使用 AsyncTask 加载图像 [英] Loading Image using AsyncTask

问题描述

我正在尝试创建一个自定义列表适配器，其中包含需要从 Internet 下载的每个项目的图像.当我第一次进入活动时 - 应用程序会冻结一段时间，直到下载图像然后加载活动列表.

I am trying to create a custom List Adapter which has an Image for every item it needs to download from the internet. When I first enter the Activity - the app freezes for a while till the images are downloaded and then the Activity List is loaded.

据我所知，在活动加载之前正在下载图像.活动加载后如何下载图像.我想我需要使用异步任务.但由于我正在自定义阵列适配器中加载图像，因此不知道该怎么做.

As I can make out the images are being downloaded before the activity loads. How can I download the images after the activity has loaded. I think I need to use Async Task. But since I am loading the images in the Custom Array Adapter not sure how to do it.

这是我的自定义适配器的getView():

This is my Custom Adapter's getView():

public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;

        if (v == null) {
            LayoutInflater vi;
            vi = LayoutInflater.from(getContext());
            v = vi.inflate(R.layout.list_item_default, null);
        }

        Item p = items.get(position);

        if (p != null) {

            TextView list_title = (TextView) v.findViewById(R.id.list_title);
            TextView list_description = (TextView) v
                    .findViewById(R.id.list_description);
            TextView list_timestamp = (TextView) v
                    .findViewById(R.id.list_timestamp);
            ImageView list_image = (ImageView) v.findViewById(R.id.list_image);

            if (list_title != null) {
                list_title.setText(p.getItemTitle());
            }

            if (list_description != null) {
                list_description.setText(p.getItemDescription());
            }

            if (list_timestamp != null) {
                list_timestamp.setText(p.getItemTimestamp());
            }

            if (list_image != null) {
                Log.d("bMobile", "inside getView() image");
                try {
                    URL imageURL = new URL(p.getItemImage());
                    HttpURLConnection con = (HttpURLConnection) imageURL
                            .openConnection();
                    InputStream inputStrem = con.getInputStream();
                    Bitmap image = BitmapFactory.decodeStream(inputStrem);
                    if (null != image)
                        list_image.setImageBitmap(image);
                    else
                        Log.d("bMobile", "Bitmap is Null");
                } catch (Exception e) {
                }
            }
        }

        return v;
    }

异步任务:

public class DownloadImagesTask extends AsyncTask<ImageView, Void, Bitmap> {

    ImageView imageView = null;

    @Override
    protected Bitmap doInBackground(ImageView... imageViews) {
        this.imageView = imageViews[0];
        return download_Image((String)imageView.getTag());
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        imageView.setImageBitmap(result);
    }

}
    private Bitmap download_Image(String url) {

        Bitmap bmp =null;
        try{
            URL ulrn = new URL(url);
            HttpURLConnection con = (HttpURLConnection)ulrn.openConnection();
            InputStream is = con.getInputStream();
            bmp = BitmapFactory.decodeStream(is);
            if (null != bmp)
                return bmp;

            }catch(Exception e){}
        return bmp;
    }

推荐答案

应用程序冻结" - 这是因为您正在事件线程或 UI 线程上下载图像.你永远不应该那样做.所有阻塞操作、长时间运行的操作、网络操作都应该在单独的线程上运行.是的，您可以使用 asynctask 进行图像加载，这应该可以解决问题.

"the app freezes" - This is because you are downloading the images on the event thread or UI Thread. You should never do that. All blocking operations,long running operations, network operations, should be run on a separate thread. Yes you can use asynctask to do the image loading which should fix the issue.

解决问题的两个选项是

1. 使用来自 http://brainflush.wordpress.com/2009/11/23/droid-fu-part-2-webimageview-and-webgalleryadapter/WebImageView 也使用缓存，因此您无需再次下载已经下载的图像.下载图像既昂贵又消耗数据.我使用了 WebImageView 并且能够在列表中加载图像.我只花了大约 20 分钟就完成了所有工作，所以你知道它很容易集成.

1. Use WebImageView from droid fu library at http://brainflush.wordpress.com/2009/11/23/droid-fu-part-2-webimageview-and-webgalleryadapter/ WebImageView uses caching as well so you dont need to download images again which you have already downloaded. Downloading images are expensive and consume data. I used WebImageView and was able to load images in list. It took me just about 20 mins to get it all working, so you know its easy integrating it.

第二个选项是使用异步任务.请检查 Android:使用 Asynctask 从网络加载图像.stackoverflow 中有更多关于如何执行此操作的信息.

Second option is to use async task . Please check Android : Loading an image from the Web with Asynctask . There are lot more info in stackoverflow regarding how to do this.

每当您的 UI 挂起时，您都应该有一种令人毛骨悚然的感觉，即您正在 UI 线程上执行某些操作.

Any time your UI hangs, you should have a creepy feeling, that you are doing something on the UI thread.

这篇关于使用 AsyncTask 加载图像的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！