java - 为什么 onAttach 不能识别正确的 instanceof 上下文？

Question

我无法让 Android 设置我的听众。不知何故，上下文不是我期望的那种类型。我不确定我哪里出错了。

下面是 AddEditCharacterFragment.java，它抛出异常，因为上下文不是我期望的类型。

public class AddEditCharacterFragment extends Fragment {

    public static final String ARG_PARAM1 = "param1";

    private InitiativeTrackerDBHelper mHelper;

    private String mParam1;

    private Character mCharacter;

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

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @return A new instance of fragment AddEditCharacterFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static AddEditCharacterFragment newInstance() {
        AddEditCharacterFragment fragment = new AddEditCharacterFragment();
        Bundle args = new Bundle();
        //args.putInt(ARG_PARAM1, id);
        fragment.setArguments(args);
        return fragment;
    }

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

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View v = inflater.inflate(R.layout.fragment_add_character, container, false);

        mHelper = new InitiativeTrackerDBHelper(getActivity());

        mCharacter = mHelper.addCharacter();

        EditText characterNameEditText = (EditText) v.findViewById(R.id.character_name_text_edit);
        characterNameEditText.setText(mCharacter.getName());
        characterNameEditText.addTextChangedListener(new TextWatcher() {
            public void onTextChanged(CharSequence c, int start, int before, int count) {
                mCharacter.setName(c.toString());
            }

            public void beforeTextChanged(CharSequence c, int start, int before, int after) {
            }

            public void afterTextChanged(Editable c) {
            }
        });

        EditText modifierPicker =
                (EditText) v.findViewById(R.id.modEditText);

        modifierPicker.setText(Integer.toString(mCharacter.getModifier()));

        modifierPicker.addTextChangedListener(new TextWatcher() {
            public void onTextChanged(CharSequence c, int start, int before, int count) {
                mCharacter.setModifier(Integer.parseInt(c.toString()));
            }

            public void beforeTextChanged(CharSequence c, int start, int before, int after) {
            }

            public void afterTextChanged(Editable c) {
            }
        });

        Button saveButton = (Button) v.findViewById(R.id.saveButton);
        saveButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(mHelper != null)
                {
                    mHelper.updateCharacter(mCharacter);
                    Toast.makeText(getActivity(), "Update complete!", Toast.LENGTH_LONG).show();
                    mListener.onCharacterSave();
                }

            }
        });

        return v;
    }

    private OnCharacterSave mListener;

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnCharacterSave) {
            mListener = (OnCharacterSave) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

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

    public interface OnCharacterSave {
        public void onCharacterSave();
    }
}

AddEditCharacterActivity 是上面 fragment 的 Activity 。

public class AddEditCharacterActivity extends SingleFragmentActivity
        implements AddEditCharacterFragment.OnCharacterSave {

    @Override
    protected Fragment createFragment() {
        return AddEditCharacterFragment.newInstance();
    }

    @Override
    public void onCharacterSave() {
        FragmentManager fm = getFragmentManager();

        // Get the container for the character list
        InitiativeListFragment initiativeListFragment = (InitiativeListFragment)
                fm.findFragmentById(R.id.fragmentContainer);

        // Update the UI
        initiativeListFragment.updateInitiativeList();
    }
}

InitiativeTrackerActivity 使用 Intent 启动 AddEditCharacterActivity 和随后的 AddEditCharacterFragment。

    public class InitiativeTrackerActivity extends SingleFragmentActivity
    implements InitiativeListFragment.OnCharacterListListener, AddEditCharacterFragment.OnCharacterSave {

    @Override
    protected Fragment createFragment() {
        return InitiativeListFragment.newInstance();
    }

    @Override
    public void onAddCharacter() {
        Intent intent = new Intent(this, AddEditCharacterActivity.class);
        startActivity(intent);
    }

    @Override
    public void onCharacterSave() {
        FragmentManager fm = getFragmentManager();

        // Get the container for the character list
        InitiativeListFragment initiativeListFragment = (InitiativeListFragment)
                fm.findFragmentById(R.id.fragmentContainer);

        // Update the UI
        initiativeListFragment.updateInitiativeList();
    }
}

以及 SingleFragmentActivity 的基类供引用:

    public abstract class SingleFragmentActivity extends AppCompatActivity {

    protected abstract Fragment createFragment();

    protected int getLayoutId() {
        return R.layout.activity_single_fragment;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(getLayoutId());

        FragmentManager fm = getFragmentManager();
        Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);

        if (fragment == null) {
            fragment = createFragment();
            fm.beginTransaction()
                    .add(R.id.fragmentContainer, fragment)
                    .commit();
        }
    }
}

和 InitiativeListFragment.java

package com.example.twistedpurpose.finalproject;

import android.content.Context;
import android.content.Intent;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.app.Fragment;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.CursorAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import android.widget.Toast;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;


/**
 * A simple {@link Fragment} subclass.
 * Activities that contain this fragment must implement the
 * {@link InitiativeListFragment.OnCharacterListListener} interface
 * to handle interaction events.
 * Use the {@link InitiativeListFragment#newInstance} factory method to
 * create an instance of this fragment.
 */
public class InitiativeListFragment extends Fragment {

    private InitiativeTrackerDBHelper.CharacterCursor mCursor;

    private CharacterCursorAdapter adapter;

    private OnCharacterListListener mListener;

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

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @return A new instance of fragment InitiativeListFragment.
     */
    public static InitiativeListFragment newInstance() {
        return new InitiativeListFragment();
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (adapter != null) {
            adapter.notifyDataSetChanged();
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View v = inflater.inflate(R.layout.fragment_initiative_list, container, false);

        //getActivity().deleteDatabase("characters.db");

        Context context = getActivity();

        // 1. Create a new InitiativeTrackerDBHelper
        InitiativeTrackerDBHelper dbHelper = new InitiativeTrackerDBHelper(context);


        // 2. Query the characters and obtain a cursor (store in mCursor).
        mCursor = dbHelper.queryCharacters();

        // Find ListView to populate
        ListView characterListView = (ListView) v.findViewById(R.id.character_listView);
        // Setup cursor adapter using cursor from last step
        adapter = new CharacterCursorAdapter(context, mCursor);
        // Attach cursor adapter to the ListView
        characterListView.setAdapter(adapter);

        Button rollButton = (Button) v.findViewById(R.id.rollBtn);
        rollButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                InitiativeTrackerDBHelper dbHelper = new InitiativeTrackerDBHelper(getContext());
                List<Character> characterList = dbHelper.getCharacters();

                InitiativeRoller.rollInitiative(characterList);

                for (Character c : characterList) {
                    dbHelper.updateCharacter(c);
                }

                updateInitiativeList();
                Toast.makeText(getContext(), "Roll initiative!", Toast.LENGTH_SHORT).show();
            }
        });

        Button addButton = (Button) v.findViewById(R.id.addBtn);
        addButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(mListener != null) {
                    mListener.onAddCharacter();
                }
            }
        });

        return v;
    }

    public void updateInitiativeList(){
        if(mCursor != null && adapter != null){
            mCursor.requery();
            adapter.notifyDataSetChanged();
        }

    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnCharacterListListener) {
            mListener = (OnCharacterListListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @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 OnCharacterListListener {
        public void onAddCharacter();
    }

    /**
     * A character cursor adaptor for adding characters
     * to a list
     */
    private static class CharacterCursorAdapter extends CursorAdapter {

        private InitiativeTrackerDBHelper.CharacterCursor mCharacterCursor;

        public CharacterCursorAdapter(Context context, InitiativeTrackerDBHelper.CharacterCursor cursor) {
            super(context, cursor, 0);
            mCharacterCursor = cursor;
        }

        @Override
        public View newView(Context context, Cursor cursor, ViewGroup parent) {
            // Use a layout inflater to get a row view
            LayoutInflater inflater = (LayoutInflater) context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            return inflater.inflate(R.layout.character_listview, parent, false);
        }

        @Override
        public void bindView(View view, Context context, Cursor cursor) {
            TextView characterName = (TextView) view.findViewById(R.id.name);
            TextView characterMod = (TextView) view.findViewById(R.id.mod);
            TextView characterInit = (TextView) view.findViewById(R.id.init);

            characterName.setText(mCharacterCursor.getCharacter().getName());
            characterMod.setText(Integer.toString(mCharacterCursor.getCharacter().getModifier()));
            characterInit.setText(Integer.toString(mCharacterCursor.getCharacter().getTotalInitiative()));
        }
    }
}

Answer 1

你的问题是你没有在你的 Activity 中实现接口(interface)......
public class AddEditCharacterActivity extends SingleFragmentActivity {
这不是 implement OnCharacterSave .

这就是我之前评论的意思。

更新 :

你误解了 fragment 。
// Get the container for the character list -> 这不是真的。您没有得到“容器”，而是试图获取 fragment 的实际实例。

    InitiativeListFragment initiativeListFragment = (InitiativeListFragment)
            fm.findFragmentById(R.id.fragmentContainer);

如果那个 fragment 在那里，那就太好了。

让我用更形象的方式说，这就是你正在做的……(给予或接受)

开始 Activity XXX (SingleFragmentActivity)。

在某个时候，InitiativeListFragment在 R.id.fragmentContainer替换为 AddEditCharacterActivity / AddEditCharacterFragment组合。

此时，R.id.fragmentContainer包含 AddEditCharacterFragment 类型的 fragment .

由于您的 Activity 实现了 OnCharacterSave ， 到目前为止，一切都很好。

仍然在相同的 Activity/fragment 组合中，您调用 onCharacterSave()这是实现的(见＃4)，所以一切都很好。

然后你告诉 fragment 管理器给你 R.id.fragmentContainer 中的 fragment 。并且您明确地说(又名:类型转换) fragment 的类型为 InitiativeListFragment ，但是……您的 Activity 应该知道情况并非如此……因为当前 fragment 是 AddEditCharacterFragment .

你应该做的是:

重新阅读 FragmentManager 和 FragmentTransactions。

如果您要将信息传递给当前不可见/启动/附加/等的另一个 fragment 。那么你必须获得一个引用(如果你有的话，通过 TAG)。

然后如果可能的话，可以将它添加到容器中，同时通过 Bundle 传递“数据”。

完全不清楚您要做什么以及顺序是什么，因为您的代码实际上并没有很多关注点分离，因此您可以看到您的 Activity 和 fragment 正在成为充满代码和业务逻辑的单体怪物。有一些解决方案和替代方案(阅读有关 Model-View-Presenter 或类似模式的信息)可以缓解困惑，同时提供更轻松的环境来测试您的代码。

话虽如此，不管您的代码有多复杂，我相信您需要了解为什么会出现异常，并且我觉得您需要稍微练习一下。

简而言之……当你这样做时 findFragmentById ，你确实得到了 fragment (如果存在的话)，但你不能把它转换成你想要的任何东西。

旧评论 :
newInstance()静态方法通常应该存在于 Fragments 中并返回 new YourFragment();
我的意思是 fragment 创建通常是通过 fragment 中的静态方法完成的。

说你有

MyFragment extends Fragment {
    public static MyFragment newInstance() {
        return new MyFragment();
    }

    public MyFragment() {
          // empty constructor is most of the time needed to restore.
    }
}

然后从 Activity 中你通常做你正在做的事情，但是 fragment 实例是通过调用 MyFragment.newInstance(); 创建的(这就是谷歌的做法)。

我建议您也通过标签添加您的 fragment (它更快)。所以你也是

final Fragment existing = getSupportFragmentManager().findFragmentByTag(tag);

if (existing == null) {
    final Fragment newInstance = MyFragment.newInstance();
       getSupportFragmentManager()
           .beginTransaction()
           .add(R.id.fragmentContainer, newInstance, tag)
           .commit();
 }

标签是 String您可以将其保存在常量中(例如 final static String MYFRAGMENT_TAG = "MYFRAGMENT_TAG";)。

你在使用 Support.V4 fragment 吗？如果是这样，您需要更改 getFragmentManager()至 getSupportFragmentManager() (看起来你是，因为你有 AppCompatActivity 。

此外， fragment 事务应由 if (savedInstaceState == null) { // do it here } 包围。