无法从 Android 的线程中加载图像答案

【问题标题】：Can't load an image from within a thread in Android无法从 Android 的线程中加载图像
【发布时间】：2013-02-01 20:52:08
【问题描述】：

我正在尝试使用以下代码从服务器加载远程图像：

imageViewClient = (ImageView) findViewById(R.id.imageViewClient);
try 
{
    URL thumb_u = new URL(c.getImage());
    Drawable thumb_d = Drawable.createFromStream(thumb_u.openStream(), "src");
    imageViewClient.setImageDrawable(thumb_d);                      

}
catch (Exception e) 
{

}

图像显示正常，但是当我将代码放入新线程时，图像未加载。

代码是：

new Thread() 
{
    @Override
    public void run()
    {
            //** Set imageview url
            try {
                URL thumb_u = new URL(c.getImage());
                Drawable thumb_d = Drawable.createFromStream(
                    thumb_u.openStream(), "src");

                imageViewClient.setImageDrawable(thumb_d);                      

            }
            catch (Exception e) {

            }

    }
}.start();

图片不加载，为什么不加载？

【问题讨论】：

标签： android multithreading image

【解决方案1】：

您不能以这种方式更改线程内的 UI。您应该使用asyncTask 或runOnUiThread 方法来正确更改用户界面。或者更好，handler 像这样post

【讨论】：

【解决方案2】：

您需要使用Activity.runOnUiThread 从后台线程更改为：

 @Override
  public void run() {
     //** Set imageview url
     try {
        URL thumb_u = new URL(c.getImage());
         Drawable thumb_d = Drawable.createFromStream(
                              thumb_u.openStream(), "src");                      

        Your_current_Activity.this.runOnUiThread(new Runnable() {
        public void run() {
         // Chnage imageView bg here
        imageViewClient.setImageDrawable(thumb_d); 
        }
    });
       }
     catch (Exception e) {

    }

【讨论】：

【解决方案3】：

作为尝试像这样手动处理线程的替代方法（在这种情况下，您会遇到无法从另一个线程更新 UI 的问题），请查看 this article 并考虑使用类似AsyncTask 已经有一些这样的基础设施，实际上有一个适合您的用例的示例：

public void onClick(View v) {
  new DownloadImageTask().execute("http://example.com/image.png");
}

private class DownloadImageTask extends AsyncTask {
     protected Bitmap doInBackground(String... urls) {
         return loadImageFromNetwork(urls[0]);
     }

     protected void onPostExecute(Bitmap result) {
         mImageView.setImageBitmap(result);
     }
 }

【讨论】：

【解决方案4】：

这是一个简单的 AsyncTask，用于从网络下载和缓存图像：

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStream;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Environment;
import android.util.Log;
import android.widget.ImageView;

/**
 * Downloads an image from a URL and saves it to disk, to be referenced if the
 * same URL is requested again
 */
public class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {

    private static final String LOG = DownloadImageTask.class.getSimpleName();

    ImageView mImageView;

    public static final String IMAGE_CACHE_DIRECTORY = YourApplication
            .getContext().getExternalFilesDir(Environment.DIRECTORY_PICTURES)
            .getAbsolutePath();

    public DownloadImageTask(ImageView bmImage) {
        this.mImageView = bmImage;

        // TODO set to default "loading" drawable
        mImageView.setImageBitmap(null);
    }

    @Override
    protected Bitmap doInBackground(String... urls) {

        String url = urls[0];

        Log.v(LOG, "url: " + url);

        String filename = url.substring(url.lastIndexOf("/") + 1, url.length());

        Log.v(LOG, "filename: " + filename);

        Bitmap bmp = getBitmapFromDisk(filename);

        if (bmp == null) {
            try {
                InputStream in = new java.net.URL(url).openStream();
                bmp = BitmapFactory.decodeStream(in);
                writeBitmapToDisk(filename, bmp);
            } catch (Exception e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
        }

        return bmp;
    }

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

    public boolean writeBitmapToDisk(String imageid, Bitmap bmp) {
        boolean saved = false;

        File path = new File(IMAGE_CACHE_DIRECTORY);
        path.mkdirs();
        File imageFile = new File(path, imageid);
        FileOutputStream imageOS = null;
        try {
            imageOS = new FileOutputStream(imageFile);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        saved = bmp.compress(Bitmap.CompressFormat.JPEG, 95, imageOS);
        return saved;
    }

    public Bitmap getBitmapFromDisk(String imageid) {
        Bitmap bmp = BitmapFactory.decodeFile(IMAGE_CACHE_DIRECTORY + "/"
                + imageid);
        return bmp;
    }

}

具体实现如下：

new DownloadImageTask(mImageView).execute("https://www.google.com/images/srpr/logo3w.png");

【讨论】：