HandlerServer: use executor service to send data to TrackRecordingService.

Fixes #329.

src/main/java/de/dennisguse/opentracks/services/handlers/HandlerServer.java

@@ -3,26 +3,46 @@ package de.dennisguse.opentracks.services.handlers;
 import android.content.Context;
 import android.content.SharedPreferences;
 
+import androidx.annotation.VisibleForTesting;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
 import de.dennisguse.opentracks.content.data.TrackPoint;
 
 public class HandlerServer {
-    private String TAG = HandlerServer.class.getSimpleName();
+
+    private static String TAG = HandlerServer.class.getSimpleName();
 
     private final LocationHandler locationHandler;
     private final HandlerServerInterface service;
+    private ExecutorService serviceExecutor;
 
     public HandlerServer(HandlerServerInterface service) {
         this.locationHandler = new LocationHandler(this);
         this.service = service;
     }
 
+    @VisibleForTesting
+    HandlerServer(LocationHandler locationHandler, HandlerServerInterface service) {
+        this.locationHandler = locationHandler;
+        this.service = service;
+    }
+
     public void start(Context context) {
+        serviceExecutor = Executors.newSingleThreadExecutor();
+
         locationHandler.onStart(context);
         locationHandler.onSharedPreferenceChanged(context, null, null);
     }
 
     public void stop(Context context) {
         locationHandler.onStop(context);
+
+        if (serviceExecutor != null) {
+            serviceExecutor.shutdownNow();
+        }
+        serviceExecutor = null;
     }
 
     public void onSharedPreferenceChanged(Context context, SharedPreferences preferences, String key) {
@@ -30,7 +50,10 @@ public class HandlerServer {
     }
 
     public void sendTrackPoint(TrackPoint trackPoint, int recordingGpsAccuracy) {
-        service.newTrackPoint(trackPoint, recordingGpsAccuracy);
+        if (serviceExecutor == null || serviceExecutor.isTerminated() || serviceExecutor.isShutdown()) {
+            return;
+        }
+        serviceExecutor.execute(() -> service.newTrackPoint(trackPoint, recordingGpsAccuracy));
     }
 
     public interface HandlerServerInterface {