[app] Introduce RepoManager for managing list of repos

Later this will also be used for adding/removing repos.

libs/database/src/main/java/org/fdroid/index/RepoManager.kt

@@ -0,0 +1,67 @@
+package org.fdroid.index
+
+import kotlinx.coroutines.DelicateCoroutinesApi
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+import org.fdroid.database.FDroidDatabase
+import org.fdroid.database.Repository
+import java.util.concurrent.CountDownLatch
+import kotlin.coroutines.CoroutineContext
+
+@OptIn(DelicateCoroutinesApi::class)
+public class RepoManager @JvmOverloads constructor(
+    db: FDroidDatabase,
+    private val coroutineContext: CoroutineContext = Dispatchers.IO,
+) {
+
+    private val _repositoriesState: MutableStateFlow<List<Repository>> =
+        MutableStateFlow(emptyList())
+    public val repositoriesState: StateFlow<List<Repository>> = _repositoriesState.asStateFlow()
+
+    /**
+     * Used internally as a mechanism to wait until repositories are loaded from the DB.
+     * This happens quite fast and the load is triggered at construction time.
+     * However, in some cases like when the app got killed and restarted by the OS,
+     * code could try to access the [repositoriesState] before they've loaded.
+     */
+    private val repoCountDownLatch = CountDownLatch(1)
+
+    init {
+        // we need to load the repositories first off the UiThread, so it doesn't deadlock
+        GlobalScope.launch(coroutineContext) {
+            _repositoriesState.value = db.getRepositoryDao().getRepositories()
+            repoCountDownLatch.countDown()
+            withContext(Dispatchers.Main) {
+                // keep observing the repos from the DB
+                // and update internal cache when changes happen
+                db.getRepositoryDao().getLiveRepositories().observeForever { repositories ->
+                    _repositoriesState.value = repositories
+                }
+            }
+        }
+    }
+
+    /**
+     * This method will block the current thread in the rare case
+     * that the repositories have not been loaded from the DB.
+     */
+    public fun getRepository(repoId: Long): Repository? {
+        repoCountDownLatch.await()
+        return repositoriesState.value.firstOrNull { repo -> repo.repoId == repoId }
+    }
+
+    /**
+     * This method will block the current thread in the rare case
+     * that the repositories have not been loaded from the DB.
+     */
+    public fun getRepositories(): List<Repository> {
+        repoCountDownLatch.await()
+        return repositoriesState.value
+    }
+
+}