使用AsyncTask在ListView中加载图像

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我有一个ListView,它可以保存一个图像。这取决于图像是否存在于sdCard中。

这里是我的示例代码:

public class MainActivity extends Activity  {

    ListView mListView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mListView = new ListView(this);
        setContentView(mListView);

        String[] arr = new String[] { 
                "/example/images/1.jpg", "/example/images/2.jpg",  
                "/example/images/3.jpg", "/example/images/4.jpg",  
                "/example/images/5.jpg", "/example/images/6.jpg", 
                "/example/images/7.jpg", "/example/images/8.jpg",  
                "/example/images/9.jpg", "/example/images/1.jpg", 
                "/example/images/2.jpg", "/example/images/3.jpg",  
                "/example/images/4.jpg", "/example/images/5.jpg",  
                "/example/images/6.jpg", "/example/images/7.jpg",  
                "/example/images/8.jpg", "/example/images/9.jpg", 
                "/example/images/1.jpg", "/example/images/2.jpg",  
                "/example/images/3.jpg", "/example/images/4.jpg",  
                "/example/images/5.jpg", "/example/images/6.jpg", 
                "/example/images/7.jpg", "/example/images/8.jpg",  
                "/example/images/9.jpg", "/example/images/1.jpg", 
                "/example/images/2.jpg", "/example/images/3.jpg",  
                "/example/images/4.jpg", "/example/images/5.jpg",  
                "/example/images/6.jpg", "/example/images/7.jpg",  
                "/example/images/8.jpg", "/example/images/9.jpg"}; 

        List<String> list = Arrays.asList(arr);

        MyAdapter adapter = new MyAdapter(this, R.layout.listitem_imv, list);

        mListView.setAdapter(adapter);
    }

    class MyAdapter extends ArrayAdapter<String>{

        List<String> mList;
        LayoutInflater mInflater;
        int mResource;

        public MyAdapter(Context context, int resource,
                List<String> objects) {
            super(context, resource, objects);

            mResource = resource;
            mInflater = getLayoutInflater();
            mList = objects;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View view;

            if(convertView == null){
                view = mInflater.inflate(mResource, null);
            }else{
                view = convertView;
            }

            ImageView imageView = (ImageView) view.findViewById(R.id.imv);
            TextView textView = (TextView) view.findViewById(R.id.txv);

                            imageView.setTag(mList.get(position));//tag of imageView == path to image
            new LoadImage().execute(imageView);
            textView.setText(mList.get(position).toString());

            return view;
        }       
    }

    class LoadImage extends AsyncTask<Object, Void, Bitmap>{

        private ImageView imv;
        private String path;


        @Override
        protected Bitmap doInBackground(Object... params) {
            imv = (ImageView)   params[0];

            path = imv.getTag().toString();

            Bitmap bitmap = null;
            File file = new File( 
                    Environment.getExternalStorageDirectory().getAbsolutePath() + path);

            if(file.exists()){
                bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
            }

            return bitmap;
        }
        @Override
        protected void onPostExecute(Bitmap result) {
            if(result != null && imv != null){
                imv.setVisibility(View.VISIBLE);
                imv.setImageBitmap(result);
            }else{
                imv.setVisibility(View.GONE);
            }
        }
    }
}

‘sdCard/样例/映像’目录有图像:1.jpg、2.jpg、3.jpg、4.jpg、6.jpg、7.jpg和9.jpg。预期的结果是:

但是,如果我快速滚动列表,一些图像会插入错误的项中。这是由于在getView()方法中使用了转换视图。

如果我使用以下代码,代码可以正常工作:

        //if(convertView == null){
        //  view = mInflater.inflate(mResource, null);
        //}else{
        //  view = convertView;
        //}
        view = mInflater.inflate(mResource, null);

当列表快速滚动时,由于使用了转换视图,两个异步任务可以引用一个相同的视图。当视图不再可见时,如何取消AsyncTask?

编辑

            @Override
    protected void onPostExecute(Bitmap result) {
        if(result != null && imv != null){

            if(imv.getTag().equals(path)){
                imv.setVisibility(View.VISIBLE);
                imv.setImageBitmap(result);
            }else{
                imv.setVisibility(View.GONE);
            }

        }else{
            imv.setVisibility(View.GONE);
        }
    }
提问于
用户回答回答于

可以将ImageView发送到任务构造函数,并在那里保留对图像路径的引用。现在在onPostExecute,检查ImageView的当前标记是否与开始使用的标记相同。如果是,那么设置图像。

然而,这意味着图像将在任何情况下被下载。你只是不会在视图上设置错误的图像。

编辑:首先,将ImageView传递给任务构造函数:

new LoadImage(imageView).execute()

然后在LoadImage构造函数中保存对ImageView和图像路径的引用。重要的是在构造函数中保存路径,而不是在doInBackplace中保存路径,以确保我们不会遇到多线程问题。然后在onPostExecute,我们检查当前的路径。

class LoadImage extends AsyncTask<Object, Void, Bitmap>{

        private ImageView imv;
        private String path;

        public LoadImage(ImageView imv) {
             this.imv = imv;
             this.path = imv.getTag().toString();
        }

    @Override
    protected Bitmap doInBackground(Object... params) {
        Bitmap bitmap = null;
        File file = new File( 
                Environment.getExternalStorageDirectory().getAbsolutePath() + path);

        if(file.exists()){
            bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
        }

        return bitmap;
    }
    @Override
    protected void onPostExecute(Bitmap result) {
        if (!imv.getTag().toString().equals(path)) {
               /* The path is not same. This means that this
                  image view is handled by some other async task. 
                  We don't do anything and return. */
               return;
        }

        if(result != null && imv != null){
            imv.setVisibility(View.VISIBLE);
            imv.setImageBitmap(result);
        }else{
            imv.setVisibility(View.GONE);
        }
    }

}
用户回答回答于

只需将http访问代码替换为SD卡文件读取即可。

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