Class Cast Exception 从子片段中检索选择的日期以显示在父片段中

Question

我有一个名为Planning 的parentfragment，我想在其中显示一个名为DatePickerfragment 的DialogFragment。DatePickerFragment 有一个button，当单击它时，将为用户显示一个datepicker .用户选择一个日期，该日期应在名为“日期”的文本视图中传递给Parent Fragment。

不幸的是，每次我点击 button 时，应用程序都会因以下错误而崩溃：

java.lang.ClassCastException: com.example.android.meat_timealpha10.Activities.MainActivity cannot be cast to android.app.DatePickerDialog$OnDateSetListener
    at com.example.android.meat_timealpha10.Fragments.DatePickerFragment.onCreateDialog(DatePickerFragment.java:29)

我知道错误在哪里，但我不知道如何解决它。 它指的是DatePickerDialogFragment中的这一行：

return new DatePickerDialog(getActivity(), (DatePickerDialog.OnDateSetListener)getActivity(), year, month, day );

问题是我关注了一个 youtube 视频，而这家伙似乎没有任何问题。再说一次，他不像我那样使用nested fragments。 有谁知道如何修复该错误，以便所选日期显示在textview 中？

这是我的代码：

XML 的parent fragment ：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".Fragments.planning">


    <Button
        android:id="@+id/chooseDate"
        style="@style/Widget.AppCompat.Button.Borderless.Colored"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginBottom="5dp"
        android:elevation="0dp"
        android:hint="touch here to select date"

        android:textSize="20sp" />

    <TextView
        android:id="@+id/date"
        android:layout_width="match_parent"
        android:layout_height="30dp"
        android:textColor="@color/blue" />
</LinearLayout>

以下是Child Fragment DatePickerFragment:

package com.example.android.meat_timealpha10.Fragments;

import android.app.DatePickerDialog;
import android.app.Dialog;
import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.DialogFragment;
import android.widget.DatePicker;
import android.widget.TextView;

import com.example.android.meat_timealpha10.R;

import org.w3c.dom.Text;

import java.text.DateFormat;
import java.util.Calendar;

public class DatePickerFragment extends DialogFragment {

    public String dateHolder;
    @NonNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Calendar c = Calendar.getInstance();
        int year = c.get(Calendar.YEAR);
        int month = c.get(Calendar.MONTH);
        int day = c.get(Calendar.DAY_OF_MONTH);
        return new DatePickerDialog(getActivity(), (DatePickerDialog.OnDateSetListener)getActivity(), year, month, day );
    }   

}

parent fragment 的代码（不含导入）

public class planning extends Fragment implements DatePickerDialog.OnDateSetListener{
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private OnFragmentInteractionListener mListener;

    public planning() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment planning.
     */
    // TODO: Rename and change types and number of parameters
    public static planning newInstance(String param1, String param2) {
        planning fragment = new planning();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }



    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }



    }



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_planning,
                container, false);
        final Button chooseDate = (Button)view.findViewById(R.id.chooseDate);




        chooseDate.setOnClickListener(new View.OnClickListener() {
                                      @Override
                                      public void onClick(View v) {

                                        DialogFragment datePicker = new DatePickerFragment();
                                        datePicker.show(getChildFragmentManager(), "date picker");

                                      }
                                      });

        return view;
    }

    @Override
    public void onDateSet(DatePicker view, int year, int month, int day) {
        Calendar c = Calendar.getInstance();
        c.set(Calendar.YEAR, year);
        c.set(Calendar.MONTH, month);
        c.set(Calendar.DAY_OF_MONTH, day);
        String currentDateString = DateFormat.getDateInstance(DateFormat.FULL).format(c.getTime());
        TextView date = (TextView)view.findViewById(R.id.date);
        date.setText(currentDateString);

    }


    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);

    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

我在这方面还很陌生，所以在回答我的问题时，请尽量让它容易理解。 在此先感谢各位！

Answer 1

正如它所建议的，您不能将活动转换为侦听器，错误发生在这一行：(DatePickerDialog.OnDateSetListener)getActivity()。

您需要在片段中实现 DatePickerDialog.OnDateSetListener 并覆盖 onDateSet 函数：

public class DatePickerFragment extends DialogFragment implements DatePickerDialog.OnDateSetListener {

@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
    ...
    return new DatePickerDialog(getActivity(), this, year, month, day);
}  

@Override
public void onDateSet(DatePicker view, int year, int month, int day) {
    ...        
}}