java - 使用 volley 避免 Android 设备旋转时的冗余网络调用

Question

我创建了一个简单的 android 应用程序，它只解析来自苹果网站的 XML。它有解析前 10 名“歌曲”、“付费应用程序”或“免费应用程序”的菜单。

对于网络调用，我使用 volley。现在，当我在旋转设备时看到日志时，processXml() - 从 URL 获取 xml 并将其放在 ListView 中 - 调用多次，在 onCreate() 和 onRestoreInstanceState() 中。恕我直言，这可能是多余的下载。

我的问题:

1. 我把 processXml() 放错地方了吗？如果是这样，避免冗余网络调用的正确位置在哪里？
2. 因为我使用 volley，有什么方法可以缓存响应并在设备旋转时使用它？我想我在VolleySingleton上写了缓存，但我不知道我的实现是否正确

非常感谢

这是日志

// start of app
04-13 09:14:36.456 MainActivity: onCreate: processXml() called

// select "songs" from menu
04-13 09:14:49.627 MainActivity: onOptionsItemSelected: processXml() called

// rotate device
04-13 09:14:55.758 MainActivity: onCreate: processXml() called

这是我在 MainACtivity.java 上的源代码公共(public)类 MainActivity 扩展 AppCompatActivity {

private static final String TAG = "MainActivity";
private static final String KEY_RSS_URL = "KEY_RSS_URL";
private static final String KEY_FEED_LIMIT = "KEY_FEED_LIMIT";
private String rssUrl = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml";
private int feedLimit = 10;

private ListView xmlListView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    this.xmlListView = (ListView) findViewById(R.id.xmlListView);

    if (savedInstanceState != null) {
        this.rssUrl = savedInstanceState.getString(KEY_RSS_URL);
        this.feedLimit = savedInstanceState.getInt(KEY_FEED_LIMIT);
    }

    // need to put it here because when app start, it should already display top 10 free aps
    processRss(String.format(this.rssUrl, this.feedLimit));
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
  ... create menu
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int itemId = item.getItemId();

    switch (itemId) {
        case R.id.mnuFree:
            this.rssUrl = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topfreeapplications/limit=%d/xml";
            break;
        case R.id.mnuPaid:
            this.rssUrl = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/toppaidapplications/limit=%d/xml";
            break;
        case R.id.mnuSongs:
            this.rssUrl = "http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topsongs/limit=%d/xml";
            break;
        default:
            return super.onOptionsItemSelected(item);
    }

    processRss(String.format(this.rssUrl, this.feedLimit));
    Log.d(TAG, "onOptionsItemSelected: processRss() called");

    return true;
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);

    outState.putString(KEY_RSS_URL, this.rssUrl);
    outState.putInt(KEY_FEED_LIMIT, this.feedLimit);
}

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);

    this.rssUrl = savedInstanceState.getString(KEY_RSS_URL);
    this.feedLimit = savedInstanceState.getInt(KEY_FEED_LIMIT);

    processRss(String.format(this.rssUrl, this.feedLimit));
    Log.d(TAG, "onRestoreInstanceState: processRss() called");
}

private void processRss(String rssUrl) {
    StringRequest stringRequest = new StringRequest(Request.Method.GET, rssUrl, new Response.Listener<String>() {
        @Override
        public void onResponse(String response) {
            ParseApplication parseApplication = new ParseApplication();
            if (parseApplication.parse(response)) {
                ArrayAdapter<FeedEntry> arrayAdapter = new FeedAdapter(
                        MainActivity.this,
                        R.layout.list_record,
                        parseApplication.getApplications(),
                        VolleySingleton.getInstance(MainActivity.this).getImageLoader());

                xmlListView.setAdapter(arrayAdapter);
            }
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            Log.e(TAG, "onErrorResponse: from volley : " + error.getMessage());
        }
    });

    VolleySingleton.getInstance(this).addToRequestQueue(stringRequest);
}

对于 VolleySingleton，我使用这段代码，getRequestQueue 使用缓存

公共(public)类 VolleySingleton {

private static VolleySingleton mInstance;
private static Context mCtx;
private RequestQueue mRequestQueue;
private ImageLoader mImageLoader;

private VolleySingleton(Context context) {
    mCtx = context;
    mRequestQueue = getRequestQueue();

    mImageLoader = new ImageLoader(mRequestQueue,
            new ImageLoader.ImageCache() {
                private final LruCache<String, Bitmap>
                        cache = new LruCache<String, Bitmap>(20);

                @Override
                public Bitmap getBitmap(String url) {
                    return cache.get(url);
                }

                @Override
                public void putBitmap(String url, Bitmap bitmap) {
                    cache.put(url, bitmap);
                }
            });
}

public static synchronized VolleySingleton getInstance(Context context) {
    if (mInstance == null) {
        mInstance = new VolleySingleton(context);
    }
    return mInstance;
}

private RequestQueue getRequestQueue() {
    if (mRequestQueue == null) {
        File cacheDir = new File(mCtx.getCacheDir(), "VOLLEY_CACHE_DIR");
        mRequestQueue = new RequestQueue(
                new DiskBasedCache(cacheDir, 512 * 512),
                new BasicNetwork(new HurlStack()));

        mRequestQueue.start();
    }
    return mRequestQueue;
}

public <T> void addToRequestQueue(Request<T> req) {
    getRequestQueue().add(req);
}

public ImageLoader getImageLoader() {
    return mImageLoader;
}

Answer 1

1. am I put processXml() on a wrong place? If so, where is the correct place to avoid redundant network call?

是的，您在两个地方调用了该方法。不要在 onRestoreInstanceState 中重复相同的调用，您应该保留对正在进行的请求的引用。有几种方法可以做到这一点。其中一些是:

• 使用 getLastNonConfigurationInstance

• 使用 Service并将网络响应广播到您的 Activity ( http://www.truiton.com/2014/09/android-service-broadcastreceiver-example/ )

• 使用 Loaders ( https://medium.com/google-developers/making-loading-data-on-android-lifecycle-aware-897e12760832 )

2. since I use volley, is there any way to cache response and use it when device rotate? I think I've write the cache on VolleySingleton, but I don't know if my implementation is correct

您正在使用 DiskBasedCache，它看起来与 Android documentation 中的示例相同.您可以通过关闭网络连接(数据和 wifi)并旋转您的设备来轻松检查它是否正常工作。您还可以检查 Volley 代码以了解它如何使用缓存。