Android：使用异步箱从网络上加载图像

Question

如何将以下代码行替换为异步箱？您如何从异步箱“恢复”位图？谢谢你。

ImageView mChart = (ImageView) findViewById(R.id.Chart);
String URL = "http://www...anything ...";

mChart.setImageBitmap(download_Image(URL));

public static Bitmap download_Image(String url) {

        //---------------------------------------------------
        Bitmap bm = null;
        try {
            URL aURL = new URL(url);
            URLConnection conn = aURL.openConnection();
            conn.connect();
            InputStream is = conn.getInputStream();
            BufferedInputStream bis = new BufferedInputStream(is);
            bm = BitmapFactory.decodeStream(bis);
            bis.close();
            is.close();
        } catch (IOException e) {
            Log.e("Hub","Error getting the image from server : " + e.getMessage().toString());
        } 
        return bm;
        //---------------------------------------------------

    }

我想到了这样的事情：

代替 ：

mChart.setImageBitmap(download_Image(graph_URL));

像：

mChart.setImageBitmap(new DownloadImagesTask().execute(graph_URL));

和

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

@Override
protected Bitmap doInBackground(String... urls) {
    return download_Image(urls[0]);
}

@Override
protected void onPostExecute(Bitmap result) {
    mChart.setImageBitmap(result);              // how do I pass a reference to mChart here ?
}


private Bitmap download_Image(String url) {
    //---------------------------------------------------
    Bitmap bm = null;
    try {
        URL aURL = new URL(url);
        URLConnection conn = aURL.openConnection();
        conn.connect();
        InputStream is = conn.getInputStream();
        BufferedInputStream bis = new BufferedInputStream(is);
        bm = BitmapFactory.decodeStream(bis);
        bis.close();
        is.close();
    } catch (IOException e) {
        Log.e("Hub","Error getting the image from server : " + e.getMessage().toString());
    } 
    return bm;
    //---------------------------------------------------
}


}

但是，如何在Onpostexecute中传递对MCHART的引用（位图结果）？？？我需要以某种方式通过URL传递它？我想替换所有代码行：

mChart1.setImageBitmap(download_Image(URL_1));
mChart2.setImageBitmap(download_Image(URL_2));

有类似的东西...但是以异步性方式！

mChart1.setImageBitmap(new DownloadImagesTask().execute(graph_URL_1));
mChart2.setImageBitmap(new DownloadImagesTask().execute(graph_URL_2));

有一个简单的解决方案吗？我在这里遇到了问题吗？

Answer 1

如果没有充分的理由自己下载图像，那么我建议使用 毕加索.

毕加索为您节省了下载，设置和缓存图像的所有问题。一个简单示例所需的整个代码是：

Picasso.with(context).load(url).into(imageView);

如果您真的想自己做所有事情，请在下面使用我的旧答案。

---

如果图像不那么大，则可以将匿名类用于异步任务。这会这样：

ImageView mChart = (ImageView) findViewById(R.id.imageview);
String URL = "http://www...anything ...";

mChart.setTag(URL);
new DownloadImageTask.execute(mChart);

任务类：

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) {
   ...
}

隐藏标签中的URL有点棘手，但是如果您想以这种方式填充很多imageviews，则在调用类中看起来更好。如果您在listView中使用imageView，并且想知道在下载图像期间是否回收了imageView，这也有所帮助。

我写的话，如果您的图像不是那么大，因为这将导致任务具有隐含的指针，直到垃圾收集器将整个活动保存在内存中，直到完成任务为止。如果用户在位图下载时移至应用程序的另一个屏幕，则无法释放内存，并且可能会使您的应用程序和整个系统较慢。

Answer 2

尝试此代码：

ImageView myFirstImage = (ImageView) findViewById(R.id.myFirstImage);
ImageView mySecondImage = (ImageView) findViewById(R.id.mySecondImage);
ImageView myThirdImage = (ImageView) findViewById(R.id.myThirdImage);

String URL1 = "http://www.google.com/logos/2013/estonia_independence_day_2013-1057005.3-hp.jpg";
String URL2 = "http://www.google.com/logos/2013/park_su-geuns_birthday-1055005-hp.jpg";
String URL3 = "http://www.google.com/logos/2013/anne_cath_vestlys_93rd_birthday-1035005-hp.jpg";


myFirstImage.setTag(URL1);
mySecondImage.setTag(URL2);
myThirdImage.setTag(URL3);


new DownloadImageTask.execute(myFirstImage);
new DownloadImageTask.execute(mySecondImage);
new DownloadImageTask.execute(myThirdImage);



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;
    }
}

Answer 3

您可以创建一个says.bkgprocess，其中包含扩展异步的内部类。实例化BKGProcess在BKGPROCESS构造函数中通过您的活动类的上下文。例如：

public class BkgProcess {

 String path;   
 Context _context;

public Download(Downloader downloader, String path2){

 this.path = path2;
    _context = downloader;

}

public void callProgressDialog(){

new BkgProcess().execute((Void)null);
}
class Downloads extends AsyncTask<Void, Void, Boolean> {
    private ProgressDialog dialog = new ProgressDialog(_context);
    protected void onPreExecute(){
        dialog.setMessage("Downloading image..");
        dialog.show();
    }

    protected void onPostExecute(Boolean success) {
        dialog.dismiss();
        if(success)
            Toast.makeText(_context, "Download complete", Toast.LENGTH_SHORT).show();
    }

@Override
protected Boolean doInBackground(Void... params) {
    return(startDownload(path));

    }


public boolean startDownload(String img_url) {

// download img..

      return true;
}
}
}

从您的活动课上..

BkgProcess dwn = new BkgProcess (Your_Activity_class.this, img_path);

dwn.callProgressDialog();

Answer 4

这将为您提供任何大小的图像...如果您不希望“进度对话框”只需在OnPreeXecute（）中注释代码;

for(int i = 0 ; i < no_of_files ; i++ )
 new FetchFilesTask().execute(image_url[i]);


private class FetchFilesTask extends AsyncTask<String, Void, Bitmap> {

    private ProgressDialog dialog = new ProgressDialog(FileExplorer.this);
    Bitmap bitmap[];
    protected void onPreExecute(){
        dialog.setMessage("fetching image from the server");
        dialog.show();
    }

     protected Bitmap doInBackground(String... args) {

             bitmap = getBitmapImageFromServer();
         return bitmap;
     }

     protected void onPostExecute(Bitmap m_bitmap) {
         dialog.dismiss();
         if(m_bitmap != null)
             //store the images in an array or do something else with all the images.   
     }
 }

public Bitmap getBitmapImageFromServer(){

    // fetch image form the url using the URL and URLConnection class
}