在主屏幕上从json下载的DIsplay文件？

Question

我正在解析json，以显示用户可以点击下载的文件列表。我想在我的主屏幕上显示用户选择下载的文件。这是他们下载文件时的代码。

public final class Main extends Activity {

// Log tag for this class
private static final String TAG = "Main";

// Callback code for request of storage permission
final private int PERMISSIONS_REQUEST_STORAGE = 735;

// JSON Node Names
private static final String TAG_TYPE = "plans";
private static final String TAG_NAME = "name";
private static final String TAG_FILENAME = "filename";
private static final String TAG_URL = "url";
private static final String TAG_SHOULD_NOT_CACHE = "shouldNotCache";

// Files to delete
private static final ArrayList<String> filesToGetDeleted = new ArrayList<>();

// Strings displayed to the user
private static String offline;
private static String noPDF;
private static String localLoc;
private static String jsonURL;

// PDF Download
private static DownloadManager downloadManager;
private static long downloadID;
private static File file;

@SuppressLint("StaticFieldLeak")
private static SwipeRefreshLayout swipeRefreshLayout;

private final BroadcastReceiver downloadReceiver = new BroadcastReceiver() {

    @Override
    public void onReceive(Context arg0, Intent arg1) {
        final DownloadManager.Query query = new DownloadManager.Query();
        query.setFilterById(downloadID);
        final Cursor cursor = downloadManager.query(query);

        if (cursor.moveToFirst()) {
            final int columnIndex = cursor
                    .getColumnIndex(DownloadManager.COLUMN_STATUS);
            final int status = cursor.getInt(columnIndex);

            switch (status) {
                case DownloadManager.STATUS_SUCCESSFUL:
                    final Uri path = Uri.fromFile(file);
                    final 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) {
                        Utils.makeLongToast(Main.this, noPDF);
                        Log.e(TAG, e.getMessage());
                    }
                    break;
                case DownloadManager.STATUS_FAILED:
                    Utils.makeLongToast(Main.this,
                            getString(R.string.down_error));
                    break;
                case DownloadManager.STATUS_PAUSED:
                    Utils.makeLongToast(Main.this,
                            getString(R.string.down_paused));
                    break;
                case DownloadManager.STATUS_PENDING:
                    Utils.makeLongToast(Main.this,
                            getString(R.string.down_pending));
                    break;
                case DownloadManager.STATUS_RUNNING:
                    Utils.makeLongToast(Main.this,
                            getString(R.string.down_running));
                    break;
            }
        }
    }
};
// Data from JSON file
private ArrayList<HashMap<String, String>> downloadList = new ArrayList<>();

private static void checkDir() {
    final File dir = new File(Environment.getExternalStorageDirectory() + "/"
            + localLoc + "/");
    if (!dir.exists()) dir.mkdir();
}


@Override
protected void onResume() {
    super.onResume();
    final IntentFilter intentFilter = new IntentFilter(
            DownloadManager.ACTION_DOWNLOAD_COMPLETE);
    registerReceiver(downloadReceiver, intentFilter);
}

@Override
protected void onPause() {
    super.onPause();
    unregisterReceiver(downloadReceiver);
}

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

    // Delete files which should not get cached.
    for (String file : filesToGetDeleted) {
        final File f = new File(file);
        if (f.exists()) {
            f.delete();
        }
    }
}

@Override
public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
    switch (requestCode) {
        case PERMISSIONS_REQUEST_STORAGE:
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                Utils.makeLongToast(Main.this, getString(R.string.permission_write_external_storage_success));
            }
            else {
                final String permission_write_external_storage_failure = getString(R.string.permission_write_external_storage_failure);
                final String app_title = getString(R.string.app_name);
                final String message = String.format(permission_write_external_storage_failure, app_title);
                Utils.makeLongToast(Main.this, message);

                boolean showRationale = ActivityCompat.shouldShowRequestPermissionRationale(Main.this, permissions[0]);
                if (!showRationale) {
                    Intent intent = new Intent(Settings.ACTION_APPLICATION_DETAILS_SETTINGS);
                    Uri uri = Uri.fromParts("package", getPackageName(), null);
                    intent.setData(uri);
                    startActivity(intent);
                }
            }
            break;
    }
}

private void update(boolean force, boolean firstLoad) {
    downloadManager = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
    swipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swipeRefreshLayout_main);

    offline = getString(R.string.status_offline);
    noPDF = getString(R.string.except_nopdf);
    localLoc = getString(R.string.gen_loc);
    jsonURL = getString(R.string.gen_json);

    checkDir();

    if (firstLoad) {
        try {
            downloadList = (ArrayList<HashMap<String, String>>) Utils.readObject(this, "downloadList");
            if (downloadList != null) {
                setList(true, true);
            }
        } catch (ClassNotFoundException | IOException e) {
            Log.e(TAG, e.getMessage());
        }
    }

    // Parse the JSON file of the plans from the URL
    JSONParse j = new JSONParse();
    j.force = force;
    j.online = !Utils.isNoNetworkAvailable(this);
    j.execute();
}

private void setList(final boolean downloadable, final boolean itemsAvailable) {
    if (itemsAvailable) {
        try {
            Utils.writeObject(this, "downloadList", downloadList);
        } catch (IOException e) {
            Log.e(TAG, e.getMessage());
        }
    }
    final ListView list = (ListView) findViewById(R.id.listView_main);
    final ListAdapter adapter = new SimpleAdapter(this, downloadList,
            android.R.layout.simple_list_item_1, new String[]{TAG_NAME},
            new int[]{android.R.id.text1});
    list.setAdapter(adapter);

    // React when user click on item in the list
    if (itemsAvailable) {
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> parent, View v, int pos,
                                    long id) {
                int hasStoragePermission = ContextCompat.checkSelfPermission(Main.this,
                        Manifest.permission.WRITE_EXTERNAL_STORAGE);
                if (hasStoragePermission != PackageManager.PERMISSION_GRANTED) {
                    ActivityCompat.requestPermissions(Main.this,
                            new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                            PERMISSIONS_REQUEST_STORAGE);
                    return;
                }
                final Uri downloadUri = Uri.parse(downloadList.get(pos).get(TAG_URL));
                final String title = downloadList.get(pos).get(TAG_NAME);
                final String shouldNotCache = downloadList.get(pos).get(TAG_SHOULD_NOT_CACHE);
                file = new File(Environment.getExternalStorageDirectory() + "/"
                        + localLoc + "/"
                        + downloadList.get(pos).get(TAG_FILENAME) + ".pdf");
                final Uri dst = Uri.fromFile(file);

                if (file.exists()) {
                    final Intent pdfIntent = new Intent(Intent.ACTION_VIEW);
                    pdfIntent.setDataAndType(dst, "application/pdf");
                    pdfIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

                    try {
                        startActivity(pdfIntent);
                    } catch (ActivityNotFoundException e) {
                        Utils.makeLongToast(Main.this, noPDF);
                        Log.e(TAG, e.getMessage());
                    }
                    return;
                }
                if (downloadable && !Utils.isNoNetworkAvailable(Main.this)) {
                    // Download PDF
                    final Request request = new Request(downloadUri);
                    request.setTitle(title).setDestinationUri(dst);
                    downloadID = downloadManager.enqueue(request);
                    if (shouldNotCache.equals("true")) {
                        filesToGetDeleted.add(file.toString());
                    }
                } else {
                    Utils.makeLongToast(Main.this, offline);
                }
            }
        });
    }

    swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
        @Override
        public void onRefresh() {
            update(true, false);
        }
    });
}

private class JSONParse extends AsyncTask<String, String, JSONObject> {

    public boolean force = false;
    public boolean online = false;

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

        swipeRefreshLayout.post(new Runnable() {
            @Override
            public void run() {
                swipeRefreshLayout.setRefreshing(true);
            }
        });
    }

    @Override
    protected JSONObject doInBackground(String... args) {
        return JSONParser.getJSONFromUrl(Main.this, jsonURL, force, online);
    }

    @Override
    protected void onPostExecute(JSONObject json) {

        swipeRefreshLayout.post(new Runnable() {
            @Override
            public void run() {
                swipeRefreshLayout.setRefreshing(false);
            }
        });

        downloadList.clear();
        if (json == null) {
            String error = getString(R.string.except_json);
            if (!online) {
                error = offline;
            }
            final HashMap<String, String> map = new HashMap<>();
            map.put(TAG_NAME, error);
            downloadList.add(map);

            setList(false, false);
            return;
        }

        try {

            // Get JSON Array from URL
            final JSONArray j_plans = json.getJSONArray(TAG_TYPE);

            for (int i = 0; i < j_plans.length(); i++) {
                final JSONObject c = j_plans.getJSONObject(i);

                // Storing JSON item in a Variable
                final String ver = c.getString(TAG_FILENAME);
                final String name = c.getString(TAG_NAME);
                final String api = c.getString(TAG_URL);
                String shouldNotCache = "";
                if (c.has(TAG_SHOULD_NOT_CACHE)) {
                    shouldNotCache = c.getString(TAG_SHOULD_NOT_CACHE);
                }

                // Adding value HashMap key => value
                final HashMap<String, String> map = new HashMap<>();
                map.put(TAG_FILENAME, ver);
                map.put(TAG_NAME, name);
                map.put(TAG_URL, api);
                map.put(TAG_SHOULD_NOT_CACHE, shouldNotCache);
                downloadList.add(map);

                setList(online, true);
            }
        } catch (JSONException e) {
            Log.e(TAG, e.getMessage());
        }
    }
}

那么我如何获得上述代码的结果，并在用户每次打开应用程序时将其显示在主屏幕上