MainActivity.这不是一个封闭类AsyncTask

Question

我试图第一次创建一个AsyncTask，但我没有太多的运气。

我的AsyncTask需要从服务器获取一些信息，然后在主布局中添加新的布局来显示这些信息。

一切似乎或多或少清楚，但是，错误信息"MainActivity不是一个封闭类"是困扰我。

似乎没有人有这个问题，所以我想我错过了一些非常明显的东西，我只是不知道它是什么。

另外，我不确定我是否使用了正确的方法来获取上下文，并且因为我的应用程序不编译所以我无法测试它。

非常感谢你的帮助。

下面是我的代码:

public class BackgroundWorker extends AsyncTask<Context, String, ArrayList<Card>> {
    Context ApplicationContext;
    @Override
    protected ArrayList<Card> doInBackground(Context... contexts) {
        this.ApplicationContext = contexts[0];//Is it this right way to get the context?
        SomeClass someClass = new SomeClass();
        return someClass.getCards();
    }
    /**
     * Updates the GUI before the operation started
     */
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }
    @Override
    /**
     * Updates the GUI after operation has been completed
     */
    protected void onPostExecute(ArrayList<Card> cards) {
        super.onPostExecute(cards);
        int counter = 0;
        // Amount of "cards" can be different each time
        for (Card card : cards) {
            //Create new view
            LayoutInflater inflater = (LayoutInflater) ApplicationContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            ViewSwitcher view = (ViewSwitcher)inflater.inflate(R.layout.card_layout, null);
            ImageButton imageButton = (ImageButton)view.findViewById(R.id.card_button_edit_nickname);
            /**
             * A lot of irrelevant operations here
             */ 
            // I'm getting the error message below
            LinearLayout insertPoint = (LinearLayout)MainActivity.this.findViewById(R.id.main);
            insertPoint.addView(view, counter++, new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));
        }
    }
}

Answer 1

Eclipse可能是正确的，并且您正在尝试从自己的文件(BackgroundWorker)中的另一个类访问位于自己的文件中的类(MainActivity)。没有办法做到这一点——一个类怎么能神奇地知道另一个类的实例呢?你能做的:

• 移动AsyncTask，所以它是一个内部类在MainActivity
• 将您的活动传递给AsyncTask(通过其构造函数)，然后使用activityVariable.findViewById();访问(我在下例中使用mActivity)或者，您的ApplicationContext(使用适当的命名约定，A需要小写)实际上是MainActivity的一个实例你很好去，所以做ApplicationContext.findViewById();

使用构造函数示例:

public class BackgroundWorker extends AsyncTask<Context, String, ArrayList<Card>>
{
    Context ApplicationContext;
    Activity mActivity;
   public BackgroundWorker (Activity activity)
   {
     super();
     mActivity = activity;
   }
//rest of code...

至于

我不确定我是否使用了正确的方法来获取上下文

Answer 2

上面的例子是内部类，这里是独立类

public class DownloadFileFromURL extends AsyncTask<String, String, String> {
ProgressDialog pd;
String pathFolder = "";
String pathFile = "";
Context ApplicationContext;
Activity mActivity;
public DownloadFileFromURL (Activity activity)
{
    super();
    mActivity = activity;
}
@Override
protected void onPreExecute() {
    super.onPreExecute();
    pd = new ProgressDialog(mActivity);
    pd.setTitle("Processing...");
    pd.setMessage("Please wait.");
    pd.setMax(100);
    pd.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
    pd.setCancelable(true);
    pd.show();
}
@Override
protected String doInBackground(String... f_url) {
    int count;
    try {
        pathFolder = Environment.getExternalStorageDirectory() + "/YourAppDataFolder";
        pathFile = pathFolder + "/yourappname.apk";
        File futureStudioIconFile = new File(pathFolder);
        if(!futureStudioIconFile.exists()){
            futureStudioIconFile.mkdirs();
        }
        URL url = new URL(f_url[0]);
        URLConnection connection = url.openConnection();
        connection.connect();
        // this will be useful so that you can show a tipical 0-100%
        // progress bar
        int lengthOfFile = connection.getContentLength();
        // download the file
        InputStream input = new BufferedInputStream(url.openStream());
        FileOutputStream output = new FileOutputStream(pathFile);
        byte data[] = new byte[1024]; //anybody know what 1024 means ?
        long total = 0;
        while ((count = input.read(data)) != -1) {
            total += count;
            // publishing the progress....
            // After this onProgressUpdate will be called
            publishProgress("" + (int) ((total * 100) / lengthOfFile));
            // writing data to file
            output.write(data, 0, count);
        }
        // flushing output
        output.flush();
        // closing streams
        output.close();
        input.close();

    } catch (Exception e) {
        Log.e("Error: ", e.getMessage());
    }
    return pathFile;
}
protected void onProgressUpdate(String... progress) {
    // setting progress percentage
    pd.setProgress(Integer.parseInt(progress[0]));
}
@Override
protected void onPostExecute(String file_url) {
    if (pd!=null) {
        pd.dismiss();
    }
    StrictMode.VmPolicy.Builder builder = new StrictMode.VmPolicy.Builder();
    StrictMode.setVmPolicy(builder.build());
    Intent i = new Intent(Intent.ACTION_VIEW);
    i.setDataAndType(Uri.fromFile(new File(file_url)), "application/vnd.android.package-archive" );
    i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    getApplicationContext().startActivity(i);
}

}