Question

这是我搜索某些内容的时间，但它似乎没有帮助.Android Studio不会启动任何错误，但屏幕仍为空白。为什么呢？

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();*/
        }
    }


}

Answer 1

问题是你使用的ArrayList只有AsyncTask.doInBackground()方法的范围。由于没有给适配器，notifyDataSetChanged()什么都不做。你需要像这样替换现有的数组：

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

话虽如此，您还需要更改AsyncTask的使用。无需在runOnUiThread()内拨打doInBackground()，因为AsyncTask会在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，调整适配器，如上所示。

Answer 2

您正在将项目传递给AsyncTask，但在AsyncTask中，您使用结果的内容声明它们是新的。您永远不会将项目传递给ArrayAdapter。使它们成为类实例的属性。

编辑 - 示例：

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    // Make sure your segue name in storyboard is the same as this line
    if ([[segue identifier] isEqualToString:@"isSecond"])
    {
        // Get reference to the destination view controller
        SecondViewController *vc = [segue destinationViewController];

        // Pass any objects to the view controller here, like...
        [vc setMyProductImage:productImage];
    }
}

之后

rssItemsArrayAdapter.clear();
rssItemsArrayAdapter.addAll(items);

Android - 在notifyDataSetChanged之后ListView保持空白

2 个答案: