Android Listview 异步图片加载

Question

我有一个 ListView ，我试图在其中执行一个异步任务，该任务将图像加载到适配器的 getView 方法内的 View 中。我已经成功地创建了这个但是我的问题是因为在 ListView 和适配器中 View 被回收，当用户滚动 ListView 时有很短的时间位于布局中的 imageView 显示 View 的图像以前的位置。然后它显示正确的图像。在调用异步任务之前，我尝试在 getView 方法中将 imageView 位图设置为 null，但它的行为仍然相同。如何使用异步任务将图像加载到 View 中，而不会在图像加载完成之前显示之前的图像？

Answer 1

嘿，按照几个步骤从服务器下载图像并显示在 ListView 中

第 1 步. 在您的项目中创建此类

OnImageDownloaded.java

public class OnImageDownloaded {

public OnImageDownloaded() {
    try {
        if (Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED)) {
            File file = new File(Environment.getExternalStorageDirectory()
                    .getAbsolutePath() + File.separator + "your_Dir_name");
            if (file.mkdirs()) {
            }
        } else {
            Log.e("testing", "External Directory is not mounted");
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public void downloadTheImages(ArrayList<String> imageUrls) {
    try {
        new downloadingTheImages(imageUrls).execute();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

class downloadingTheImages extends AsyncTask<Void, Void, Void> {
    ArrayList<String> imageUrls;

    public downloadingTheImages(ArrayList<String> imageUrls) {
        this.imageUrls = imageUrls;
    }

    @Override
    protected Void doInBackground(Void... params) {
        try {
            for (int i = 0; i < imageUrls.size(); i++) {
                if (imageUrls.get(i).equals("0")) {
                } else
                    downloadTheImageIfRequired(imageUrls.get(i));
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

public void downloadTheImageIfRequired(String imageName) {
    try {
        String dirPath = Environment.getExternalStorageDirectory()
                .getAbsolutePath()
                + File.separator
                + "your_Dir_name"
                + File.separator;
        String CompleteFilePath = dirPath + imageName;
        File f = new File(CompleteFilePath);
        if (f.exists()) {
        } else {
            URL url = new URL(
                    "http image URL ::"
                            + imageName);
            URLConnection conexion = url.openConnection();
            conexion.connect();

            InputStream input = new BufferedInputStream(url.openStream());
            OutputStream output = new FileOutputStream(CompleteFilePath);
            byte data[] = new byte[1024];
            int count;
            while ((count = input.read(data)) != -1) {
                output.write(data, 0, count);
            }
            output.flush();
            output.close();
            input.close();
        }

    } catch (Exception e) {
        e.printStackTrace();
    }
}

第 2 步。

看看你是如何得到那张图片的

  private class getImage extends AsyncTask<Void, Void, String> {
    Dialog dialog;
    String url;

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

    }

    @Override
    protected String doInBackground(Void... params) {
        url = getResources().getString(R.string.baseurl) + "getNews";

        JSONParser jParser = new JSONParser();
        String json = jParser.getJSONFromUrl(url);
        try {
            JSONObject jobject = new JSONObject(json);
            Log.e("testing", "url: " + url + " " + json);
            int success = jobject.getInt("success");
            Log.e("testing", "json length" + jobject.length());
            for (int i = 0; i < jobject.length() - 1; i++) {
                JSONObject jobj = jobject
                        .getJSONObject(Integer.toString(i));
                if (success == 1) {
                    HashMap<String, String> hm = new HashMap<String, String>();
                    ArrayList<String> tempAl1 = new ArrayList<String>();

                    tempAl1.add(jobj.getString("image"));
                    if (tempAl1.size() > 0) {
                        new OnImageDownloaded().downloadTheImages(tempAl1);
                    }

                    Log.e("test", "image" + jobj.getString("image"));

                    hm.put(image, jobj.getString("image"));

                    aldata.add(hm);
                }
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        if (dialog != null)
            if (dialog.isShowing())
                dialog.dismiss();
         Custom_Adapter adapter = new Custom_Adapter (
                (Activity) context, aldata);
        lv.setAdapter(adapter);
    }
}

第 3 步。

像这样在你的适配器中显示该图像

在适配器中的 getview 中调用这些方法

   @Override
   public View getView(int position, View convertView, ViewGroup parent) {
    View view = convertView;
    view = null;
    if (view == null) {
        LayoutInflater inflator = context.getLayoutInflater();
        view = inflator.inflate(R.layout.news_view, null);
        final ViewHolder viewHolder = new ViewHolder();
        initAll(view, viewHolder);
        view.setTag(viewHolder);
    }
    ViewHolder holder = (ViewHolder) view.getTag();
    fillAll(holder, position);
    return view;

}

 public void fillAll(final ViewHolder holder, final int position) {

    String dirPath = Environment.getExternalStorageDirectory()
            .getAbsolutePath()
            + File.separator
            + "your_Dir_name"
            + File.separator;
    String CompleteFilePath = dirPath + allData.get(position).get("image");
    File f = new File(CompleteFilePath);
    if (f.exists()) {
        Log.e("testingTag", "if part");
        holder.ivimage.setVisibility(View.VISIBLE);
        catchOutOfMemory(holder.ivimage, CompleteFilePath);
    } else {
        Log.e("testingTag", "else part");
        holder.ivimage.setVisibility(View.GONE);
    }

    Log.e("test", "image" + allData.get(position).get("image"));



}

void catchOutOfMemory(ImageView iv, String path) {
    try {
        iv.setImageURI(Uri.parse(path));
    } catch (OutOfMemoryError e) {
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inSampleSize = 8;
        Bitmap preview_bitmap = BitmapFactory.decodeFile(path, options);
        iv.setImageBitmap(preview_bitmap);
    }
}

谢谢了