Block SyncAdapter.onPerformSync() until the javascript task is executed

android/src/main/java/com/fnp/reactnativesyncadapter/HeadlessService.java

@@ -3,16 +3,58 @@
 import android.app.ActivityManager;
 import android.content.Context;
 import android.content.Intent;
+import android.os.Binder;
+import android.os.IBinder;
+import android.support.annotation.Nullable;
 
 import com.facebook.react.HeadlessJsTaskService;
 import com.facebook.react.jstasks.HeadlessJsTaskConfig;
 
+import java.util.ArrayList;
 import java.util.List;
 
 public class HeadlessService extends HeadlessJsTaskService {
 
     private static final String TASK_ID = "TASK_SYNC_ADAPTER";
 
+    private final IBinder mBinder = new LocalBinder();
+    private List<Callback> mCallbacks = new ArrayList<>();
+
+    public class LocalBinder extends Binder {
+        HeadlessService getService() {
+            return HeadlessService.this;
+        }
+    }
+
+    public interface Callback {
+        void onTaskCompletion();
+    }
+
+    @Override
+    public @Nullable
+    IBinder onBind(Intent intent) {
+        return mBinder;
+    }
+
+    public void notifyOnTaskCompletion(Callback cb) {
+        mCallbacks.add(cb);
+    }
+
+    @Override
+    public void onHeadlessJsTaskFinish(int taskId) {
+        super.onHeadlessJsTaskFinish(taskId);
+        for (Callback cb : mCallbacks) {
+            cb.onTaskCompletion();
+        }
+        mCallbacks.clear();
+    }
+
+    public void startHeadlessTask(Intent intent) {
+        HeadlessJsTaskConfig taskConfig = getTaskConfig(intent);
+        if (taskConfig != null) {
+            startTask(taskConfig);
+        }
+    }
     @Override
     protected HeadlessJsTaskConfig getTaskConfig(Intent intent) {
         boolean allowForeground = Boolean.valueOf(getString(R.string.rnsb_allow_foreground));

android/src/main/java/com/fnp/reactnativesyncadapter/SyncAdapter.java

@@ -3,25 +3,62 @@
 import android.accounts.Account;
 import android.accounts.AccountManager;
 import android.content.AbstractThreadedSyncAdapter;
+import android.content.ComponentName;
 import android.content.ContentProviderClient;
 import android.content.ContentResolver;
 import android.content.Context;
 import android.content.Intent;
+import android.content.ServiceConnection;
 import android.content.SyncRequest;
 import android.content.SyncResult;
 import android.os.Build;
 import android.os.Bundle;
+import android.os.IBinder;
 
-class SyncAdapter extends AbstractThreadedSyncAdapter {
+import java.util.concurrent.CountDownLatch;
+
+class SyncAdapter extends AbstractThreadedSyncAdapter implements HeadlessService.Callback {
+
+    private CountDownLatch doneSignal = new CountDownLatch(1);
+    private Intent mIntent;
+    private ServiceConnection mConnection = new ServiceConnection() {
+        @Override
+        public void onServiceConnected(ComponentName componentName, IBinder binder) {
+            HeadlessService service = ((HeadlessService.LocalBinder)binder).getService();
+            service.notifyOnTaskCompletion(SyncAdapter.this);
+            service.startHeadlessTask(mIntent);
+        }
+
+        @Override
+        public void onServiceDisconnected(ComponentName componentName) {
+        }
+    };
 
     public SyncAdapter(Context context, boolean autoInitialize) {
         super(context, autoInitialize);
     }
 
+    @Override
+    public void onTaskCompletion() {
+        getContext().unbindService(mConnection);
+        doneSignal.countDown();
+    }
+
     @Override
     public void onPerformSync(Account account, Bundle extras, String authority, ContentProviderClient provider, SyncResult syncResult) {
-        Intent service = new Intent(getContext(), HeadlessService.class);
-        getContext().startService(service);
+        mIntent = new Intent(getContext(), HeadlessService.class);
+        Context context = getContext();
+
+        if (context.getString(R.string.rnsb_user_visible).equals("true")) {
+            context.bindService(mIntent, mConnection, Context.BIND_AUTO_CREATE);
+            try {
+                doneSignal.await();
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        } else {
+            context.startService(mIntent);
+        }
     }
 
     /**