update checker: finds new releases, downloads and installs them

Checks GitHub's latest release once a day when the app opens and after a
drive. A new version shows as a card on Home, a notification after the
drive, and a card on the car screen at plug-in while parked. Downloads are
checked against GitHub's SHA-256 and never install while the car is
connected. Update notifications can be turned off in Settings.

gearslip/build.gradle.kts

@@ -33,7 +33,8 @@ android {
         minSdk = 31
         targetSdk = 37
         versionCode = 8
-        versionName = "0.2.00-alpha"
+        // -PgearslipVersionName=0.1.99-test makes a build that thinks it's older, to try the updater.
+        versionName = project.findProperty("gearslipVersionName")?.toString() ?: "0.2.00-alpha"
         manifestPlaceholders["appLabel"] = "Gearslip"
         // Where opted-in phones send their daily usage note: the stats-worker/ Worker's address,
         // set as gearslipStatsUrl in gradle.properties. Left empty, nothing is ever sent.

gearslip/src/main/AndroidManifest.xml

@@ -4,6 +4,7 @@
 
     <!-- The projected WebView loads real pages; without this every load fails silently. -->
     <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
 
     <!--
       Phone mirroring. Android refuses to hand over a MediaProjection unless a foreground

gearslip/src/main/java/app/seb3thehacker/gearslip/GearslipActivity.kt

@@ -152,6 +152,7 @@ class GearslipActivity : ComponentActivity(), Projection {
         super.onStart()
         Prefetch.warm(this)
         UsageStats.onAppOpened(this)
+        app.seb3thehacker.gearslip.update.UpdateChecker.onAppOpened(this)
     }
 
     override fun onNewIntent(intent: Intent) {

gearslip/src/main/java/app/seb3thehacker/gearslip/SessionReport.kt

@@ -146,6 +146,7 @@ object SessionReport {
                 else -> category.label
             }
             runCatching { UsageStats.recordSession(context, headUnitInfo, protocolVersion, outcome, carScreen, carDpi) }
+            runCatching { app.seb3thehacker.gearslip.update.UpdateChecker.onDriveEnded(context) }
         }
         GearslipLog.flush()
     }

gearslip/src/main/java/app/seb3thehacker/gearslip/car/CarUi.kt

@@ -126,6 +126,8 @@ fun CarUi() {
     // a video in the web browser or a map's voice needs it too. Asks once, after the car offers
     // somewhere to play it.
     LaunchedEffect(Unit) { if (CarSettings.pipeAudio.value) app.seb3thehacker.gearslip.audio.CarAudio.request() }
+    // A new version gets one card in the top right, a few seconds in and only while parked.
+    LaunchedEffect(Unit) { app.seb3thehacker.gearslip.update.UpdateChecker.onCarConnected(appContext) }
     // The map follows the light outside, whatever the app's own theme is set to.
     LaunchedEffect(darkOutside) { CarServices.nav.pushConfiguration() }
 

gearslip/src/main/java/app/seb3thehacker/gearslip/ui/HomeScreen.kt

@@ -91,6 +91,7 @@ fun HomeScreen(
                 Spacer(Modifier.height(16.dp))
                 StatusCard(status)
                 Spacer(Modifier.height(16.dp))
+                UpdateCard()
             }
 
             // Sits at the bottom, where a thumb reaches it.

gearslip/src/main/java/app/seb3thehacker/gearslip/ui/SettingsScreen.kt

@@ -106,7 +106,7 @@ private fun Arrow() = Icon(
     tint = MaterialTheme.colorScheme.onSurfaceVariant,
 )
 
-/** What's new and the setup guide, one tap from the top. */
+/** What's new, updates and the setup guide, one tap from the top. */
 @Composable
 private fun AboutSection(onOpenWhatsNew: () -> Unit, onReplayTutorial: () -> Unit) {
     SettingsSection {
@@ -118,6 +118,10 @@ private fun AboutSection(onOpenWhatsNew: () -> Unit, onReplayTutorial: () -> Uni
             trailing = { Arrow() },
         )
         SettingsDivider()
+        UpdateRow()
+        SettingsDivider()
+        UpdateNotifyRow()
+        SettingsDivider()
         SettingsRow(
             "Replay the setup guide",
             Modifier.clickable(onClick = onReplayTutorial),

gearslip/src/main/java/app/seb3thehacker/gearslip/ui/UpdateViews.kt

@@ -0,0 +1,146 @@
+package app.seb3thehacker.gearslip.ui
+
+import android.content.Intent
+import android.net.Uri
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.selection.toggleable
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.CheckCircle
+import androidx.compose.material.icons.filled.Notifications
+import androidx.compose.material3.Button
+import androidx.compose.material3.LinearProgressIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Switch
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.LocalContext
+import androidx.compose.ui.semantics.Role
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import app.seb3thehacker.gearslip.BuildConfig
+import app.seb3thehacker.gearslip.SessionStatus
+import app.seb3thehacker.gearslip.update.UpdateChecker
+import app.seb3thehacker.gearslip.update.UpdateChecker.State
+
+private const val UNPLUG_FIRST = "Unplug from the car to install. Installing closes Gearslip."
+
+/** Shown on Home once a newer version is found; hidden otherwise. */
+@Composable
+fun UpdateCard() {
+    val context = LocalContext.current
+    val state by UpdateChecker.state.collectAsStateWithLifecycle()
+    SessionStatus.state.collectAsStateWithLifecycle().value // redraw when the car comes or goes
+    val release = when (val s = state) {
+        is State.Available -> s.release
+        is State.Downloading -> s.release
+        is State.Ready -> s.release
+        is State.Failed -> s.release
+        else -> null
+    } ?: return
+
+    Surface(
+        color = MaterialTheme.colorScheme.tertiaryContainer,
+        contentColor = MaterialTheme.colorScheme.onTertiaryContainer,
+        shape = MaterialTheme.shapes.extraLarge,
+    ) {
+        Column(Modifier.fillMaxWidth().padding(20.dp)) {
+            Text("Gearslip ${release.version} is out", style = MaterialTheme.typography.titleLarge, fontWeight = FontWeight.SemiBold)
+            Spacer(Modifier.height(4.dp))
+            when (val s = state) {
+                is State.Downloading -> {
+                    Text("Downloading… ${(s.fraction * 100).toInt()}%", style = MaterialTheme.typography.bodyMedium)
+                    Spacer(Modifier.height(12.dp))
+                    LinearProgressIndicator(progress = { s.fraction }, modifier = Modifier.fillMaxWidth())
+                }
+                is State.Ready -> {
+                    val connected = UpdateChecker.carConnected
+                    Text(
+                        if (connected) UNPLUG_FIRST else "Downloaded. Android asks before it installs.",
+                        style = MaterialTheme.typography.bodyMedium,
+                    )
+                    Spacer(Modifier.height(12.dp))
+                    Button(onClick = { UpdateChecker.install(context) }, enabled = !connected) { Text("Install") }
+                }
+                else -> {
+                    Text(
+                        (s as? State.Failed)?.message ?: "Download it now. Android asks before it installs.",
+                        style = MaterialTheme.typography.bodyMedium,
+                    )
+                    Spacer(Modifier.height(12.dp))
+                    Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                        Button(onClick = { UpdateChecker.download(context, release) }) {
+                            Text(if (s is State.Failed) "Try again" else "Update")
+                        }
+                        if (release.pageUrl.isNotBlank()) {
+                            TextButton(onClick = {
+                                context.startActivity(Intent(Intent.ACTION_VIEW, Uri.parse(release.pageUrl)))
+                            }) { Text("What's new") }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+/** The Settings row: checks on tap, and carries on to download and install once something is found. */
+@Composable
+internal fun UpdateRow() {
+    val context = LocalContext.current
+    val state by UpdateChecker.state.collectAsStateWithLifecycle()
+    SessionStatus.state.collectAsStateWithLifecycle().value
+    val subtitle = when (val s = state) {
+        State.Idle -> if (BuildConfig.DEBUG) "Dev builds check only when you tap" else "Gearslip checks once a day"
+        State.Checking -> "Checking…"
+        State.UpToDate -> "You have the latest version"
+        is State.Available -> "Version ${s.release.version} is out. Tap to download."
+        is State.Downloading -> "Downloading… ${(s.fraction * 100).toInt()}%"
+        is State.Ready -> if (UpdateChecker.carConnected) UNPLUG_FIRST else "Tap to install version ${s.release.version}"
+        is State.Failed -> s.message
+    }
+    SettingsRow(
+        "Check for updates",
+        Modifier.clickable {
+            when (val s = state) {
+                is State.Available -> UpdateChecker.download(context, s.release)
+                is State.Ready -> UpdateChecker.install(context)
+                is State.Failed -> s.release?.let { UpdateChecker.download(context, it) } ?: UpdateChecker.check(context)
+                else -> UpdateChecker.check(context)
+            }
+        },
+        subtitle = subtitle,
+        icon = Icons.Filled.CheckCircle,
+    )
+}
+
+/** Whether a drive's end may notify about a new version. */
+@Composable
+internal fun UpdateNotifyRow() {
+    val context = LocalContext.current
+    var on by remember { mutableStateOf(UpdateChecker.notifyEnabled(context)) }
+    SettingsRow(
+        "Update notifications",
+        Modifier.toggleable(on, role = Role.Switch) {
+            on = it
+            UpdateChecker.setNotifyEnabled(context, it)
+        },
+        subtitle = "On the car screen when you plug in, and on the phone after a drive",
+        icon = Icons.Filled.Notifications,
+        trailing = { Switch(checked = on, onCheckedChange = null) },
+    )
+}

gearslip/src/main/java/app/seb3thehacker/gearslip/update/UpdateChecker.kt

@@ -0,0 +1,374 @@
+package app.seb3thehacker.gearslip.update
+
+import android.Manifest
+import android.app.NotificationChannel
+import android.app.NotificationManager
+import android.app.PendingIntent
+import android.content.Context
+import android.content.Intent
+import android.content.pm.PackageManager
+import android.net.Uri
+import android.provider.Settings
+import androidx.core.app.NotificationCompat
+import androidx.core.app.NotificationManagerCompat
+import androidx.core.content.FileProvider
+import androidx.core.graphics.drawable.toBitmap
+import app.seb3thehacker.gearslip.BuildConfig
+import app.seb3thehacker.gearslip.GearslipActivity
+import app.seb3thehacker.gearslip.GearslipLog
+import app.seb3thehacker.gearslip.SessionStatus
+import app.seb3thehacker.gearslip.notify.CarMotion
+import app.seb3thehacker.gearslip.notify.CarNotification
+import app.seb3thehacker.gearslip.notify.CarNotifications
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import org.json.JSONObject
+import java.io.File
+import java.net.HttpURLConnection
+import java.net.URL
+import java.security.MessageDigest
+
+/**
+ * Looks for a newer Gearslip on GitHub and installs it through Android's own installer.
+ *
+ * Only GitHub's "latest" release counts, which leaves out pre-releases and drafts. The app checks
+ * at most once a day: when it opens, and when a drive ends. After a drive a new version also gets
+ * a notification, once per version, since the driver is parked and the app is likely closed. The
+ * car screen shows the same news once per version, at plug-in and only while parked.
+ * Nothing runs in the background otherwise, and nothing else is sent: the request is the same one
+ * a browser makes for the release page. Dev builds never check on their own.
+ *
+ * The APK is checked against the size and SHA-256 GitHub lists for it, and Android refuses an
+ * update signed with a different key, so a bad download can't replace the app.
+ */
+object UpdateChecker {
+
+    data class Release(val version: String, val apkUrl: String, val size: Long, val sha256: String?, val pageUrl: String)
+
+    sealed interface State {
+        data object Idle : State
+        data object Checking : State
+        data object UpToDate : State
+        data class Available(val release: Release) : State
+        data class Downloading(val release: Release, val fraction: Float) : State
+        data class Ready(val release: Release, val apk: File) : State
+        data class Failed(val message: String, val release: Release? = null) : State
+    }
+
+    private val flow = MutableStateFlow<State>(State.Idle)
+    val state: StateFlow<State> = flow.asStateFlow()
+
+    private const val PREFS = "updates"
+    private const val KEY_CHECKED_AT = "checked_at"
+    private const val KEY_FOUND = "found"
+    private const val KEY_NOTIFY = "notify"
+    private const val KEY_NOTIFIED = "notified_version"
+    private const val KEY_CAR_NOTIFIED = "car_notified_version"
+    /** Lets the car screen settle after plug-in, so the card isn't lost behind the first frames. */
+    private const val CAR_DELAY_MS = 8_000L
+    private const val CHANNEL = "updates"
+    private const val NOTIFICATION_ID = 4100
+    private const val DAY_MS = 24 * 60 * 60 * 1000L
+    private const val LATEST = "https://api.github.com/repos/Seb3thehacker/gearslip/releases/latest"
+
+    /** Set when the driver tapped Install but first had to allow installs from Gearslip. */
+    @Volatile private var installWhenAllowed = false
+
+    private fun prefs(context: Context) = context.applicationContext.getSharedPreferences(PREFS, Context.MODE_PRIVATE)
+
+    /** Whether a drive's end may post a notification about a new version. On by default: it sends nothing. */
+    fun notifyEnabled(context: Context): Boolean = prefs(context).getBoolean(KEY_NOTIFY, true)
+
+    fun setNotifyEnabled(context: Context, on: Boolean) {
+        prefs(context).edit().putBoolean(KEY_NOTIFY, on).apply()
+        if (!on) NotificationManagerCompat.from(context).cancel(NOTIFICATION_ID)
+    }
+
+    private fun dueForCheck(context: Context) =
+        !BuildConfig.DEBUG && System.currentTimeMillis() - prefs(context).getLong(KEY_CHECKED_AT, 0) >= DAY_MS
+
+    /** Gearslip came to the front: check if a day has passed, and finish an install that waited on permission. */
+    fun onAppOpened(context: Context) {
+        val current = flow.value
+        if (installWhenAllowed && current is State.Ready && context.packageManager.canRequestPackageInstalls()) {
+            installWhenAllowed = false
+            install(context)
+            return
+        }
+        if (current != State.Idle) return
+        // A release found earlier still shows after Android closed the app.
+        savedRelease(context)?.let { flow.value = State.Available(it) }
+        if (dueForCheck(context)) check(context)
+    }
+
+    /**
+     * A car session ended, so the driver is parked. Checks if a day has passed, then posts a
+     * notification for a new version they haven't been told about yet.
+     */
+    fun onDriveEnded(context: Context) {
+        if (!notifyEnabled(context)) return
+        val app = context.applicationContext
+        Thread({
+            val current = flow.value
+            if (dueForCheck(app) && current !is State.Checking && current !is State.Downloading) {
+                flow.value = State.Checking
+                flow.value = checkNow(app)
+            }
+            val release = foundRelease(app) ?: return@Thread
+            if (prefs(app).getString(KEY_NOTIFIED, null) == release.version) return@Thread
+            if (notify(app, release)) prefs(app).edit().putString(KEY_NOTIFIED, release.version).apply()
+        }, "gearslip-update").start()
+    }
+
+    /**
+     * The car screen just came up. A new version gets a card in the top right, like a message,
+     * once per version. Skipped while moving; the next plug-in tries again.
+     */
+    fun onCarConnected(context: Context) {
+        if (!notifyEnabled(context)) return
+        val app = context.applicationContext
+        Thread({
+            val current = flow.value
+            if (dueForCheck(app) && current !is State.Checking && current !is State.Downloading) {
+                flow.value = State.Checking
+                flow.value = checkNow(app)
+            }
+            val release = foundRelease(app) ?: return@Thread
+            if (prefs(app).getString(KEY_CAR_NOTIFIED, null) == release.version) return@Thread
+            Thread.sleep(CAR_DELAY_MS)
+            if (!carConnected || CarMotion.moving) return@Thread
+            CarNotifications.post(
+                CarNotification(
+                    key = "gearslip-update",
+                    id = CarNotifications.nextId(),
+                    packageName = app.packageName,
+                    appLabel = "Gearslip",
+                    title = "Gearslip ${release.version} is out",
+                    text = "Update in the Gearslip app or from GitHub",
+                    postedAt = System.currentTimeMillis(),
+                    icon = runCatching { app.packageManager.getApplicationIcon(app.packageName).toBitmap(96, 96) }.getOrNull(),
+                    reply = null,
+                ),
+                quiet = false,
+            )
+            prefs(app).edit().putString(KEY_CAR_NOTIFIED, release.version).apply()
+            GearslipLog.i("update: showed ${release.version} on the car screen")
+        }, "gearslip-update").start()
+    }
+
+    private fun foundRelease(app: Context): Release? = when (val s = flow.value) {
+        is State.Available -> s.release
+        is State.Downloading -> s.release
+        is State.Ready -> s.release
+        else -> savedRelease(app)
+    }
+
+    /** Posts "Gearslip X is out"; tapping it opens the app, where the Home card has the button. */
+    private fun notify(context: Context, release: Release): Boolean {
+        if (context.checkSelfPermission(Manifest.permission.POST_NOTIFICATIONS) != PackageManager.PERMISSION_GRANTED) return false
+        context.getSystemService(NotificationManager::class.java).createNotificationChannel(
+            NotificationChannel(CHANNEL, "Updates", NotificationManager.IMPORTANCE_DEFAULT),
+        )
+        val open = PendingIntent.getActivity(
+            context, NOTIFICATION_ID,
+            Intent(context, GearslipActivity::class.java).addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TOP),
+            PendingIntent.FLAG_IMMUTABLE,
+        )
+        val notification = NotificationCompat.Builder(context, CHANNEL)
+            .setSmallIcon(android.R.drawable.stat_sys_download_done)
+            .setContentTitle("Gearslip ${release.version} is out")
+            .setContentText("Tap to update.")
+            .setContentIntent(open)
+            .setAutoCancel(true)
+            .build()
+        return runCatching { NotificationManagerCompat.from(context).notify(NOTIFICATION_ID, notification) }
+            .onSuccess { GearslipLog.i("update: told the driver about ${release.version}") }
+            .onFailure { GearslipLog.w("update: could not post the notification (${it.message})") }
+            .isSuccess
+    }
+
+    /** Asks GitHub now. Ignored while a check or download is under way. */
+    fun check(context: Context) {
+        val current = flow.value
+        if (current is State.Checking || current is State.Downloading) return
+        val app = context.applicationContext
+        flow.value = State.Checking
+        Thread({ flow.value = checkNow(app) }, "gearslip-update").start()
+    }
+
+    private fun checkNow(app: Context): State = runCatching { fetchLatest() }.fold(
+        onSuccess = { release ->
+            prefs(app).edit().putLong(KEY_CHECKED_AT, System.currentTimeMillis()).apply()
+            if (release != null && isNewer(release.version, BuildConfig.VERSION_NAME)) {
+                GearslipLog.i("update: ${release.version} is out (this is ${BuildConfig.VERSION_NAME})")
+                save(app, release)
+                State.Available(release)
+            } else {
+                prefs(app).edit().remove(KEY_FOUND).apply()
+                updatesDir(app).deleteRecursively()
+                State.UpToDate
+            }
+        },
+        onFailure = {
+            GearslipLog.w("update: could not check: ${it.message}")
+            State.Failed("Couldn't reach GitHub. Check the connection and try again.")
+        },
+    )
+
+    /** Downloads [release], then opens the installer once the file checks out. */
+    fun download(context: Context, release: Release) {
+        if (flow.value is State.Downloading) return
+        val app = context.applicationContext
+        flow.value = State.Downloading(release, 0f)
+        Thread({
+            val dir = updatesDir(app).apply { deleteRecursively(); mkdirs() }
+            val apk = File(dir, "Gearslip-${release.version}.apk")
+            flow.value = runCatching {
+                fetchApk(release, apk)
+                GearslipLog.i("update: downloaded ${release.version}")
+                State.Ready(release, apk)
+            }.getOrElse {
+                apk.delete()
+                GearslipLog.w("update: download failed: ${it.message}")
+                State.Failed(it.message ?: "The download failed. Try again.", release)
+            }
+            if (flow.value is State.Ready) install(app)
+        }, "gearslip-update").start()
+    }
+
+    /**
+     * Hands the downloaded APK to Android's installer. The first time, Android wants the driver to
+     * allow installs from Gearslip; that opens its settings page, and the install carries on when
+     * they come back.
+     */
+    fun install(context: Context) {
+        val ready = flow.value as? State.Ready ?: return
+        // Installing closes Gearslip, which would drop the car screen mid-drive.
+        if (carConnected) return
+        val pm = context.packageManager
+        if (!pm.canRequestPackageInstalls()) {
+            installWhenAllowed = true
+            context.startActivity(
+                Intent(Settings.ACTION_MANAGE_UNKNOWN_APP_SOURCES, Uri.parse("package:${context.packageName}"))
+                    .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK),
+            )
+            return
+        }
+        val uri = FileProvider.getUriForFile(context, "${context.packageName}.logshare", ready.apk)
+        context.startActivity(
+            Intent(Intent.ACTION_VIEW)
+                .setDataAndType(uri, "application/vnd.android.package-archive")
+                .addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION or Intent.FLAG_ACTIVITY_NEW_TASK),
+        )
+    }
+
+    private fun fetchLatest(): Release? {
+        val conn = (URL(LATEST).openConnection() as HttpURLConnection).apply {
+            connectTimeout = 15_000
+            readTimeout = 15_000
+            setRequestProperty("Accept", "application/vnd.github+json")
+            setRequestProperty("User-Agent", "Gearslip/${BuildConfig.VERSION_NAME}")
+        }
+        try {
+            if (conn.responseCode == 404) return null // no stable release yet
+            if (conn.responseCode !in 200..299) error("GitHub answered ${conn.responseCode}")
+            val json = JSONObject(conn.inputStream.bufferedReader().use { it.readText() })
+            val assets = json.optJSONArray("assets") ?: return null
+            val apk = (0 until assets.length()).map { assets.getJSONObject(it) }
+                .firstOrNull { it.optString("name").endsWith(".apk") && !it.optString("name").contains("debug") }
+                ?: return null
+            return Release(
+                version = json.getString("tag_name").removePrefix("v"),
+                apkUrl = apk.getString("browser_download_url"),
+                size = apk.optLong("size", -1),
+                sha256 = apk.optString("digest").takeIf { it.startsWith("sha256:") }?.removePrefix("sha256:"),
+                pageUrl = json.optString("html_url"),
+            )
+        } finally {
+            conn.disconnect()
+        }
+    }
+
+    private fun fetchApk(release: Release, into: File) {
+        val conn = (URL(release.apkUrl).openConnection() as HttpURLConnection).apply {
+            connectTimeout = 15_000
+            readTimeout = 30_000
+            setRequestProperty("User-Agent", "Gearslip/${BuildConfig.VERSION_NAME}")
+        }
+        try {
+            if (conn.responseCode !in 200..299) error("GitHub answered ${conn.responseCode}")
+            val total = release.size.takeIf { it > 0 } ?: conn.contentLengthLong
+            val digest = MessageDigest.getInstance("SHA-256")
+            var done = 0L
+            var shown = -1
+            conn.inputStream.use { input ->
+                into.outputStream().use { output ->
+                    val buffer = ByteArray(64 * 1024)
+                    while (true) {
+                        val n = input.read(buffer)
+                        if (n < 0) break
+                        output.write(buffer, 0, n)
+                        digest.update(buffer, 0, n)
+                        done += n
+                        // Whole percents only, so the screen isn't redrawn for every buffer.
+                        val percent = if (total > 0) (done * 100 / total).toInt() else 0
+                        if (percent != shown) {
+                            shown = percent
+                            flow.value = State.Downloading(release, percent / 100f)
+                        }
+                    }
+                }
+            }
+            if (release.size > 0 && done != release.size) error("The download stopped early. Try again.")
+            val hex = digest.digest().joinToString("") { "%02x".format(it) }
+            if (release.sha256 != null && !hex.equals(release.sha256, ignoreCase = true)) {
+                error("The download came out damaged. Try again.")
+            }
+        } finally {
+            conn.disconnect()
+        }
+    }
+
+    /** Installing waits until the car is unplugged. */
+    val carConnected: Boolean
+        get() = SessionStatus.state.value.phase.let { it == SessionStatus.Phase.CONNECTING || it == SessionStatus.Phase.PROJECTING }
+
+    private fun updatesDir(context: Context) = File(context.cacheDir, "updates")
+
+    private fun save(context: Context, release: Release) {
+        val json = JSONObject()
+            .put("version", release.version)
+            .put("apkUrl", release.apkUrl)
+            .put("size", release.size)
+            .put("sha256", release.sha256 ?: "")
+            .put("pageUrl", release.pageUrl)
+        prefs(context).edit().putString(KEY_FOUND, json.toString()).apply()
+    }
+
+    /** The release found last time, if it is still newer than this build (it isn't after updating). */
+    private fun savedRelease(context: Context): Release? {
+        val json = prefs(context).getString(KEY_FOUND, null)?.let { runCatching { JSONObject(it) }.getOrNull() } ?: return null
+        val release = Release(
+            version = json.optString("version"),
+            apkUrl = json.optString("apkUrl"),
+            size = json.optLong("size", -1),
+            sha256 = json.optString("sha256").ifBlank { null },
+            pageUrl = json.optString("pageUrl"),
+        )
+        return release.takeIf { isNewer(it.version, BuildConfig.VERSION_NAME) }
+    }
+
+    /** "0.2.00-alpha" beats "0.1.10-alpha": the numbers before the dash compare one by one. */
+    internal fun isNewer(candidate: String, installed: String): Boolean {
+        fun parts(v: String) = v.removePrefix("v").substringBefore('-').split('.').map { it.toIntOrNull() ?: 0 }
+        val a = parts(candidate)
+        val b = parts(installed)
+        for (i in 0 until maxOf(a.size, b.size)) {
+            val x = a.getOrElse(i) { 0 }
+            val y = b.getOrElse(i) { 0 }
+            if (x != y) return x > y
+        }
+        return false
+    }
+}

gearslip/src/main/res/xml/share_paths.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <paths>
-    <!-- Only the log copies made for sharing; nothing else in the app is exposed. -->
+    <!-- Only log copies made for sharing and downloaded updates; nothing else in the app is exposed. -->
     <cache-path name="shared_logs" path="shared/" />
+    <cache-path name="updates" path="updates/" />
 </paths>