java - Android-notifyDataSetChanged ListView保持空白后

我已经花了几个小时搜索内容，但似乎无济于事。Android Studio不会启动任何错误，但屏幕仍然空白。为什么？

package org.newapp;

import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import org.xml.sax.SAXException;

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;

import saxrssreader.*;


public class MainActivity extends Activity {

    private ArrayAdapter<RssItem> rssItemsArrayAdapter;
    ListView codeLearnLessons;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        RssItem[] rssItems = new RssItem[]{};
        rssItemsArrayAdapter = new ArrayAdapter<RssItem>(this, android.R.layout.simple_list_item_1, rssItems);
        new WebCall().execute(rssItems);
        codeLearnLessons = (ListView)findViewById(R.id.listView1);
        codeLearnLessons.setAdapter(rssItemsArrayAdapter);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    private class WebCall extends AsyncTask<RssItem, Void, Void>  {

        @Override
        protected Void doInBackground(RssItem... items) {
            URL url = null;
            try {
                url = new URL("http://www.somewpsite.com/feed");
            } catch (MalformedURLException e) {
                e.printStackTrace();
            }
            try {
                ArrayList<RssItem> listItems = RssReader.read(url).getRssItems();
                items = listItems.toArray(new RssItem[listItems.size()]);
                final int l = items.length;
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        codeLearnLessons.invalidateViews();
                        rssItemsArrayAdapter.notifyDataSetChanged();
                        Log.i("rec", "rec" + l);
                    }
                });
            } catch (SAXException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPreExecute() {
            /*ProgressDialog mDialog = new ProgressDialog(MainActivity.this);
            mDialog.setMessage("Please wait...");
            mDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
            mDialog.setIndeterminate(true);
            mDialog.setCancelable(false);
            mDialog.show();*/
        }
    }


}

最佳答案

问题是您使用的ArrayList仅在AsyncTask.doInBackground()方法内部具有作用域。由于未提供给适配器，因此notifyDataSetChanged()不执行任何操作。您需要像这样替换现有的数组：

rssItemsArrayAdapter.clear();
rssItemsArrayAdapter.addAll(listItems);
rssItemsArrayAdapter.notifyDataSetChanged();

话虽如此，您实际上还需要更改对AsyncTask的使用。无需从runOnUiThread()内部调用doInBackground()，因为AsyncTask将在UI线程上自动运行其onPostExecute()。让您的doInBackground()方法返回新的ArrayList<RssItem>并创建一个onPostExecute()覆盖方法，该方法将调整适配器，如上所示。