如何在 Asynctask 中调用 Mainactivity 方法进行后台工作？

Question

我想要一个方法（在 MainActivity 中）从 MyAsynctask 类（在 MainActivity 中）的 doInBackground(File... file) 方法中调用，并且该方法必须在后台工作，因为它需要时间并且我的应用程序会暂时停止工作并且没有出现我在onPreExecute() 方法中调用的任何对话框如何解决这里的问题是我的代码它工作得很好但花费了太多时间而且我的应用程序看起来挂起但实际上并非如此。

ArrayList<HashMap<String,String>> getPlayList(File rootFolder) {
    ArrayList<HashMap<String,String>> fileList = new ArrayList<>();

    try {
        File[] files = rootFolder.listFiles();
         //here you will get NPE if directory doesn't contains  any file,handle it like this.
        if (files != null) {
            for (File file : files) {
                if (file.isDirectory() && !file.isHidden()) {
                    if (true) {
                        fileList.addAll(getPlayList(file));
                    }
                    else {
                        break;
                    }
                } else if (file.getName().endsWith(".pdf")) {
                    HashMap<String, String> song = new HashMap<>();
                    song.put("file_path", file.getAbsolutePath());
                    song.put("file_name", file.getName());
                    fileList.add(song);
                }
            }
        }
        return fileList;
    } catch (Exception e) {
        return null;
    }
}

Asynctask 类在下面..

private class AsyncTaskExample extends AsyncTask<File, String, ArrayList> {


    @Override
    protected ArrayList doInBackground(File... file) {

        ArrayList<HashMap<String,String>> songList=getPlayList(folder);
        if(songList!=null){
            for(int i=0;i<songList.size();i++){
                final String fileName=songList.get(i).get("file_name");
                final String filePath=songList.get(i).get("file_path");

            //saving filePath and filName in SQLite Database..
                saveFileToDatabase(filePath, fileName);

            }
        }
        return songList;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        p = new ProgressDialog(MainActivity.this);
        p.setMessage("Please wait...Loading..");
        p.setIndeterminate(false);
        p.setCancelable(false);
        p.show();
    }

    @Override
    protected void onPostExecute(ArrayList arrayList) {
        super.onPostExecute(arrayList);
        if (mFiles != null)
        {
            p.dismiss();
        }
        else
        {
            p.show();
        }
    }
}

如何高效地完成工作？

解决方案

问题在于下面给出的这些代码行：

AsyncTaskExample taskExample = new AsyncTaskExample();
        folder = new File(Environment.getExternalStorageDirectory().getAbsolutePath());
      //I was writing this below line..
        taskExample.doInBackground(folder);

      //but it should be like in line below..
      taskExample.execute(folder);

Answer 1

我建议你定义一个回调，并在你的异步任务的构造函数中传递它，这样你就可以通知你的主要活动在需要时显示对话框。

回调

public interface AsyncTaskExampleCallback {
    public void onShowDialog();
    public void onHideDialog();
}

异步任务

class AsyncTaskExample extends AsyncTask<File, String, ArrayList> {

    @Nullable
    private AsyncTaskExampleCallback callback = null;

    public AsyncTaskExample(@NotNull AsyncTaskExampleCallback callback) {
        this.callback = callback;
    }

    @Override
    protected ArrayList doInBackground(File... file) {

        ArrayList<HashMap<String, String>> songList = getPlayList(folder);
        if (songList != null) {
            for (int i = 0; i < songList.size(); i++) {
                final String fileName = songList.get(i).get("file_name");
                final String filePath = songList.get(i).get("file_path");

                //saving filePath and filName in SQLite Database..
                saveFileToDatabase(filePath, fileName);

            }
        }
        return songList;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        if (callback != null) {
            callback.onShowDialog();
        }
    }

    @Override
    protected void onPostExecute(ArrayList arrayList) {
        super.onPostExecute(arrayList);
        if (mFiles != null) {
            if (callback != null) {
                callback.onHideDialog();
            }
        } else {
            if (callback != null) {
                callback.onShowDialog();
            }
        }
    }
}

主要活动

public class MainActivity extends AppCompatActivity implements AsyncTaskExampleCallback {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        new AsyncTaskExample(this).execute();
    }

    @Override
    public void onShowDialog() {
        //Show dialog here
    }

    @Override
    public void onHideDialog() {
        //Hide dialog here
    }
}