Fixes issue #2 Race Condition on refresh, getView

Partial fix only.
replacing the underlying books after all books are loaded,
so the danger time is shortened.  Now the code looks cleaner.

Also, the item of listview is now recycled.

app/src/main/java/com/loopbook/cuhk_loopbook/BookFragment.java

@@ -49,11 +49,11 @@ public static class BookAdapter extends BaseAdapter {
         private int viewResourceId;
         private Context ctx;
 
-        public BookAdapter(Context ctx, int viewResourceId, ArrayList<LibConn.Book> books) {
+        public BookAdapter(Context ctx, int viewResourceId) {
             this.ctx = ctx;
             mInflater = (LayoutInflater)ctx.getSystemService(
                         Context.LAYOUT_INFLATER_SERVICE);
-            this.books = books;
+            this.books = new ArrayList<LibConn.Book>();
             this.viewResourceId = viewResourceId;
         }
 
@@ -70,37 +70,41 @@ public int getCount() {
         }
 
         public View getView(int position, View convertView, ViewGroup parent) {
-            View view = mInflater.inflate(viewResourceId, null);
+            if (convertView == null) { convertView = mInflater.inflate(viewResourceId, null); }
             LibConn.Book book = books.get(position);
 
-            ImageView iv = (ImageView)view.findViewById(R.id.option_icon);
+            ImageView iv = (ImageView)convertView.findViewById(R.id.option_icon);
             iv.setImageResource(book.remainDays() >= DueChecker.getAlertDays(ctx) ?
                                 R.drawable.green_circle :
                                 R.drawable.red_circle);
 
-            TextView tv = (TextView)view.findViewById(R.id.option_text);
+            TextView tv = (TextView)convertView.findViewById(R.id.option_text);
             tv.setText(book.name + "\n" + formater.format(book.dueDate.getTime()));
 
-            return view;
+            return convertView;
+        }
+
+        public void setBooks(ArrayList<LibConn.Book> booksGot) {
+            /* I am afraid race condition may happen in these line (issue #2),
+             * but probability is very low */
+            books = booksGot;
+            notifyDataSetChanged();
         }
     }
 
     private static class Data {
 
-        private ArrayList<LibConn.Book> books = new ArrayList<>();
-
-        private class AsyncBookLoader extends AsyncTask<Context, String, Void> {
+        private class AsyncBookLoader extends AsyncTask<Context, String, ArrayList<LibConn.Book>> {
             private Exception caughtException = null;
             private Context context;
-            private BookAdapter bookAdapter;
+            public BookAdapter bookAdapter;
 
             @Override
-            protected Void doInBackground(Context... context) {
+            protected ArrayList<LibConn.Book> doInBackground(Context... context) {
                 this.context = context[0];
                 String msg = LibConn.isConnectable(this.context) ? "connecting" : "No connection";
                 publishProgress(msg);
 
-                books.clear();
                 ArrayList<LibConn.Book> booksGot;
                 try {
                     booksGot = DataIO.getBooks(this.context);
@@ -109,10 +113,7 @@ protected Void doInBackground(Context... context) {
                     return null;
                 }
 
-                for (LibConn.Book book: booksGot) {  /* make deep copy */
-                    books.add(book);
-                }
-                return null;
+                return booksGot;
             }
 
             @Override
@@ -121,26 +122,22 @@ protected void onProgressUpdate(String... msgs) {
             }
 
             @Override
-            protected void onPostExecute(Void nothing) {
+            protected void onPostExecute(ArrayList<LibConn.Book> booksGot) {
                 if (caughtException != null) {
                     Toast.makeText(context,
                             caughtException.getMessage(),
                             Toast.LENGTH_SHORT).show();
                 } else {
-                    if (books.size() == 0) {
+                    if (booksGot.size() == 0) {
                         Toast.makeText(context,
                                 context.getString(R.string.no_books),
                                 Toast.LENGTH_SHORT).show();
                     }
-                    bookAdapter.notifyDataSetChanged();
+                    bookAdapter.setBooks(booksGot);
                 }
             }
         }
 
-        public ArrayList<LibConn.Book> getbooks() {
-            return books;
-        }
-
         public void refresh(BookAdapter adapter, Context context) {
             AsyncBookLoader bookLoader = new AsyncBookLoader();
             bookLoader.bookAdapter = adapter;
@@ -159,8 +156,7 @@ public void onCreate(Bundle savedInstanceState) {
 
         bookAdapter = new BookAdapter(
                 getActivity(),
-                R.layout.list_item,
-                data.getbooks());
+                R.layout.list_item);
         if  (getArguments() != null ? getArguments().getBoolean("firstRun", false) : false) {
             getArguments().putBoolean("firstRun", false);
         } else {