安卓改造 : Wait for response

Question

我正在我的应用程序中实现改造 2 以调用网络服务。我的代码如下:

注册.java

ConnectionDetector connectionDetector = new ConnectionDetector(SignUpActivity.this);
if (connectionDetector.isConnectingToInternet()) {
    ArrayList<HashMap<String, String>> arrayListCountryDetails = new ArrayList<>();
    GetCountryList getCountryList = new GetCountryList();
    arrayListCountryDetails = getCountryList.CallWebServiceForCountryDetails(this);

    // The app should wait here till the above retrofit web service calling returns response


    CountryDetailsAdapter countryDetailsAdapter = new CountryDetailsAdapter(SignUpActivity.this, arrayListCountryDetails);
    spinnerCountryName.setAdapter(countryDetailsAdapter);
} else {
    String message = "No internet connection.";
    AlertDialog alertDialog = new AlertDialog.Builder(this).create();
    alertDialog.setTitle(getResources().getString(R.string.app_name));
    alertDialog.setMessage(message);
    alertDialog.setCancelable(false);
    alertDialog.setButton(AlertDialog.BUTTON_NEUTRAL, "OK",
            new DialogInterface.OnClickListener() {
               public void onClick(DialogInterface dialog, int which) {
                   dialog.dismiss();
               }
            });
    alertDialog.show();
}

GetCountryList.java

public class GetCountryList {
    ProgressDialog dialog;
    APIService mAPIService;
    ArrayList<HashMap<String, String>> arrayListCountryDetails;

    public ArrayList<HashMap<String, String>> CallWebServiceForCountryDetails(final Context context) {
        dialog = new ProgressDialog(context);
        dialog.setMessage("Please wait...");
        dialog.setCancelable(false);
        dialog.show();

        arrayListCountryDetails = new ArrayList<>();

        mAPIService = ApiUtils.getAPIService();
        mAPIService.getCountryDetails().enqueue(new Callback<CountryDetailsResponseModel>() {
            @Override
            public void onResponse(Call<CountryDetailsResponseModel> call, Response<CountryDetailsResponseModel> response) {

                if (response.isSuccessful()) {
                    HashMap<String, String> cntDetails = new HashMap<>();
                    cntDetails.put("airLineID", "0");
                    cntDetails.put("airLineName", "Select Airline");
                    arrayListCountryDetails.add(cntDetails);

                    // Get response
                    try {
                        if (response.body().getStatus() == 200 && response.body().getMessage().equalsIgnoreCase("success")) {
                            for (int count = 0; count < response.body().getCountry().size(); count++) {
                                cntDetails = new HashMap<>();
                                String countryID = response.body().getCountry().get(count).getCountryId();
                                String countryName = response.body().getCountry().get(count).getCountryName();

                                cntDetails.put("countryID", countryID);
                                cntDetails.put("countryName", countryName);
                                arrayListCountryDetails.add(cntDetails);
                            }

                            // do UI work here
                            if (dialog.isShowing()) {
                                dialog.dismiss();
                            }
                        } else {
                            // do UI work here
                            if (dialog.isShowing()) {
                                dialog.dismiss();
                            }
                        }
                    } catch (Exception e) {
                        // do UI work here
                        if (dialog.isShowing()) {
                            dialog.dismiss();
                        }
                    }
                }
            }

            @Override
            public void onFailure(Call<AirLineDetailsResponseModel> call, Throwable t) {
                // do UI work here
                if (dialog.isShowing()) {
                    dialog.dismiss();
                }
            }
        });

        return arrayListCountryDetails;
    }
}

当我执行代码时，出现空指针异常错误:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setAdapter(android.widget.SpinnerAdapter)' on a null object reference
    at com.abc.xyz.SignUpActivity.initializeScreen(SignUpActivity.java:176)
    at com.abc.xyz.SignUpActivity.onCreate(SignUpActivity.java:147)
    at android.app.Activity.performCreate(Activity.java:6575)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1134)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3121)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3278) 
    at android.app.ActivityThread.access$1000(ActivityThread.java:211) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1705) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:145) 
    at android.app.ActivityThread.main(ActivityThread.java:6912) 
    at java.lang.reflect.Method.invoke(Native Method)

我知道这是因为微调器初始化执行发生在改造响应之前。

请建议我如何等待改造响应。我需要在上面的代码中做哪些更改。由于这个问题，我无法继续前进。提前致谢。

Answer 1

非常粗略地做类似下面的事情。我只是将必要的代码部分放在 AsyncTask 方法中。根据需要进行修改。

if (connectionDetector.isConnectingToInternet()) {


        // The app should wait here till the above retrofit web service calling returns response

    AsyncTask task = new AsyncTask<Void, Void, List<Map<String, String>>>() {
        @Override
        protected String doInBackground(Void... params) {
            List<Map<String, String>> arrayListCountryDetails = new ArrayList<>();
            GetCountryList getCountryList = new GetCountryList();
            arrayListCountryDetails = getCountryList.CallWebServiceForCountryDetails(this);
            return arrayListCountryDetails;
        }

        @Override
        protected void onPostExecute(List<Map<String, String>> arrayListCountryDetails) {
            CountryDetailsAdapter countryDetailsAdapter = new CountryDetailsAdapter(SignUpActivity.this, arrayListCountryDetails);
            spinnerCountryName.setAdapter(countryDetailsAdapter);
        }

        @Override
        protected void onPreExecute() {}

        @Override
        protected void onProgressUpdate(Void... values) {}
    }
    task.execute();

}

同时从您的 GetCountryList 中删除所有 UI 调用，因为它将在“后台”运行