Android：使用 Asynctask 从 Web 加载图像

Question

如何用 Asynctask 替换以下代码行？ 你如何从 Asynctask 中“取回”位图？谢谢。

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;
        //---------------------------------------------------

    }

我想过这样的事情：

替换：

mChart.setImageBitmap(download_Image(graph_URL));

类似：

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(Bitmap result) 中传递对 mChart 的引用？ 我是否需要以某种方式将其与 URL 一起传递？ 我想替换我所有的代码行：

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

使用类似的东西......但以 Asynctask 方式！

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

有没有简单的解决方案？ 我这里有什么问题吗？

Answer 1

如果没有充分的理由自己下载图像，那么我建议使用Picasso。

Picasso 为您省去了下载、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，请使用下面我的旧答案。

---

如果图像不是那么大，您可以使用匿名类来执行异步任务。 这会是这样的：

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) {
   ...
}

在标签中隐藏 URL 有点棘手，但如果您有很多要以这种方式填充的图像视图，它在调用类中看起来会更好。如果您在 ListView 中使用 ImageView 并且您想知道 ImageView 在下载图像期间是否被回收，这也会有所帮助。

我写了如果你的图像不是那么大，因为这将导致任务具有指向底层活动的隐式指针，导致垃圾收集器将整个活动保存在内存中，直到任务完成。如果用户在下载位图时移动到应用程序的另一个屏幕，则无法释放内存，这可能会使您的应用程序和整个系统变慢。

Answer 2

试试这个代码：

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

您可以创建一个类 say..BkgProcess，其中包含一个扩展 AsyncTask 的内部类。在实例化 BkgProcess 时，在 BkgProcess 构造函数中传递 Activity 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;
}
}
}

来自你的活动课..

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

dwn.callProgressDialog();

Answer 4

这将为您提供任何尺寸的图像... 如果您不想要进度对话框，只需在 onPreExecute() 中注释代码即可；

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
}