Question

我有一个可以从服务器下载图像的活动但是应用程序在完成下载图像之前没有加载UI，我正在使用此代码：

private Toolbar toolbar;
private ImageView downloadedImg;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    downloadedImg = (ImageView) findViewById(R.id.Main_Image_ID);
    toolbar = (Toolbar) findViewById(R.id.Main_Appbar_ID);

    setSupportActionBar(toolbar);

    getSupportActionBar().setDisplayShowHomeEnabled(true);

    NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
            getSupportFragmentManager().findFragmentById(R.id.Main_Activity_navigation_drawer_ID);
    drawerFragment.setUp(R.id.Main_Activity_navigation_drawer_ID, (DrawerLayout) findViewById(R.id.Main_DrawerLayout_ID), toolbar);


    ImageDownloader task = new ImageDownloader();
    Bitmap myImage;

    try {
        myImage = task.execute("https://drive.google.com/uc?id=0B51TujFYa0RBUUYzSE15WHZRR3c").get();

        downloadedImg.setImageBitmap(myImage);

    } catch (Exception e) {
        e.printStackTrace();
    }
    Log.i("Interaction", "Button Tapped");
}

public void ViewMone(View view) {
    startActivity(new Intent(MainActivity.this,MainWebViewActivity.class));
}


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

    @Override
    protected Bitmap doInBackground(String... urls) {
        try {
            URL url = new URL(urls[0]);
            HttpURLConnection connection = (HttpsURLConnection) url.openConnection();
            connection.connect();

            InputStream inputStream = connection.getInputStream();
            //to convert Image from string data to bitmap
            Bitmap myBitmap = BitmapFactory.decodeStream(inputStream);

            return myBitmap;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}

Answer 1

您未正确使用 AysncTask 类。通过调用get()，您可以暂停主线程，直到下载图像。这就是用户界面感觉缓慢的原因。

相反，您应该从任务的onPostExecute方法设置图像：

private Toolbar toolbar;
private ImageView downloadedImg;

@Override
protected void onCreate(Bundle savedInstanceState) {

    // ... omitted ...

    ImageDownloader task = new ImageDownloader();    
    task.execute("https://drive.google.com/uc?id=0B51TujFYa0RBUUYzSE15WHZRR3c");

}

// ... omitted ...    

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

    @Override
    protected Bitmap doInBackground(String... urls) {
        try {
            URL url = new URL(urls[0]);
            HttpURLConnection connection = (HttpsURLConnection) url.openConnection();
            connection.connect();

            InputStream inputStream = connection.getInputStream();
            //to convert Image from string data to bitmap
            Bitmap myBitmap = BitmapFactory.decodeStream(inputStream);

            return myBitmap;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    protected void onPostExecute(Bitmap result) {

        downloadedImg.setImageBitmap(result);
    }
}

Answer 2

您必须异步执行此操作，而不是在主活动中执行此操作，否则在下载完成之前，页面将被阻止。有很多选择可以做到这一点。我认为在这个问题的接受答案中表示最好：

Android : Loading an image from the Web with Asynctask

Answer 3

如果您查看异步任务的文档，您会发现get()具有以下描述

如果需要等待计算完成，然后检索其结果。

您应该在AsyncTask子类中实现onPostExecute并在那里处理您的图像，而不是调用get()。 Android开发者页面上甚至有一个例子。

public class ImageDownloader extends AsyncTask<String, Void, Bitmap> {
   <...>

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

我的Android应用程序中的UI速度很慢

3 个答案: