Question

我想使用接口从AsyncTask类返回一个值。问题是我的以下代码在Activity中工作正常但在片段类中没有。

我这样得ClassCastException：

java.lang.ClassCastException: com.demo.HomeActivity cannot be cast to com.demo.helper.OnTaskCompleteListener
at com.demo.util.JSONParseAsync.<init>(JSONParseAsync.java:33)
at com.demo.fragment.PersonalDetailFragment.loadProfileAction(PersonalDetailFragment.java:93)
at com.demo.fragment.PersonalDetailFragment.onCreate(PersonalDetailFragment.java:81)
at android.support.v4.app.Fragment.performCreate(Fragment.java:1942)

接口类：

public interface OnTaskCompleteListener {

void onTaskComplete(JSONObject jsonObject);

}

PersonalDetailFragment类：

public class PersonalDetailFragment extends Fragment  implements OnTaskCompleteListener {
private View view;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.fragment_personal_detail, container,
            false);
    loadProfileAction();
    return view;
}
private void loadProfileAction() {

    SessionPreference preference = new SessionPreference(getActivity());
    try {
        String encodedUrl = URLEncoder.encode(preference.getSessionId(), "UTF-8")
                + ","
                + URLEncoder.encode(Constants.URL_TOKEN, "UTF-8");
        // URL base64Encode
        String processUrl = Base64.encodeToString(encodedUrl.getBytes("UTF-8"), Base64.DEFAULT);

        JSONParseAsync parseAsync = new JSONParseAsync(getContext()); // also try getActivity()
        parseAsync.execute((URLConstants.GET_USER_DETAIL_URL+processUrl).trim());

    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (Exception e) {
        e.printStackTrace();
    }

}
@Override
public void onTaskComplete(JSONObject jsonObject) {
    try {

        boolean status = jsonObject.getBoolean(URLConstants.TAG_STATUS);

        Log.e(Constants.DEBUG_TAG, "Status:- " + status);

        if (status == true) {
            JSONArray dataarray = jsonObject.getJSONArray(URLConstants.TAG_DATA);
            JSONObject data = dataarray.getJSONObject(0);
            fillProfileData(data);

        } else if (status == false) {
            Snackbar.make(view,
                    "Incorrect User Name OR Password",
                    Snackbar.LENGTH_LONG).show();
        }

        Log.i("GARG", "Excution Line Finish ");
    } catch (Exception e) {
        e.printStackTrace();
    }
}

JSONParseAsync类：

public class JSONParseAsync extends AsyncTask<String, String, JSONObject>{

private Context mContext;

ProgressDialog mProgress;
private OnTaskCompleteListener mCallback;

public JSONParseAsync(Context context){
    this.mContext = (AppCompatActivity)context;
    this.mCallback = (OnTaskCompleteListener) mContext;
}

@Override
protected JSONObject doInBackground(String... URL) {
    JSONObject jsonObj = null;

    try{
    Log.d(Constants.DEBUG_TAG, "line excucation 2 doInBackground");
    ServiceHandler sh = new ServiceHandler();
    String url = URL[0];
    Log.d(Constants.ACTIVITY_TAG, "...." + url);
    // Making a request to url and getting response.

    String jsonStr = sh.makeServiceCall(url, ServiceHandler.GET);

    Log.d(Constants.JSON_TAG, "" + jsonStr);

    if (jsonStr != null) {

            jsonObj = new JSONObject(jsonStr);
            Log.e(Constants.JSON_TAG, "" + jsonObj);

    }
    } catch (JSONException e) {
        e.printStackTrace();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return jsonObj;
}

@Override
protected void onPreExecute() {
    Log.d(Constants.DEBUG_TAG, "line excucation 1 onPreexcute");
    mProgress = new ProgressDialog(mContext);
    mProgress.setMessage("Downloading nPlease wait...");
    mProgress.show();
}

@Override
protected void onProgressUpdate(String... values) {
    Log.d(Constants.DEBUG_TAG, "line excucation 3 onProgressUpdate");
    mProgress.setMessage(values[0]);
}


@Override
protected void onPostExecute(JSONObject result) {
    Log.d(Constants.DEBUG_TAG, "line excucation 3 onPostExecute");
    mProgress.dismiss();
    //This is where you return data back to caller
    Log.d(Constants.APP_TAG, " final result:- "+result);
    mCallback.onTaskComplete(result);
}

}

请帮帮我：

Answer 1

通过这样做

JSONParseAsync parseAsync = new JSONParseAsync(getContext());

您已将Activity发送到AsyncTask，但实施Fragment的是OnTaskCompleteListener。

要么Activity实施您的界面，或

这样做：

JSONParseAsync parseAsync = new JSONParseAsync(this, getContext());

并将您的AsyncTask构造函数更改为

public JSONParseAsync(OnTaskCompleteListener listener, Context context){
    this.mContext = context;
    //this.mContext = (AppCompatActivity)context; -> you don't need that cast, AppCompatActivity is a subclass of Context
    this.mCallback = listener;
}

Answer 2

为任务构造函数使用2个参数：

public JSONParseAsync(Context context, OnTaskCompleteListener listener){
    this.mContext = context;
    this.mCallback = listener;
}

并使用

创建一个新实例

    JSONParseAsync parseAsync = new JSONParseAsync(getContext(), this);

因为当您传递Context时，您不会传递片段而是传递基础活动。因此，如果您想在片段中使用任务，那么您可能需要2个参数。

Answer 3

您的问题是，因为您已将活动上下文传递给ul，并且在其构造函数内部，您将其投射到活动JSONParseAsync，然后将活动投射到您的AppCompatActivity ..所以，你的活动应该是实现监听器，而不是你的片段。

让OnTaskCompleteListener实现您的界面HomeActivity。

示例：

OnTaskCompleteListener

在我看来，除非并且直到需要，否则不要创建自定义类。在你的情况下你也应该用匿名类来完成你的任务..没有必要创建单独的类..

我希望你明白我的观点。

但是，你可以做的是，在构造函数中传递上下文和OnTaskCompleteListener

public class HomeActivity extends AppCompatActivity implements OnTaskCompleteListener{ ... }

并将构造函数更改为：

private void loadProfileAction() { ... JSONParseAsync parseAsync = new JSONParseAsync(getContext(), this); // also try getActivity() ... }

如何将Fragment的上下文转换为接口

3 个答案: