java - 在Android中执行一次异步任务

Question

嗨，我在这里需要帮助，我已经完成了一个应用程序，其中包含来自 Apple rss feed 的十大下载应用程序，并且我陷入了困境，以避免每次设备旋转时通过 onCreate 方法从 bundle 中获取 url 和限制值来下载数据

我已经实现了这一点，并将数据保存在 onSaveInstanceState​，但它不断在设备旋转上下载数据

这是我的 MainActivity 代码

package com.ksamj.top10downloader;

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListView;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    private ListView listApps;
    private String feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml";
    private int feedLimit = 10;
    private String feedCachedURL = "INVALID";
    public static final String STATE_URL = "feedURL";
    public static final String STATE_LIMIT = "feedLimit";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        listApps = (ListView) findViewById(R.id.xmlListView);


        if(savedInstanceState != null) // if null means run at 1st time
        {

            feedURL = savedInstanceState.getString(STATE_URL);
            feedLimit = savedInstanceState.getInt(STATE_LIMIT);
            Log.d(TAG, "from Bundle feedURL: "+ feedURL);
            Log.d(TAG, "from Bundle feedLimit: "+ feedLimit);
        }
        downloadURL(String.format(feedURL,feedLimit));
    }

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.menuFreeApps:
                feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml";
                break;
            case R.id.menuPaidApps:
                feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/toppaidapplications/limit=%d/xml";
                break;
            case R.id.menuSongs:
                feedURL = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topsongs/limit=%d/xml";
                break;

            case R.id.menu10:
                feedLimit = 10;
                item.setChecked(true);
                break;

            case R.id.menu25:
                feedLimit = 25;
                item.setChecked(true);
                break;


            case R.id.menuRefresh:
                feedCachedURL = "INVALID";
                break;

            default:
                return super.onOptionsItemSelected(item);
        }
        downloadURL(String.format(feedURL,feedLimit));
        return true;
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        outState.putString(STATE_URL, feedURL);
        outState.putInt(STATE_LIMIT, feedLimit);
        super.onSaveInstanceState(outState);
    }



    private void downloadURL(String feedURL) {
        if (!feedURL.equalsIgnoreCase(feedCachedURL)) {
            Log.d(TAG, "downloadURL: Downloading...");
            DownloadData downloadData = new DownloadData();
            downloadData.execute(feedURL);
            feedCachedURL = feedURL;
            Log.d(TAG, "downloadURL: feedURL " + feedURL);
            Log.d(TAG, "downloadURL: feedCachedURL " + feedCachedURL);
        }else
            Log.d(TAG, "downloadURL: Nothing to Download");
    }

    private class DownloadData extends AsyncTask<String, Void, String> {
        private static final String TAG = "DownloadData";

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            //Log.d(TAG, "onPostExecute: params are" + s);
            ParseApplications parseApplications = new ParseApplications();
            parseApplications.parse(s);

            FeedAdapter feedAdapter = new FeedAdapter(MainActivity.this, R.layout.list_record, parseApplications.getApplications());
            listApps.setAdapter(feedAdapter);
        }

        @Override
        protected String doInBackground(String... strings) {
            Log.d(TAG, "doInBackground: starts with " + strings[0]);
            String rssFeed = downloadXML(strings[0]);
            if (rssFeed == null) {
                Log.e(TAG, "doInBackground: Error Downloading");
            }

            return rssFeed;
        }

        private String downloadXML(String urlPath) {
            StringBuilder xmlResult = new StringBuilder();
            try {
                URL url = new URL(urlPath);
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                int response = httpURLConnection.getResponseCode();
                Log.d(TAG, "downloadXML: The response Code is " + response);
                BufferedReader reader = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream()));

                int charsRead;
                char[] inputBuffer = new char[5500];
                while (true) {
                    charsRead = reader.read(inputBuffer);
                    if (charsRead < 0)
                        break;
                    if (charsRead > 0)
                        xmlResult.append(String.copyValueOf(inputBuffer, 0, charsRead));
                }
                reader.close();

                return xmlResult.toString();

            } catch (MalformedURLException ex) {
                Log.e(TAG, "downloadXML: Malformated URL Exp: " + ex.getMessage());

            } catch (IOException ex) {
                Log.e(TAG, "downloadXML: IO Exp: " + ex.getMessage());
            }

            return null;
        }

    }

}

这是我的 logcat，用于执行后的简单旋转

03-28 10:01:56.373 21047-21047/com.ksamj.top10downloader W/art: Verification of java.lang.Object com.ksamj.top10downloader.MainActivity.access$super(com.ksamj.top10downloader.MainActivity, java.lang.String, java.lang.Object[]) took 100.789ms
03-28 10:01:56.515 21047-21047/com.ksamj.top10downloader W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
03-28 10:01:56.805 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: Downloading...
03-28 10:01:56.809 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:01:56.809 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedCachedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:01:56.809 21047-21272/com.ksamj.top10downloader D/DownloadData: doInBackground: starts with http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:01:56.813 21047-21272/com.ksamj.top10downloader D/NetworkSecurityConfig: No Network Security Config specified, using platform default
03-28 10:01:56.953 21047-21274/com.ksamj.top10downloader I/OpenGLRenderer: Initialized EGL, version 1.4
03-28 10:01:56.953 21047-21274/com.ksamj.top10downloader D/OpenGLRenderer: Swap behavior 1
03-28 10:01:56.957 21047-21274/com.ksamj.top10downloader W/OpenGLRenderer: Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
03-28 10:01:56.957 21047-21274/com.ksamj.top10downloader D/OpenGLRenderer: Swap behavior 0
03-28 10:01:57.002 21047-21272/com.ksamj.top10downloader D/DownloadData: downloadXML: The response Code is 200
03-28 10:01:57.098 21047-21047/com.ksamj.top10downloader W/art: Before Android 4.1, method int android.support.v7.widget.ListViewCompat.lookForSelectablePosition(int, boolean) would have incorrectly overridden the package-private method in android.widget.ListView
03-28 10:01:57.403 21047-21047/com.ksamj.top10downloader W/InputMethodManager: Ignoring onBind: cur seq=94, given seq=93
03-28 10:02:02.331 21047-21047/com.ksamj.top10downloader D/MainActivity: from Bundle feedURL: http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml
03-28 10:02:02.332 21047-21047/com.ksamj.top10downloader D/MainActivity: from Bundle feedLimit: 10
03-28 10:02:02.332 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: Downloading...
03-28 10:02:02.333 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:02:02.333 21047-21047/com.ksamj.top10downloader D/MainActivity: downloadURL: feedCachedURL http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:02:02.336 21047-21345/com.ksamj.top10downloader D/DownloadData: doInBackground: starts with http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=10/xml
03-28 10:02:02.354 21047-21345/com.ksamj.top10downloader D/DownloadData: downloadXML: The response Code is 200

你能帮我找出我在这里做错或遗漏的地方吗？

Answer 1

使用加载器模式。加载器允许您注册一个异步进程，这样您就不会启动额外的任务，因此当它完成时存在的 Activity 将获得实际结果。请参阅https://developer.android.com/reference/android/app/LoaderManager.html了解更多详情。