feat(android,ios): stop a crawl from the phones, as the desktop can

The bridge gains stop_crawl(). A running crawl watches for it and returns
without an error. Shares already read stay stored, and a queue of new
shares stops instead of going on to the next. The activity banner shows
Stop while a crawl runs, because listing a large share can take minutes
and holds the connection the player wants.

android/app/src/main/java/io/narl/protonstream/ui/AppViewModel.kt

@@ -47,6 +47,8 @@ import kotlinx.coroutines.isActive
 data class AppUiState(
     val loading: Boolean = true,
     val refreshing: Boolean = false,
+    /** A crawl is running, which the banner offers to stop. */
+    val crawling: Boolean = false,
     val query: String = "",
     val titles: List<TitleRecord> = emptyList(),
     /**
@@ -118,6 +120,12 @@ class AppViewModel(context: Context, private val workManager: WorkManager) : Vie
     private val activities = mutableListOf<Pair<Long, String>>()
     private var nextActivity = 0L
 
+    /** Crawls running now; main thread only, like [activities]. */
+    private var crawlsRunning = 0
+
+    /** Set by [stopCrawl], so a queue of shares stops rather than going on to the next. */
+    private var crawlStopped = false
+
     init {
         if (!NativeRuntime.tlsReady) {
             mutableState.update {
@@ -187,7 +195,7 @@ class AppViewModel(context: Context, private val workManager: WorkManager) : Vie
         viewModelScope.launch {
             busy("Refreshing the library…") {
                 mutableState.update { it.copy(message = null) }
-                runCatching { withContext(Dispatchers.IO) { NativeRuntime.engine().crawl(null) } }
+                runCatching { crawlShares(null) }
                     .onFailure { error -> mutableState.update { it.copy(message = error.message) } }
             }
             reloadNow()
@@ -225,6 +233,27 @@ class AppViewModel(context: Context, private val workManager: WorkManager) : Vie
         }
     }
 
+    /** Crawl one share, or all with null, with the banner offering Stop meanwhile. */
+    private suspend fun crawlShares(id: String?) {
+        crawlsRunning++
+        mutableState.update { it.copy(crawling = true) }
+        try {
+            withContext(Dispatchers.IO) { NativeRuntime.engine().crawl(id) }
+        } finally {
+            crawlsRunning--
+            mutableState.update { it.copy(crawling = crawlsRunning > 0) }
+        }
+    }
+
+    /**
+     * Stop the crawls running now. Shares already read stay in the library;
+     * the one being read keeps what it had.
+     */
+    fun stopCrawl() {
+        crawlStopped = true
+        viewModelScope.launch(Dispatchers.IO) { NativeRuntime.engine().stopCrawl() }
+    }
+
     private fun showActivity() {
         val newest = activities.lastOrNull()?.second
         mutableState.update { it.copy(activity = newest, refreshing = newest != null) }
@@ -243,7 +272,7 @@ class AppViewModel(context: Context, private val workManager: WorkManager) : Vie
             val name = mutableState.value.shares.firstOrNull { it.id == id }?.name ?: "the share"
             busy("Reading $name…") {
                 mutableState.update { it.copy(message = null) }
-                runCatching { withContext(Dispatchers.IO) { NativeRuntime.engine().crawl(id) } }
+                runCatching { crawlShares(id) }
                     .onFailure { error -> mutableState.update { it.copy(message = error.message) } }
             }
             reloadNow()
@@ -309,10 +338,12 @@ class AppViewModel(context: Context, private val workManager: WorkManager) : Vie
         if (ids.isEmpty()) return
         val before = mutableState.value.titles.map { it.key }.toSet()
         val failed = mutableListOf<String>()
+        crawlStopped = false
         for (id in ids) {
+            if (crawlStopped) break
             val name = mutableState.value.shares.firstOrNull { it.id == id }?.name ?: "the new share"
             busy("Reading $name…") {
-                runCatching { withContext(Dispatchers.IO) { NativeRuntime.engine().crawl(id) } }
+                runCatching { crawlShares(id) }
                     .onFailure { failed += "$name: ${it.message}" }
             }
         }

android/app/src/main/java/io/narl/protonstream/ui/ProtonStreamApp.kt

@@ -76,6 +76,7 @@ import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.viewmodel.compose.viewModel
 import androidx.work.WorkManager
+import io.narl.protonstream.ui.theme.QuietButton
 import io.narl.protonstream.ui.theme.accentNavigationColors
 import uniffi.pstr_android.TitleRecord
 import uniffi.pstr_android.EpisodeRecord
@@ -355,7 +356,8 @@ fun ProtonStreamApp(
             }
             ActivityBanner(
                 state.activity,
-                Modifier
+                onStop = model::stopCrawl.takeIf { state.crawling },
+                modifier = Modifier
                     .align(Alignment.BottomCenter)
                     .padding(bottom = body.calculateBottomPadding() + 8.dp),
             )
@@ -437,7 +439,7 @@ private fun MiniTransport(
  * Not dismissible and not timed — it goes when the work does.
  */
 @Composable
-private fun ActivityBanner(activity: String?, modifier: Modifier = Modifier) {
+private fun ActivityBanner(activity: String?, onStop: (() -> Unit)?, modifier: Modifier = Modifier) {
     AnimatedVisibility(
         visible = activity != null,
         modifier = modifier,
@@ -458,7 +460,10 @@ private fun ActivityBanner(activity: String?, modifier: Modifier = Modifier) {
             verticalAlignment = Alignment.CenterVertically,
         ) {
             CircularProgressIndicator(Modifier.size(18.dp), strokeWidth = 2.dp)
-            Text(shown, style = MaterialTheme.typography.bodyMedium, maxLines = 2)
+            Text(shown, style = MaterialTheme.typography.bodyMedium, maxLines = 2, modifier = Modifier.weight(1f))
+            // A crawl of a large share can take minutes, and is the viewer's
+            // to call off; what it has read so far stays.
+            onStop?.let { QuietButton(onClick = it) { Text("Stop") } }
         }
     }
 }