在doinbackground方法的异步任务中，我得到了feeditem的null返回值

Question

在doinbackground方法的异步任务中，我获得了null List FeedItems的{​​{1}}返回值。

以下是我的asynctask班级代码。如何归还List FeedItems？我已经在for循环中向List<FeedItem>添加了项目。如何从for循环内部（或任何其他方式）返回List的值？

private class AsynFetchTask extends AsyncTask<Void, FeedItem, List<FeedItem>> {
        private int count = 0;

        @Override
        protected List<FeedItem> doInBackground(Void... params) {


            ParseQuery<ParseObject> parseQuery = ParseQuery.getQuery("NewsAndUpdates");
            parseQuery.findInBackground(new FindCallback<ParseObject>() {
                @Override
                public void done(final List<ParseObject> list, ParseException e) {
                    if (e==null){
                        List<FeedItem> feedItems= new ArrayList<>();
                        for (int i = 0; i < list.size(); i++) {
                            final FeedItem item = new FeedItem();
                            item.setTitle(list.get(i).getString("Title"));
                            item.setDescription(list.get(i).getString("Description"));
                            item.setTime(list.get(i).getDate("createdAt"));
                            ParseFile file = list.get(i).getParseFile("File");
                            file.getDataInBackground(new GetDataCallback() {
                                @Override
                               public void done(byte[] bytes, ParseException e) {
                                   if (e==null){
                                        Bitmap bmp = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
                                        item.setThumbnail(bmp);
                                       publishProgress(item);
                                   }else {
                                        Toast.makeText(MainActivity.this, R.string.No_Internet,Toast.LENGTH_LONG).show();
                                    }


                                }
                            });
                            if (item != null) {
                                feedItems.add(item);
                            }

                        }

                    }else {
                        Toast.makeText(MainActivity.this, R.string.No_Internet,Toast.LENGTH_LONG).show();
                    }

                }
            });


            return null;
        }

        @Override
        protected void onProgressUpdate(FeedItem... values) {

            super.onProgressUpdate(values);


        }

        @Override
        protected void onPostExecute(List<FeedItem> feedItems) {

            super.onPostExecute(feedItems);
            adapter=new RecyclerViewAdapter(MainActivity.this,feedItems);
            mRecyclerView.setAdapter(adapter);
            mRecyclerView.setLayoutManager(new LinearLayoutManager(MainActivity.this));
        }
    }

Answer 1

不要在doInBackground中返回null，而是返回这样的feedItem：

private class AsynFetchTask extends AsyncTask<Void, FeedItem, List<FeedItem>> {
        private int count = 0;

        @Override
        protected List<FeedItem> doInBackground(Void... params) {


            ParseQuery<ParseObject> parseQuery = ParseQuery.getQuery("NewsAndUpdates");
            parseQuery.findInBackground(new FindCallback<ParseObject>() {
                @Override
                public void done(final List<ParseObject> list, ParseException e) {
                    if (e==null){
                        List<FeedItem> feedItems= new ArrayList<>();
                        for (int i = 0; i < list.size(); i++) {
                            final FeedItem item = new FeedItem();
                            item.setTitle(list.get(i).getString("Title"));
                            item.setDescription(list.get(i).getString("Description"));
                            item.setTime(list.get(i).getDate("createdAt"));
                            ParseFile file = list.get(i).getParseFile("File");
                            file.getDataInBackground(new GetDataCallback() {
                                @Override
                               public void done(byte[] bytes, ParseException e) {
                                   if (e==null){
                                        Bitmap bmp = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
                                        item.setThumbnail(bmp);
                                       publishProgress(item);
                                   }else {
                                        Toast.makeText(MainActivity.this, R.string.No_Internet,Toast.LENGTH_LONG).show();
                                    }


                                }
                            });
                            if (item != null) {
                                feedItems.add(item);
                            }

                        }

                    }else {
                        Toast.makeText(MainActivity.this, R.string.No_Internet,Toast.LENGTH_LONG).show();
                    }

                }
            });


            return feedItems;
        }

        @Override
        protected void onProgressUpdate(FeedItem... values) {

            super.onProgressUpdate(values);


        }

        @Override
        protected void onPostExecute(List<FeedItem> feedItems) {

            super.onPostExecute(feedItems);
            adapter=new RecyclerViewAdapter(MainActivity.this,feedItems);
            mRecyclerView.setAdapter(adapter);
            mRecyclerView.setLayoutManager(new LinearLayoutManager(MainActivity.this));
        }
    }