Android 从 AsyncTask 更新 UI 线程的最佳方式

Question

我今天发布了一个应用程序，我收到了一些崩溃报告，表明以下内容:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference

我有 Activity 和几个 Fragments。这个错误发生在我有一个 AsyncTask 的 fragment 中:

private ProgressDialog dialog;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    View view = inflater.inflate(R.layout.fragment_dial, container, false);
    dialog = new ProgressDialog(getActivity());
    new CountryCodeTask().execute();
    return view;
}

private class CountryCodeTask extends AsyncTask<Void, Void, ArrayList<String>>
{
    @Override
    protected void onPreExecute()
    {
        if (!dialog.isShowing()) {
            dialog.show();
        }
    }

    @Override
    protected void onPostExecute(ArrayList<String> result)
    {
        if (dialog.isShowing()) {
            dialog.dismiss();
        }

        if (result != null) {
            Spinner countryCodeSpinner = (Spinner) getActivity().findViewById(R.id.country_code_spinner);
            countryCodeSpinner.setAdapter(new CountryCodeAdapter(getActivity(), result));
            countryCodeSpinner.setSelection(countryCodeSpinnerValue);
            countryCodes = result;
        }
    }

    @Override
    protected ArrayList<String> doInBackground(Void... params)
    {
        return MainActivity.apiService.getCountryCodes();
    }
}

我猜这不是从 ASyncTask 更新 UI 线程的最佳方式。当我使用后退/主页按钮然后重新启动应用程序时(但并非始终如此)，会发生此错误。我在这里做错了什么？

错误发生在这一行:Spinner countryCodeSpinner = (Spinner) getActivity().findViewById(R.id.country_code_spinner);

Answer 1

您实际上并没有做错什么 - 可以预料的是，当您通过 back/home 离开 Activity 时，AsyncTask 将不再具有对该 Activity 的引用。我会修改您的代码来处理这种常见情况:

@Override
protected void onPostExecute(ArrayList<String> result)
{
    if (dialog != null && dialog.isShowing()) {
        dialog.dismiss();
    }
    Activity activity = getActivity();
    if (activity != null) {
        if (result != null) {
            Spinner countryCodeSpinner = (Spinner) activity.findViewById(R.id.country_code_spinner);
            countryCodeSpinner.setAdapter(new CountryCodeAdapter(activity, result));
            countryCodeSpinner.setSelection(countryCodeSpinnerValue);
            countryCodes = result;
        } else {
            cancelTaskAndShowDialog();
        }
   }
}