缓存 JSON 响应以在设备离线时使用

Question

我正在构建一个新闻应用程序，我想缓存一个 JSON 响应以在没有网络时使用。我尝试了很多方法，但都不适合我。

我的异步任务和我的适配器是另一个问题，当列表加载新项目时，它会进入屏幕顶部，然后添加项目。我该如何解决这个问题？

private class jsontask extends AsyncTask<String, String, List<newsmodel>> {
    @Override
    protected List<newsmodel> doInBackground(String... params) {
        BufferedReader reader = null;
        HttpURLConnection connection = null;
        try {
            URL url = new URL(params[0]);

            connection = (HttpURLConnection) url.openConnection();

            connection.connect();
            InputStream stream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));
            StringBuffer buffer = new StringBuffer();
            String line = "";
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }

            String finaljson = buffer.toString();
            JSONObject parentobject = new JSONObject(finaljson);

            JSONArray parentarray = parentobject.getJSONArray("articles");

            for (int i = 0; i < parentarray.length(); i++) {
                JSONObject finalobject = parentarray.getJSONObject(i);
                newsmodel newsmodel = new newsmodel();
                newsmodel.setAuthor(finalobject.getString("author"));
                if (finalobject.isNull("author")) {
                }
                newsmodel.setDescription(finalobject.getString("description"));
                newsmodel.setTitle(finalobject.getString("title"));
                newsmodel.setImage(finalobject.getString("urlToImage"));
                newsmodel.setUrl(finalobject.getString("url"));
                newsmodel.setPublishedAt(finalobject.getString("publishedAt"));

                moviemodelList.add(newsmodel);
            }

            return moviemodelList;

        } catch (MalformedURLException e) {
            e.printStackTrace();
return moviemodelList;
        } catch (IOException e) {
            e.printStackTrace();
            return moviemodelList;
        } catch (JSONException e) {

        } finally {
            if (connection != null) {
                connection.disconnect();
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return null;
    }

    @Override
    protected void onPostExecute(List<newsmodel> result) {

        super.onPostExecute(result);
        newsadapter adapter = new newsadapter(getApplicationContext(), R.layout.row, result);
        lvnews.setAdapter(adapter);
    }
}

public class newsadapter extends ArrayAdapter {
    private List<newsmodel> moviemodelList;
    private int resource;
    private LayoutInflater inflater;

    public newsadapter(Context context, int resource, List<newsmodel> objects) {
        super(context, resource, objects);
        moviemodelList = objects;
        this.resource = resource;
        inflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        viewholder holder = null;
        if (convertView == null) {
            holder = new viewholder();
            convertView = inflater.inflate(resource, null);
            holder.newsimage = (ImageView) convertView.findViewById(R.id.imageView2);
            holder.title = (TextView) convertView.findViewById(R.id.textView2);
            holder.description = (TextView) convertView.findViewById(R.id.textView3);
            holder.author = (TextView) convertView.findViewById(R.id.textView4);
            holder.publishdate = (TextView) convertView.findViewById(R.id.textView5);
            holder.dotsmenu = (ImageButton) convertView.findViewById(R.id.dots);
            convertView.setTag(holder);
        } else {
            holder = (viewholder) convertView.getTag();
        }

        final ProgressBar progressBar;
        progressBar = (ProgressBar) convertView.findViewById(R.id.progressBar);
        ImageLoader.getInstance().displayImage(moviemodelList.get(position).getImage(), holder.newsimage, new ImageLoadingListener() {
            @Override
            public void onLoadingStarted(String imageUri, View view) {
                progressBar.setVisibility(view.VISIBLE);
            }

            @Override
            public void onLoadingFailed(String imageUri, View view, FailReason failReason) {
                progressBar.setVisibility(view.GONE);
            }

            @Override
            public void onLoadingComplete(String imageUri, View view, Bitmap loadedImage) {
                progressBar.setVisibility(view.GONE);
            }

            @Override
            public void onLoadingCancelled(String imageUri, View view) {
                progressBar.setVisibility(view.GONE);
            }
        });
        holder.title.setText(moviemodelList.get(position).getTitle());
        holder.description.setText(moviemodelList.get(position).getDescription());
        holder.author.setText(moviemodelList.get(position).getAuthor());
        holder.publishdate.setText(moviemodelList.get(position).getPublishedAt());
        final viewholder finalHolder = holder;
        holder.dotsmenu.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                PopupMenu popup = new PopupMenu(page.this, finalHolder.dotsmenu);
                //Inflating the Popup using xml file
                popup.getMenuInflater().inflate(R.menu.dots_menu, popup.getMenu());
                popup.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {
                    @Override
                    public boolean onMenuItemClick(MenuItem item) {
                        switch (item.getItemId()) {

                            case R.id.share:
                                Intent sendIntent = new Intent();
                                sendIntent.setAction(Intent.ACTION_SEND);
                                sendIntent.putExtra(Intent.EXTRA_TEXT, moviemodelList.get(position).getUrl());
                                sendIntent.setType("text/plain");
                                startActivity(sendIntent);
                                return true;
                        }
                        return false;
                    }

                });
                popup.show();
            }
        });
        lvnews.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

                Intent intent = new Intent(page.this, webview.class);
                intent.putExtra("Link", moviemodelList.get(position).getUrl());
                startActivity(intent);
            }
        });

        return convertView;
    }

Answer 1

创建一个类来测试网络连接是否可用。

ConnectionDetector.java

public class ConnectionDetector {
public  Context context;
public ConnectionDetector(Context context)
{
    this.context=context;
}
public boolean isConnecting() {
    ConnectivityManager check=(ConnectivityManager)context.getSystemService(Context.CONNECTIVITY_SERVICE);
    if (check!=null){
        NetworkInfo[] infos=check.getAllNetworkInfo();
        for (int i=0;i<infos.length;i++)
        {
            if (infos[i].getState()== NetworkInfo.State.CONNECTED)
                return true;
        }}

    return false;

}
}

创建一个上述类的对象ob。

ConnectionDetector ob=new  ConnectionDetector(getApplicationContext());

将这些变量声明为全局变量：

  File httpCacheDir;
  long httpCacheSize = 5 * 1024 * 1024;// In place of 5 you can use size in mb
  HttpResponseCache cache;

制作一个函数cacher()：

 public void cacher()
 {
    httpCacheDir = getExternalCacheDir();

    // Cache Size of 5MB


    try {
        // Install the custom Cache Implementation
        HttpResponseCache.install(httpCacheDir, httpCacheSize);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

在 onCreate 的 setContentView 下方，添加以下代码：

   cacher();
   cache = HttpResponseCache.getInstalled();
    if(cache != null) {
        //   If cache is present, flush it to the filesystem.
        //   Will be used when activity starts again.
        cache.flush();
    }

这是最后一件事，在 url.openConnection 行下面放置以下 sn-p:

 if (ob.isConnecting()){
            connection.addRequestProperty("Cache-Control", "max-age=0");}
            else{
            connection.addRequestProperty("Cache-Control", "max-stale=" +  60*60*36);//In place of 36 , you can put hours for which cache is available 
            connection.setUseCaches(true);
}