Fix not working scheduled reconnect

Documentation for postDelayed:
> Causes the Runnable r to be added to the message queue, to be run
> after the specified amount of time elapses.
> The runnable will be run on the thread to which this handler
> is attached.
> <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
> Time spent in deep sleep will add an additional delay to execution.

TL;DR: if the CPU is in deep sleep, the postDelayed runnable won't be executed.

app/src/main/java/com/github/gotify/service/WebSocketConnection.java

@@ -1,14 +1,17 @@
 package com.github.gotify.service;
 
+import android.app.AlarmManager;
 import android.net.ConnectivityManager;
 import android.net.NetworkInfo;
+import android.os.Build;
 import android.os.Handler;
 import com.github.gotify.SSLSettings;
 import com.github.gotify.Utils;
 import com.github.gotify.api.Callback;
 import com.github.gotify.api.CertUtils;
 import com.github.gotify.client.model.Message;
 import com.github.gotify.log.Log;
+import java.util.Calendar;
 import java.util.concurrent.TimeUnit;
 import okhttp3.HttpUrl;
 import okhttp3.OkHttpClient;
@@ -17,8 +20,9 @@
 import okhttp3.WebSocket;
 import okhttp3.WebSocketListener;
 
-public class WebSocketConnection {
+class WebSocketConnection {
     private final ConnectivityManager connectivityManager;
+    private final AlarmManager alarmManager;
     private OkHttpClient client;
 
     private final Handler reconnectHandler = new Handler();
@@ -41,8 +45,10 @@ public class WebSocketConnection {
             String baseUrl,
             SSLSettings settings,
             String token,
-            ConnectivityManager connectivityManager) {
+            ConnectivityManager connectivityManager,
+            AlarmManager alarmManager) {
         this.connectivityManager = connectivityManager;
+        this.alarmManager = alarmManager;
         OkHttpClient.Builder builder =
                 new OkHttpClient.Builder()
                         .readTimeout(0, TimeUnit.MILLISECONDS)
@@ -119,14 +125,25 @@ public synchronized void close() {
         }
     }
 
-    public synchronized void scheduleReconnect(long millis) {
-        reconnectHandler.removeCallbacks(reconnectCallback);
-
-        Log.i(
-                "WebSocket: scheduling a restart in "
-                        + TimeUnit.SECONDS.convert(millis, TimeUnit.MILLISECONDS)
-                        + " second(s)");
-        reconnectHandler.postDelayed(reconnectCallback, millis);
+    public synchronized void scheduleReconnect(long seconds) {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+            Log.i(
+                    "WebSocket: scheduling a restart in "
+                            + seconds
+                            + " second(s) (via alarm manager)");
+            final Calendar future = Calendar.getInstance();
+            future.add(Calendar.SECOND, (int) seconds);
+            alarmManager.setExact(
+                    AlarmManager.RTC_WAKEUP,
+                    future.getTimeInMillis(),
+                    "reconnect-tag",
+                    this::start,
+                    null);
+        } else {
+            Log.i("WebSocket: scheduling a restart in " + seconds + " second(s)");
+            reconnectHandler.removeCallbacks(reconnectCallback);
+            reconnectHandler.postDelayed(reconnectCallback, TimeUnit.SECONDS.toMillis(seconds));
+        }
     }
 
     private class Listener extends WebSocketListener {
@@ -191,7 +208,7 @@ public void onFailure(WebSocket webSocket, Throwable t, Response response) {
                 int minutes = Math.min(errorCount * 2 - 1, 20);
 
                 onNetworkFailure.execute(minutes);
-                scheduleReconnect(TimeUnit.MINUTES.toMillis(minutes));
+                scheduleReconnect(TimeUnit.MINUTES.toSeconds(minutes));
             }
 
             super.onFailure(webSocket, t, response);

app/src/main/java/com/github/gotify/service/WebSocketService.java

@@ -1,5 +1,6 @@
 package com.github.gotify.service;
 
+import android.app.AlarmManager;
 import android.app.Notification;
 import android.app.NotificationManager;
 import android.app.PendingIntent;
@@ -30,7 +31,6 @@
 import com.github.gotify.messages.MessagesActivity;
 import java.util.List;
 import java.util.Map;
-import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 
 public class WebSocketService extends Service {
@@ -92,10 +92,15 @@ private void startPushService() {
 
         ConnectivityManager cm =
                 (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
+        AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
 
         connection =
                 new WebSocketConnection(
-                                settings.url(), settings.sslSettings(), settings.token(), cm)
+                                settings.url(),
+                                settings.sslSettings(),
+                                settings.token(),
+                                cm,
+                                alarmManager)
                         .onOpen(this::onOpen)
                         .onClose(() -> foreground(getString(R.string.websocket_closed)))
                         .onBadRequest(this::onBadRequest)
@@ -121,7 +126,7 @@ private void doReconnect() {
             return;
         }
 
-        connection.scheduleReconnect(TimeUnit.SECONDS.toMillis(5));
+        connection.scheduleReconnect(5);
     }
 
     private void onBadRequest(String message) {