Question

我在主要活动中使用asynctask并从服务器数据填充列表项。现在我在每个列表项旁边放置了一个下载符号，现在我想为它实现onclick监听器。我没有使用任何自定义适配器将下载图标放在列表项旁边，我在xml中使用了imageview。现在我又制作了一个“下载”类，它从服务器下载一个特定的文件。当我在主要活动中点击图片（下载图标）时，它显示一个空指针异常。我的主要活动：

public class MainActivity extends Activity {
TextView uid;

Button Btngetdata;
//URL to get JSON Array
private static String url = "myurl";
//JSON Node Names
private static final String TAG_POSTS = "allposts";
private static final String TAG_SINGLEPOST = "post";

JSONArray user = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Btngetdata = (Button)findViewById(R.id.getdata);
    Btngetdata.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            new JSONParse().execute();
        }
    });




}
private class JSONParse extends AsyncTask<String, String, JSONObject> {
    private ProgressDialog pDialog;
    @Override
    protected void onPreExecute() {
        super.onPreExecute();


        pDialog = new ProgressDialog(MainActivity.this);
        pDialog.setMessage("Getting Data ...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(true);
        pDialog.show();
    }
    @Override
    protected JSONObject doInBackground(String... args) {
        JSONParser jParser = new JSONParser();
        // Getting JSON from URL
        JSONObject json = jParser.getJSONFromUrl(url);
        return json;
    }
    @Override
    protected void onPostExecute(JSONObject json) {
        pDialog.dismiss();
        try {
            // Getting JSON Array
            user = json.getJSONArray(TAG_POSTS);
            String[] allposts = new String[user.length()];
            for(int i = 0; i < user.length(); i++) {
                JSONObject c = user.getJSONObject(i);

                allposts[i]=c.getString(TAG_SINGLEPOST);
                ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                  MainActivity.this,
                       R.layout.singlepost,
                        R.id.singlepostid,
                        allposts
                );
                ListView list = (ListView) findViewById(R.id.allposts);
                list.setAdapter(adapter);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }
}
}

我的下载课程：

public class download extends Activity {

public static final int DIALOG_DOWNLOAD_PROGRESS = 0;

private ProgressDialog mProgressDialog;


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
   // startBtn = (Button)findViewById(R.id.downloadid);
    ImageView img = (ImageView) findViewById(R.id.downloadid);
    img.setOnClickListener(new View.OnClickListener(){
        public void onClick(View v) {

             startDownload();
        }
    });

}

public void startDownload() {
    String url = "urlforfile";
    new DownloadFileAsync().execute(url);
}
@Override
protected Dialog onCreateDialog(int id) {
    switch (id) {
        case DIALOG_DOWNLOAD_PROGRESS:
            mProgressDialog = new ProgressDialog(this);
            mProgressDialog.setMessage("Downloading file..");
            mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            mProgressDialog.setCancelable(false);
            mProgressDialog.show();
            return mProgressDialog;
        default:
            return null;
    }
}

class DownloadFileAsync extends AsyncTask<String, String, String> {

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

    @Override
    protected String doInBackground(String... aurl) {
        int count;

        try {

            URL url = new URL(aurl[0]);
            URLConnection conexion = url.openConnection();
            conexion.connect();

            int lenghtOfFile = conexion.getContentLength();
            Log.d("ANDRO_ASYNC", "Lenght of file: " + lenghtOfFile);

            InputStream input = new BufferedInputStream(url.openStream());
            OutputStream output = new FileOutputStream("/sdcard/fileonserver.pdf");

            byte data[] = new byte[1024];

            long total = 0;

            while ((count = input.read(data)) != -1) {
                total += count;
                publishProgress(""+(int)((total*100)/lenghtOfFile));
                output.write(data, 0, count);
            }

            output.flush();
            output.close();
            input.close();
        } catch (Exception e) {}
        return null;

    }
    protected void onProgressUpdate(String... progress) {
        Log.d("ANDRO_ASYNC",progress[0]);
        mProgressDialog.setProgress(Integer.parseInt(progress[0]));
    }

    @Override
    protected void onPostExecute(String unused) {
        dismissDialog(DIALOG_DOWNLOAD_PROGRESS);
    }
}
}

现在我想为listview中的每个下载图标实现onclick监听器。有人可以帮忙吗？

Answer 1

对于listview项目视图的子项，您希望每次回调的次数（或多或少）相同吗？您可以采取几种方法。您已经在适配器中实现了getView()，对吗？让我们在那里设置回调。

public View getView (int position, View convertView, ViewGroup parent){
    ...
    converView.findViewbyId(R.id.ID_OF_YOUR_DOWNLOAD_ICON).setOnClickListener(new OnClickListener() {
        //new DownloadFileAsync().execute(url);?
    });
    return convertView;
}

我不知道您的NPE在哪里，如果您发布堆栈跟踪，我们也可以提供帮助。

Android：列表视图中的图像的Onclick监听器

1 个答案: