无法使用一个AsyncTask类显示进度对话框

public class MyTask extends AsyncTask<Void, Integer, String> {
    private Context mContext;
    private ProgressDialog mProgressDialog;

    public MyTask(Context context) {
        mContext = context;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        mProgressDialog = new ProgressDialog(mContext);
        mProgressDialog.setMessage("正在执行任务...");
        mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        mProgressDialog.show();
    }

    @Override
    protected String doInBackground(Void... voids) {
        // 执行耗时操作
        for (int i = 0; i < 100; i++) {
            // 模拟进度更新
            publishProgress(i);
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        return "任务执行完成";
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
        mProgressDialog.setProgress(values[0]);
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        mProgressDialog.dismiss();
        // 处理执行结果
        Toast.makeText(mContext, result, Toast.LENGTH_SHORT).show();
    }
}