Add background task

src/kotlin/flwr/src/main/java/dev/flower/android/Worker.kt

@@ -0,0 +1,58 @@
+package dev.flower.android
+
+import android.content.Context
+import androidx.work.Constraints
+import androidx.work.CoroutineWorker
+import androidx.work.Data
+import androidx.work.ExistingPeriodicWorkPolicy
+import androidx.work.PeriodicWorkRequest
+import androidx.work.PeriodicWorkRequestBuilder
+import androidx.work.WorkManager
+import androidx.work.WorkerParameters
+import java.util.UUID
+import java.util.concurrent.TimeUnit
+
+class FlwrWorker(private val context: Context,
+                 workerParams: WorkerParameters,
+                 private val flwrClient: Client,
+                 private val grpcRere: Boolean) : CoroutineWorker(context, workerParams) {
+    override suspend fun doWork(): Result {
+        if (grpcRere) {
+            inputData.getString("serverAddress")?.let { startFlowerRere(it, false, flwrClient) }
+        } else {
+            inputData.getString("serverAddress")?.let { startClient(it, false, flwrClient) }
+        }
+        val workManager: WorkManager = WorkManager.getInstance(context)
+        workManager.cancelWorkById(id)
+        return Result.success()
+    }
+
+    /**
+     * Start a PeriodicWorker that runs Flower client in the background.
+     *
+     * @param interval The interval for the PeriodicWorker to resume its work periodically in minutes.
+     * @param serverAddress The IPv4 or IPv6 address of the server. If the Flower server runs on the
+     * same machine on port 8080, then server_address would be “[::]:8080”.
+     */
+    fun startFlwrPeriodicWorker(interval: Long, serverAddress: String) {
+        val constraints: Constraints = Constraints.Builder().build()
+
+        val workRequest: PeriodicWorkRequest = PeriodicWorkRequestBuilder<FlwrWorker>(
+             interval, TimeUnit.MINUTES
+        )
+            .setInitialDelay(0, TimeUnit.MILLISECONDS)
+            .setInputData(
+                Data.Builder()
+                    .putString("serverAddress", serverAddress)
+                    .build()
+            )
+            .setConstraints(constraints)
+            .build()
+
+        val uniqueWorkName = "FlwrWorker${UUID.randomUUID()}"
+
+        WorkManager.getInstance(context)
+            .enqueueUniquePeriodicWork(uniqueWorkName, ExistingPeriodicWorkPolicy.KEEP, workRequest)
+    }
+
+}