Styjun

I am developing an Android Application (Android Studio - Java) which includes a sign in and registration process. I took care of the sign in and registration processes by implementing a connection between PHP files and a MySQL database through http. In short, I just created an AsyncTask class in java --called from another class-- and used it to post data to a PHP file, from there I just used the appropriate SQL commands. This part works fine.

This first part with the login and registration is important because every user will see a slightly different layout once they login. The layout is a RecyclerView composed of detailed CardView elements. Each CardView has a few TextViews with some details. The details are held by an Object i created in a separate class. To fill in the CardView elements a fetched some JSON data using a separate PHP file (and one more http connection). Parsing the data from JSON into Strings and ints was a straightforward endeavor, as was adding them to the list of custom objects. There is some code below showing how I fetched the data and added it to the Object list.

This is the complete AsyncTask class:

public class ScheduleWorker extends AsyncTask<String, Void, String> 
Context context;
AlertDialog alertDialog;
ScheduleWorker (Context ctx) context = ctx; 

@Override
protected void onPreExecute() super.onPreExecute(); 

@Override
protected void onPostExecute (String s) 
 super.onPostExecute(s);
 Toast.makeText(context, s, Toast.LENGTH_SHORT).show();
 try 
 loadJSON(s);
 catch (JSONException e) 
 e.printStackTrace();
 

@Override
protected String doInBackground(String... params) 
 final String fetch_url = "http://192.168.1.70/newfetcher.php";

 try 
 String ussr_name = params[0];
 URL url = new URL(fetch_url);
 HttpURLConnection con = (HttpURLConnection) url.openConnection();
 con.setRequestMethod("POST");
 con.setDoOutput(true);
 con.setDoInput(true);
 OutputStream outputStream = con.getOutputStream();
 BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
 String post_data = URLEncoder.encode("user_name", "UTF-8")+"="+URLEncoder.encode(ussr_name, "UTF-8");
 bufferedWriter.write(post_data);
 bufferedWriter.flush();
 bufferedWriter.close();
 outputStream.close();
 StringBuilder sb = new StringBuilder();
 BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));
 String json;

 while ((json = bufferedReader.readLine()) != null) 
 sb.append(json + "n");
 

 return sb.toString().trim();
 catch (Exception e) 
 return null;
 


private void loadJSON(String json) throws JSONException 
 JSONArray jsonArray = new JSONArray(json);
 Course course;
 List<Course> courses = new ArrayList<Course>();
 int len = jsonArray.length();

 String[] titles = new String[len];
 String[] types = new String[len];
 String[] teachers = new String[len];
 int[] pics = new int[len];

 for (int i = 0; i < len; i++) 
 JSONObject obj = jsonArray.getJSONObject(i);

 titles[i] = obj.getString("title");
 types[i] = obj.getString("type");
 teachers[i] = obj.getString("teacher");
 pics[i] = obj.getInt("pic");
 course = new Course(titles[i], types[i], teachers[i], pics[i]);
 courses.add(course);
 

Now, where I encountered problems is when I tried saving the List of Objects (and their details) to a Room Persistent Database. I based my code for the Room Database off an example from Google Developer CodeLabs. I adapted the code for my particular needs but I kept the underlying class structure. The structure includes: an Entity, a DAO, a RoomDatabase, a Repository, a ViewModel, a ViewHolder, an Adapter for the RecyclerView, and a class to populate the database. Everything seems fine except for the part where I populate the database. The example populates the database by using a callback and an AsyncTask within the RoomDatabase class.

Here is populating AsyncTask:

private static RoomDatabase.Callback sRoomDatabaseCallback = new RoomDatabase.Callback() 

 @Override
 public void onOpen(@NonNull SupportSQLiteDatabase db) 
 super.onOpen(db);
 // If you want to keep the data through app restarts,
 // comment out the following line.
 new PopulateDbAsync(INSTANCE).execute();
 
;

/**
 * Populate the database in the background.
 * If you want to start with more words, just add them.
 */
private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> 

 private final WordDao mDao;

 PopulateDbAsync(WordRoomDatabase db) 
 mDao = db.wordDao();
 

 @Override
 protected Void doInBackground(final Void... params) 
 // Start the app with a clean database every time.
 // Not needed if you only populate on creation.

 Word word = new Word("Hello");
 mDao.insert(word);
 word = new Word("World");
 mDao.insert(word);
 return null;
 

My question is how do I populate the database with the detail arrays and/or the Object list? The example executes the callback every time a adding activity is called. I just want to populate the database when the user registers.