java - 如何将 Fragment 的上下文转换为界面

Question

我想通过 interface 从 AsyncTask 类返回一个值。问题是我的以下代码在 Activity 中运行良好，但在 fragment 类中运行不正常。

我得到了这样的 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(interface)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());

您正在将 Activity 发送到您的 AsyncTask，但实现 OnTaskCompleteListener 的是您的 Fragment。

或者让您的Activity实现您的界面，或者

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;
}