Android fundamentals 10.1 Part B: Deleting data from a Room database

Task 1: Initialize data only if the database is empty

1.1 Add a method to the DAO to get a single word

• In the WordDao interface, add a method to get any word:

@Query("SELECT * from word_table LIMIT 1")
    Word[] getAnyWord();

1.2 Update the initialization method to check whether data exists

PopulateDBAsync is an inner class in WordRoomDatbase. In PopulateDBAsync, update the doInBackground() method to check whether the database has any words before initializing the data:

@Override
protected Void doInBackground(final Void... params) {

       // If we have no words, then create the initial list of words
       if (mDao.getAnyWord().length < 1) {
           for (int i = 0; i <= words.length - 1; i++) {
               Word word = new Word(words[i]);
               mDao.insert(word);
           }
       }
   return null;
}

• Run your app and add several new words. Close the app and restart it. You should see the new words that you added, as the words should now persist when the app is closed and opened again.

Task 2: Delete all words

2.1 Add deleteAll() to the WordDao interface and annotate it

• In WordDao, check that the deleteAll() method is defined and annotated with the SQL that runs when the method executes:

@Query("DELETE FROM word_table")
void deleteAll();

2.2 Add deleteAll() to the WordRepository class

• In WordRepository, define deleteAllWordsAsyncTask as an inner class. Implement doInBackground() to delete all the words by calling deleteAll() on the DAO :

private static class deleteAllWordsAsyncTask extends AsyncTask<Void, Void, Void> {
   private WordDao mAsyncTaskDao;

   deleteAllWordsAsyncTask(WordDao dao) {
       mAsyncTaskDao = dao;
   }

   @Override
   protected Void doInBackground(Void... voids) {
       mAsyncTaskDao.deleteAll();
       return null;
   }
}

• In the WordRepository class, add the deleteAll() method to invoke the AsyncTaskthat you defined.

public void deleteAll()  {
   new deleteAllWordsAsyncTask(mWordDao).execute();
}

2.3 Add deleteAll() to the WordViewModel class

Make the deleteAll() method available to the MainActivity by adding it to the WordViewModel.

• In the WordViewModel class, add the deleteAll() method:

public void deleteAll() {mRepository.deleteAll();}

Task 3: Add an Options menu item to delete all data

3.1 Add the Clear all data menu option

• In menu_main.xml, change the menu option title and id, as follows:

<item
   android:id="@+id/clear_data"
   android:orderInCategory="100"
   android:title="@string/clear_all_data"
   app:showAsAction="never" />

• In MainActivity, implement the onOptionsItemSelected() method to invoke the deleteAll() method on the WordViewModelobject.

@Override
public boolean onOptionsItemSelected(MenuItem item) {
   int id = item.getItemId();

   if (id == R.id.clear_data) {
       // Add a toast just for confirmation
       Toast.makeText(this, "Clearing the data...",
               Toast.LENGTH_SHORT).show();

       // Delete the existing data
       mWordViewModel.deleteAll();
       return true;
   }

   return super.onOptionsItemSelected(item);
}

• Run your app. In the Options menu, select Clear all data. All words should disappear.
• Restart the app. (Restart it from your device or the emulator; don’t run it again from Android Studio) You should see the initial set of words.

Task 4: Delete a single word

4.1 Add deleteWord() to the DAO and annotate it

• In WordDao, add the deleteWord() method:

@Delete
void deleteWord(Word word);

4.2 Add deleteWord() to the WordRepository class

• In WordRepository, define another AsyncTaskcalled deleteWordAsyncTask as an inner class. Implement doInBackground() to delete a word by calling deleteWord() on the DAO:

private static class deleteWordAsyncTask extends AsyncTask<Word, Void, Void> {
   private WordDao mAsyncTaskDao;

   deleteWordAsyncTask(WordDao dao) {
       mAsyncTaskDao = dao;
   }

   @Override
   protected Void doInBackground(final Word... params) {
       mAsyncTaskDao.deleteWord(params[0]);
       return null;
   }
}

• In WordRepository, add the deleteWord()method to invoke the AsyncTask you defined.

public void deleteWord(Word word)  {
   new deleteWordAsyncTask(mWordDao).execute(word);
}

4.3 Add deleteWord() to the WordViewModel class

To make the deleteWord() method available to other classes in the app, in particular, MainActivity, add it to WordViewModel.

• In WordViewModel, add the deleteWord()method:

public void deleteWord(Word word) {mRepository.deleteWord(word);}

Task 5: Enable users to swipe words away

5.1 Enable the adapter to detect the swiped word

• In WordListAdapter, add a method to get the word at a given position.

public Word getWordAtPosition (int position) {
   return mWords.get(position);
}

• In MainActivity, in onCreate(), create the ItemTouchHelper. Attach the ItemTouchHelper to the RecyclerView.

// Add the functionality to swipe items in the 
// recycler view to delete that item
ItemTouchHelper helper = new ItemTouchHelper(
    new ItemTouchHelper.SimpleCallback(0,
        ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT) {
           @Override
           public boolean onMove(RecyclerView recyclerView,
                                 RecyclerView.ViewHolder viewHolder,
                                 RecyclerView.ViewHolder target) {
               return false;
           }

           @Override
           public void onSwiped(RecyclerView.ViewHolder viewHolder, 
                                int direction) {
               int position = viewHolder.getAdapterPosition();
               Word myWord = adapter.getWordAtPosition(position);
               Toast.makeText(MainActivity.this, "Deleting " + 
                       myWord.getWord(), Toast.LENGTH_LONG).show();

               // Delete the word
               mWordViewModel.deleteWord(myWord);
            }
       });

helper.attachToRecyclerView(recyclerView);

Things to notice in the code:

onSwiped() gets the position of the ViewHolder that was swiped:

int position = viewHolder.getAdapterPosition();

Given the position, you can get the word displayed by the ViewHolder by calling the getWordAtPosition()method that you defined in the adapter:

Word myWord = adapter.getWordAtPosition(position);

Delete the word by calling deleteWord() on the WordViewModel:

mWordViewModel.deleteWord(myWord);

Now run your app and delete some words

• Run your app. You should be able to delete words by swiping them left or right.

Thank you :))