Android ListView：在基本适配器中使用异步类

我有一个类Download_格式，它是一个列出pdf文件的列表视图活动，我正在使用异步任务填充列表

下载\u格式

public class Download_format extends Activity {

    JSONObject jsonobject;
    JSONArray jsonarray;
    ArrayList<HashMap<String, String>> arraylist;
    static String url = "http://192.168.170.89/bbau_download.php";
    static String down_desc = "down_desc";
    static String down_url = "down_url";
    ListView listview;
    DownloadListViewAdapter adapter;

    @Override
    public void onCreate(Bundle savedInstantState) {
        super.onCreate(savedInstantState);
        // Get the view from listview_main.xml
        setContentView(R.layout.down_layout);
        if (url != null && url.trim().length() > 0) {
            //System.out.println("Before exec: "+url);
            new DownloadJSON().execute(url);
        }
    }

    private class DownloadJSON extends AsyncTask<String, Void, Void> {
        @Override
        protected Void doInBackground(String... params) {
            // Create an array
            arraylist = new ArrayList<HashMap<String, String>>();
            String url1 = params[0];
            // Retrieve JSON Objects from the given URL address
            jsonobject = JSONfunctions.getJSONfromURL(url1);

            try {
                // Locate the array name in JSON
                jsonarray = jsonobject.getJSONArray("Download");

                for (int i = 0; i < jsonarray.length(); i++) {
                    HashMap<String, String> map = new HashMap<String, String>();
                    jsonobject = jsonarray.getJSONObject(i);
                    // Retrive JSON Objects
                    System.out.println(jsonobject.getString("Down_desc"));
                    System.out.println(jsonobject.getString("Down_link"));
                    map.put("down_desc", jsonobject.getString("Down_desc"));
                    map.put("down_url", jsonobject.getString("Down_link"));
                    // Set the JSON Objects into the array
                    arraylist.add(map);
                }
            } catch (JSONException e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void args) {
            // Locate the listview in listview_main.xml
            listview = (ListView) findViewById(R.id.down_list);
            //listview.addItemDecoration(new HorizontalDividerItemDecoration.Builder(this).color(Color.BLACK).build());
            // Pass the results into ListViewAdapter.java
            adapter = new DownloadListViewAdapter(Download_format.this, arraylist);
            // Set the adapter to the ListView
            listview.setAdapter(adapter);

        }
    }
}

---

或者，如果有其他方法可以在单击列表视图项时下载pdf文件…请提供帮助。

您需要传入对

上下文的引用，因为
startActivity（）
是在
上下文中定义的。例如，
活动
是一个
上下文
。将上下文传递给构造函数中的DownloadFile，然后调用Context.startActivity（pdfIntent）；
public class DownloadListViewAdapter extends BaseAdapter {

    public static final int progress_bar_type = 0;
    Context context;
    LayoutInflater inflater;
    ArrayList<HashMap<String, String>> data;
    HashMap<String, String> resultp = new HashMap<String, String>();
    static int Down=0;
    private ProgressDialog pDialog = null;
    private static final int MEGABYTE = 1024 * 1024;

    public DownloadListViewAdapter(Context context,
                           ArrayList<HashMap<String, String>> arraylist) {
        this.context = context;
        data = arraylist;
    }

    @Override
    public int getCount() {
        return data.size();
    }

    @Override
    public Object getItem(int position) {
        return position;
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    public View getView(final int position, View convertView, ViewGroup parent) {
        // Declare Variables
        TextView down_desc;
        TextView down_url;


        inflater = (LayoutInflater) context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        View itemView = inflater.inflate(R.layout.down_format_item, parent, false);
        // Get the position
        resultp = data.get(position);

        // Locate the TextViews in listview_item.xml
        down_desc = (TextView) itemView.findViewById(R.id.down_text);
        down_url = (TextView) itemView.findViewById(R.id.down_url);


        // Capture position and set results to the TextViews
        down_desc.setText(resultp.get(Download_format.down_desc));
        down_url.setText(resultp.get(Download_format.down_url));

        // Capture ListView item click
        itemView.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // Get the position
                resultp = data.get(position);
                new DownloadFile().execute(resultp.get(Download_format.down_url), resultp.get(Download_format.down_desc+".pdf"));


            }
        });
        return itemView;
    }

private class DownloadFile extends AsyncTask<String, String, String> {


        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            showDialog(progress_bar_type);
        }

        @Override
        protected String doInBackground(String... strings) {
            String fileUrl = strings[0];   // -> http://maven.apache.org/maven-1.x/maven.pdf
            String fileName = strings[1];  // -> maven.pdf
            File pdfFile = null;
            //if there is no SD card, create new directory objects to make directory on device
            if (Environment.getExternalStorageState() == null) {
                System.out.println("SD card not installed");
                //create new file directory object
                String extStorageDirectory = Environment.getDataDirectory().toString();
                File folder = new File(extStorageDirectory, "BBAU_Downloads");
                System.out.println(extStorageDirectory);

                if (folder.exists()) {
                    File[] dirFiles = folder.listFiles();
                    if (dirFiles.length != 0) {
                        for (int ii = 0; ii < dirFiles.length; ii++) {
                            if(dirFiles[ii].getName().equals(fileName)){
                                System.out.println("File is present");
                                pDialog.dismiss();
                                Down=1;}
                        }
                    }
                }

                if (!folder.exists()) {
                    folder.mkdir();
                }
                pdfFile = new File(folder, fileName);

                try {
                    pdfFile.createNewFile();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }else if(Environment.getExternalStorageState() != null){
                System.out.println("SD card installed");
                String extStorageDirectory = Environment.getExternalStorageDirectory().toString();
                File folder = new File(extStorageDirectory, "MCAcourse");
                System.out.println(extStorageDirectory);

                if (folder.exists()) {
                    File[] dirFiles = folder.listFiles();
                    System.out.println(dirFiles.length);
                    if (dirFiles.length != 0) {
                        for (int ii = 0; ii < dirFiles.length; ii++) {
                            Log.d("PDF name:", dirFiles[ii].getName());
                            if(dirFiles[ii].getName().equals(fileName)){
                                System.out.println("File is present");
                                pDialog.dismiss();
                                Down=1;
                            }
                        }
                    }
                }

                if (!folder.exists()) {
                    folder.mkdir();
                }
                pdfFile = new File(folder, fileName);

                try {
                    pdfFile.createNewFile();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(Down==0) {
                try {
                    System.out.println(Down);
                    System.out.println("In Download file");
                    URL url = new URL(fileUrl);
                    URLConnection urlConnection = url.openConnection();
                    urlConnection.connect();

                    InputStream inputStream = urlConnection.getInputStream();
                    FileOutputStream fileOutputStream = new FileOutputStream(pdfFile);
                    int totalSize = urlConnection.getContentLength();
                    System.out.println(totalSize);
                    byte[] buffer = new byte[MEGABYTE];
                    long total = 0;
                    int bufferLength = 0;
                    while ((bufferLength = inputStream.read(buffer)) > 0) {
                        if (isCancelled()) {
                            inputStream.close();
                            return null;
                        }
                        total += bufferLength;
                        // publishing the progress....
                        if (totalSize > 0) // only if total length is known
                            publishProgress("" + (int) ((total * 100) / totalSize));
                        fileOutputStream.write(buffer, 0, bufferLength);
                    }
                    fileOutputStream.close();
                } catch (FileNotFoundException e) {
                    System.out.println("Some error");
                    e.printStackTrace();
                } catch (MalformedURLException e) {
                    System.out.println("Some error occured");
                    e.printStackTrace();
                } catch (IOException e) {
                    System.out.println("Some error occured again");
                    e.printStackTrace();
                }
            }
            return null;
        }
        @Override
        protected void onPostExecute(String s){

            File pdfFile = null;
            if(Environment.getExternalStorageDirectory()!=null) {
                pdfFile = new File(Environment.getExternalStorageDirectory() + "/MCAcourse/" + "syllabus.pdf");  // -> filename = maven.pdf
            }else if(Environment.getExternalStorageDirectory()==null){
                pdfFile = new File(Environment.getDataDirectory() + "/MCAcourse/" + "syllabus.pdf");
            }
            Uri path = Uri.fromFile(pdfFile);
            Intent pdfIntent = new Intent(Intent.ACTION_VIEW);
            pdfIntent.setDataAndType(path, "application/pdf");
            pdfIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

            try {
                startActivity(pdfIntent);
            } catch (ActivityNotFoundException e) {
                Toast.makeText(DownloadListViewAdapter.this, "No Application available to view PDF", Toast.LENGTH_SHORT).show();
            }

        }

        protected void onProgressUpdate(String... progress) {
            // setting progress percentage
            pDialog.setProgress(Integer.parseInt(progress[0]));
            if (Integer.parseInt(progress[0]) >= 100) {
                // close the progress bar dialog
                pDialog.dismiss();
            }
        }

    }
    @Override
    protected Dialog onCreateDialog(int id) {
        switch (id) {
            case progress_bar_type:
                pDialog = new ProgressDialog(this);
                pDialog.setMessage("Downloading file..");
                pDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
                pDialog.setCancelable(false);
                pDialog.show();
                return pDialog;
            default:
                return null;
        }
    }

try {
                    startActivity(pdfIntent);
                } catch (ActivityNotFoundException e) {
                    Toast.makeText(DownloadListViewAdapter.this, "No Application available to view PDF", Toast.LENGTH_SHORT).show();