android - Volley - 如何将加载的数据缓存到磁盘并在没有可用连接或应用程序启动时加载它们

Question

我想使用 Volley 构建我的休息客户端。我找到了一些很好的例子(在下面发布)，除了缓存之外，它们完全符合我的要求。我想将结果缓存到 sdcard 并在启动应用程序时加载缓存的结果。关于如何修改下面的代码以启用对 ListView 项的文本和图像的缓存的任何线索？

/**
 * Demonstrates: 1. ListView which is populated by HTTP paginated requests; 2. Usage of NetworkImageView; 
 * 3. "Endless" ListView pagination with read-ahead
 * 
 * Please note that for production environment you will need to add functionality like handling rotation, 
 * showing/hiding (indeterminate) progress indicator while loading, indicating that there are no more records, etc...
 *   
 * @author Ognyan Bankov (ognyan.bankov@bulpros.com)
 *
 */
public class Act_NetworkListView extends Activity {
    private static final int RESULTS_PAGE_SIZE = 20;

    private ListView mLvPicasa;
    private boolean mHasData = false;
    private boolean mInError = false;
    private ArrayList<PicasaEntry> mEntries = new ArrayList<PicasaEntry>();
    private PicasaArrayAdapter mAdapter;


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

        mLvPicasa = (ListView) findViewById(R.id.lv_picasa);

        mAdapter = new PicasaArrayAdapter(this, 0, mEntries, new ImageLoader(queue, new BitmapCache(4)));

        mLvPicasa.setAdapter(mAdapter);
        mLvPicasa.setOnScrollListener(new EndlessScrollListener());
    }


    @Override
    protected void onResume() {
        super.onResume();

        if (!mHasData && !mInError) {
            loadPage();
        }
    }

    RequestQueue queue;
    private void loadPage() {
        queue = MyVolley.getRequestQueue();

        int startIndex = 1 + mEntries.size();
        JsonObjectRequest myReq = new JsonObjectRequest(Method.GET,
                "https://picasaweb.google.com/data/feed/api/all?q=kitten&max-results="
                        +
                        RESULTS_PAGE_SIZE
                        +
                        "&thumbsize=160&alt=json"
                        + "&start-index="
                        + startIndex,
                        null,
                        createMyReqSuccessListener(),
                        createMyReqErrorListener());


        if (myReq.getCacheEntry().data != null) {

        }
        queue.add(myReq);
    }


    private Response.Listener<JSONObject> createMyReqSuccessListener() {
        return new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                try {
                    JSONObject feed = response.getJSONObject("feed");
                    JSONArray entries = feed.getJSONArray("entry");
                    JSONObject entry;
                    for (int i = 0; i < entries.length(); i++) {
                        entry = entries.getJSONObject(i);

                        String url = null;

                        JSONObject media = entry.getJSONObject("media$group");
                        if (media != null && media.has("media$thumbnail")) {
                            JSONArray thumbs = media.getJSONArray("media$thumbnail");
                            if (thumbs != null && thumbs.length() > 0) {
                                url = thumbs.getJSONObject(0).getString("url");
                            }
                        }

                        mEntries.add(new PicasaEntry(entry.getJSONObject("title").getString("$t"), url));
                    }
                    mAdapter.notifyDataSetChanged();
                } catch (JSONException e) {
                    showErrorDialog();
                }
            }
        };
    }


    private Response.ErrorListener createMyReqErrorListener() {
        return new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                showErrorDialog();
            }
        };
    }


    private void showErrorDialog() {
        mInError = true;

        AlertDialog.Builder b = new AlertDialog.Builder(Act_NetworkListView.this);
        b.setMessage("Error occured");
        b.show();
    }


    /**
     * Detects when user is close to the end of the current page and starts loading the next page
     * so the user will not have to wait (that much) for the next entries.
     * 
     * @author Ognyan Bankov (ognyan.bankov@bulpros.com)
     */
    public class EndlessScrollListener implements OnScrollListener {
        // how many entries earlier to start loading next page
        private int visibleThreshold = 5;
        private int currentPage = 0;
        private int previousTotal = 0;
        private boolean loading = true;

        public EndlessScrollListener() {
        }
        public EndlessScrollListener(int visibleThreshold) {
            this.visibleThreshold = visibleThreshold;
        }

        @Override
        public void onScroll(AbsListView view, int firstVisibleItem,
                int visibleItemCount, int totalItemCount) {
            if (loading) {
                if (totalItemCount > previousTotal) {
                    loading = false;
                    previousTotal = totalItemCount;
                    currentPage++;
                }
            }
            if (!loading && (totalItemCount - visibleItemCount) <= (firstVisibleItem + visibleThreshold)) {
                // I load the next page of gigs using a background task,
                // but you can call any function here.
                loadPage();
                loading = true;
            }
        }

        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {

        }


        public int getCurrentPage() {
            return currentPage;
        }
    }

适配器:

public class PicasaArrayAdapter extends ArrayAdapter<PicasaEntry> {
    private ImageLoader mImageLoader;

    public PicasaArrayAdapter(Context context, 
                              int textViewResourceId, 
                              List<PicasaEntry> objects,
                              ImageLoader imageLoader
                              ) {
        super(context, textViewResourceId, objects);
        mImageLoader = imageLoader;
    }


    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;

        if (v == null) {
            LayoutInflater vi = (LayoutInflater) this.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(R.layout.lv_picasa_row, null);
        }

        ViewHolder holder = (ViewHolder) v.getTag(R.id.id_holder);       

        if (holder == null) {
            holder = new ViewHolder(v);
            v.setTag(R.id.id_holder, holder);
        }        

        PicasaEntry entry = getItem(position);
        if (entry.getThumbnailUrl() != null) {
            holder.image.setImageUrl(entry.getThumbnailUrl(), mImageLoader);
        } else {
            holder.image.setImageResource(R.drawable.no_image);
        }

        holder.title.setText(entry.getTitle());

        return v;
    }


    private class ViewHolder {
        NetworkImageView image;
        TextView title; 

        public ViewHolder(View v) {
            image = (NetworkImageView) v.findViewById(R.id.iv_thumb);
            title = (TextView) v.findViewById(R.id.tv_title);

            v.setTag(this);
        }
    }
}

Answer 1

这对于 volley 来说非常简单，因为缓存是内置的。您只需使用请求的 url 调用 getCache 即可。当然请记住，服务器负责设置缓存的最长使用期限，如果时间到了，缓存将被清除

例如试试这个:

if(queue.getCache().get(url)!=null){
  //response exists
  String cachedResponse = new String(queue.getCache().get(url).data);
}else{
  //no response
  queue.add(stringRequest);
}

缓存非常适合使用 VolleyExtended:eu.the4thfloor.volleyextended 有更新的响应监听器，您可以在其中获得额外的 bool 值更改，这意味着响应是否与缓存中的最后一个响应不同。并且在错误响应中有额外的参数响应(缓存)，您可以在服务器或网络中断的情况下使用它。当然如果只是缓存的话。

private class ResponseListenerYYY extends Api.ResponseListener {
@Override
public void onResponse(String response, boolean changed) {

}

@Override
public void onErrorResponse(VolleyError error, String response) {


}

}