2-safari-services covers only the macOS extension API, so the class is reached by name and the SafariServices framework is linked by hand to put it in the process at all. The controller and its delegate are both retained for the length of the flow: UIKit holds a delegate weakly, and a controller nothing retains deallocates mid-sign-in.

On Android a Custom Tab belongs to Chrome and cannot be closed by the app that launched it. What works instead is starting MainActivity with FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_SINGLE_TOP, which brings this app back to the front of the task the tab was launched into and pops the tab off on the way. Same move AppAuth makes.

Closing the browser by hand is the other exit, and it has to be noticed or the accounts panel waits on a sign-in that will never arrive. iOS gets it directly from safariViewControllerDidFinish:. Android has no such callback, so it reads a process resume while a consent attempt is in flight, which means the tab is gone: the tab is in this app's own task, and dismissal is the only way out of it. Either way await_code sees the flag on its next poll and gives up.

ASWebAuthenticationSession needs none of that. It takes itself away when the callback scheme matches and reports a cancel as error code 1, so the completion handler is the whole of it. What it does need is to be retained: a session nothing holds deallocates and then never calls back, which is the classic way to lose an afternoon to that API, and its presentation context provider is held weakly so it goes the same way. Both are kept in browser.rs until the next attempt replaces them, rather than being released inside the completion handler that the session itself owns.

One thing the Rust side cannot do anything about: a cancel still reaches the accounts panel as a failed connect, because AuthEvent has no third shape, so it reads as "Sign-in was cancelled." under a toast rather than as nothing at all.

Toolchain

iOS needs Xcode and the iOS Rust targets:

rustup target add aarch64-apple-ios aarch64-apple-ios-sim x86_64-apple-ios

Android needs the SDK, an NDK of r25 or newer, a JDK that the generated Gradle build accepts (21 works, 25 does not), and the four Android Rust targets:

rustup target add aarch64-linux-android armv7-linux-androideabi i686-linux-android x86_64-linux-android
export ANDROID_HOME=/opt/homebrew/share/android-commandlinetools
export NDK_HOME=$ANDROID_HOME/ndk/27.2.12479018
export JAVA_HOME=/opt/homebrew/opt/openjdk@21

Building

pnpm tauri ios init          # once, generates the Xcode project
pnpm tauri ios dev           # simulator, with the Vite dev server
pnpm tauri ios build

pnpm tauri android init      # once, generates the Gradle project
pnpm tauri android dev       # emulator or attached device
pnpm tauri android build

The generated projects are committed, and three files in the Android one are edited by hand. MainActivity.kt publishes the window insets and launches the consent tab, app/build.gradle.kts carries the androidx.browser dependency the tab needs, and AndroidManifest.xml carries a <queries> element without which Android 11 and up hide every browser from CustomTabsClient.getPackageName and the consent page quietly falls back to a separate browser app. Re-running android init overwrites all three and nothing else supplies any of them, so check for them afterwards: without the insets the bars overlap the page, and without the bridge tapping Connect opens nothing at all.

Both dev commands start Vite themselves and fail outright if port 1430 is already taken, which it will be if a browser dev server is still up. Kill it, or pass a different port through --config.

Three failures that look like bugs and are not. Xcode refusing to build with "Entitlements file was modified during the build" is a stale mtime in DerivedData, fixed once by deleting ~/Library/Developer/Xcode/DerivedData/margin-calendar-*. An Android run panicking with "failed to build WebSocket client, Connection refused" is a stale $TMPDIR/studio.margin.calendar-server-addr pointing at a dead port; delete it. And an emulator that will not boot by name usually means avdmanager and emulator disagree about where AVDs live, which ANDROID_AVD_HOME settles.

Finally, an empty calendar on a device is correct. The browser fixture is gated on not being inside Tauri, so on a phone the real backend answers and there is nothing to show until an account is connected. Events without signing in only ever happen in a browser.

What is different on a phone

The desktop header carries three groups of controls across one row, which does not fit in 390 points. Under data-phone it becomes a top bar with the date and the day arrows and a bottom tab bar with the views, and everything the trailing icon row used to hold moves into an overflow sheet. Overlays become bottom sheets. Both bars pad themselves out of the way of the notch and the home indicator with env(safe-area-inset-*).

Under data-touch, which a tablet gets and a narrow desktop window does not, interaction changes rather than layout. Anything that only appeared on hover is always visible instead, because a finger cannot hover. Dragging out a new event waits for a long press, because on a touchscreen the alternative is that every tap on an empty afternoon starts creating something.

Navigation still moves one day at a time. A swipe is one day, not one week.

Both attributes are also set by the boot script in index.html before first paint, so a phone does not render the desktop layout for a frame and then jump.

Keeping out from under the system bars

--safe-top and --safe-bottom are env(safe-area-inset-*) by default, which is correct on iOS and wrong on Android in a way that is easy to miss. Android's WebView derives those values from the display cutout alone and never from the system bars, so it reports 0 at the bottom while the navigation bar really occupies 24dp of gesture pill or 48dp of buttons, and the tab bar renders underneath it. targetSdk is 36, so edge-to-edge is mandatory and there is nothing to opt out of. The top only looked right on a test device by luck, because that device had a cutout; a phone without one would have tucked the top bar under the status bar for the same reason.

So Android measures the bars natively. MainActivity.kt reads systemBars() or displayCutout() from WindowInsetsCompat, exposes them over a JavaScript bridge, and re-fires on every inset change, which covers rotation, the keyboard, and switching between gesture and button navigation live. src/safeArea.ts converts device pixels to CSS pixels and writes the two variables onto the root, where they beat the env() defaults. Off Android the bridge is simply absent and the defaults stand, so iOS and desktop are untouched.

Two consequences worth knowing. tauri android init regenerates MainActivity.kt, and nothing else on Android supplies these values, so check the bridge is still there after re-running it. And installing the inset listener makes env(safe-area-inset-*) read 0 inside that WebView, which does not matter only because those two token lines are the sole consumers and the bridge overrides both with better numbers. Anything new that reaches for env() directly on Android will get zero.

iOS needed one line of native code for the same class of problem, in the opposite direction.

UIKit hands a scroll view the safe areas as content insets unless told otherwise, and wry never tells it otherwise: it touches the scroll view only to switch bounces off. WebKit then lays the page out in what is left. On an iPhone 17 Pro that meant a layout viewport 778pt tall against an 874pt screen, still anchored at y 0, so the bottom 96pt of the display was outside the page altogether and showed as a dead band of shell colour under the tab bar. body is position: fixed, so nothing could paint down there whatever the stylesheet said.

stop_uikit_shrinking_the_viewport in lib.rs sets contentInsetAdjustmentBehavior to never through with_webview, which gives the page the whole screen back. The insets are not lost, they arrive as env(safe-area-inset-*) instead, which is where both bars already read them from.

Worth knowing before anyone tries to solve that in CSS, because it looks like a CSS problem and the obvious fix is worse than the bug. While the viewport was short, 100dvh was the only unit reporting the real box; vh, svh and lvh all reported the full screen. Sizing the root 100svh therefore did not reclaim the bottom of the screen, it laid the tab bar out past the clip where it was invisible and, less obviously, untappable. The root is height: 100%, which inherits whatever the box is and cannot drift if the native line ever regresses.

Limits worth knowing

There is no auto-updater and no process restart on mobile: the store is the update channel, and both plugins are compiled out rather than merely hidden.

The initial sync pulls the whole calendar history, because Google forbids timeMin alongside a syncToken. That is already the desktop behaviour and it is slower on a phone radio.