java - 将 Activity 转换为使用 Database 和 Adapter 类的 Fragment

Question

我正在从在线加载 json 文件并将其保存在 Sqlite 上，这样当应用程序离线时......然后用户仍然可以看到数据。它在 MainActivity 中运行良好。

但是当我尝试将其转换为 fragment 时，我收到错误 Fragment Cannot be Cast in to FlowerAdapter$FlowerClickListener

这是错误文件

我的 fragment 名称是尼泊尔语。

这是主要 Activity

public class MainActivity extends AppCompatActivity implements FlowerAdapter.FlowerClickListener, FlowerFetchListener {

    private static final String TAG = MainActivity.class.getSimpleName();
    private RecyclerView mRecyclerView;
    private RestManager mManager;
    private FlowerAdapter mFlowerAdapter;
    private FlowerDatabase mDatabase;
    private Button mReload;
    private ProgressDialog mDialog;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        configViews();

        mManager = new RestManager();
        mDatabase = new FlowerDatabase(this);

        loadFlowerFeed();

        mReload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                loadFlowerFeed();
            }
        });
    }

    private void loadFlowerFeed() {

        mDialog = new ProgressDialog(MainActivity.this);
        mDialog.setMessage("Loading Flower Data...");
        mDialog.setCancelable(true);
        mDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
        mDialog.setIndeterminate(true);

        mFlowerAdapter.reset();

        mDialog.show();

        if (getNetworkAvailability()) {
            getFeed();
        } else {
            getFeedFromDatabase();
        }
    }

    private void getFeedFromDatabase() {
        mDatabase.fetchFlowers(this);
    }

    private void configViews() {
        mReload = (Button) findViewById(R.id.reload);
        mRecyclerView = (RecyclerView) findViewById(R.id.recyclerView);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setRecycledViewPool(new RecyclerView.RecycledViewPool());
        mRecyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext(), LinearLayoutManager.VERTICAL, false));

        mFlowerAdapter = new FlowerAdapter(this);

        mRecyclerView.setAdapter(mFlowerAdapter);
    }

    @Override
    public void onClick(int position) {

    }

    public void getFeed() {

        Call<List<Flower>> listCall = mManager.getFlowerService().getAllFlowers();
        listCall.enqueue(new Callback<List<Flower>>() {

            @Override
            public void onResponse(Call<List<Flower>> call, Response<List<Flower>> response) {

                if (response.isSuccessful()) {
                    List<Flower> flowerList = response.body();

                    for (int i = 0; i < flowerList.size(); i++) {
                        Flower flower = flowerList.get(i);

                        SaveIntoDatabase task = new SaveIntoDatabase();
                        task.execute(flower);

                        mFlowerAdapter.addFlower(flower);
                    }
                } else {
                    int sc = response.code();
                    switch (sc) {
                        case 400:
                            Log.e("Error 400", "Bad Request");
                            break;
                        case 404:
                            Log.e("Error 404", "Not Found");
                            break;
                        default:
                            Log.e("Error", "Generic Error");
                    }
                }
                mDialog.dismiss();

            }

            @Override
            public void onFailure(Call<List<Flower>> call, Throwable t) {
                mDialog.dismiss();
                Toast.makeText(getApplicationContext(), t.getMessage(), Toast.LENGTH_SHORT).show();
            }




        });
    }

    public boolean getNetworkAvailability() {
        return Utils.isNetworkAvailable(getApplicationContext());
    }

    @Override
    public void onDeliverAllFlowers(List<Flower> flowers) {

    }

    @Override
    public void onDeliverFlower(Flower flower) {
        mFlowerAdapter.addFlower(flower);
    }

    @Override
    public void onHideDialog() {
        mDialog.dismiss();
    }

    public class SaveIntoDatabase extends AsyncTask<Flower, Void, Void> {


        private final String TAG = SaveIntoDatabase.class.getSimpleName();

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Flower... params) {

            Flower flower = params[0];

            try {


            } catch (Exception e) {
                Log.d(TAG, e.getMessage());
            }

            return null;
        }
    }
}

FlowerDatabase 类是

public class FlowerDatabase extends SQLiteOpenHelper {

    private static final String TAG = FlowerDatabase.class.getSimpleName();

    public FlowerDatabase(Context context) {
        super(context, Constants.DATABASE.DB_NAME, null, Constants.DATABASE.DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        try {
            db.execSQL(Constants.DATABASE.CREATE_TABLE_QUERY);
        } catch (SQLException ex) {
            Log.d(TAG, ex.getMessage());
        }
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL(Constants.DATABASE.DROP_QUERY);
        this.onCreate(db);
    }

    public void addFlower(Flower flower) {

        Log.d(TAG, "Values Got " + flower.getName());

        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(Constants.DATABASE.PRODUCT_ID, flower.getProductId());
        values.put(Constants.DATABASE.CATEGORY, flower.getCategory());
        values.put(Constants.DATABASE.PRICE, Double.toString(flower.getPrice()));
        values.put(Constants.DATABASE.INSTRUCTIONS, flower.getInstructions());
        values.put(Constants.DATABASE.NAME, flower.getName());
        try {
            db.insert(Constants.DATABASE.TABLE_NAME, null, values);
        } catch (Exception e) {

        }
        db.close();
    }

    public void fetchFlowers(FlowerFetchListener listener) {
        FlowerFetcher fetcher = new FlowerFetcher(listener, this.getWritableDatabase());
        fetcher.start();
    }

    public class FlowerFetcher extends Thread {

        private final FlowerFetchListener mListener;
        private final SQLiteDatabase mDb;

        public FlowerFetcher(FlowerFetchListener listener, SQLiteDatabase db) {
            mListener = listener;
            mDb = db;
        }

        @Override
        public void run() {
            Cursor cursor = mDb.rawQuery(Constants.DATABASE.GET_FLOWERS_QUERY, null);

            final List<Flower> flowerList = new ArrayList<>();

            if (cursor.getCount() > 0) {

                if (cursor.moveToFirst()) {
                    do {
                        Flower flower = new Flower();
                        flower.setFromDatabase(true);
                        flower.setName(cursor.getString(cursor.getColumnIndex(Constants.DATABASE.NAME)));

                        flower.setPrice(Double.parseDouble(cursor.getString(cursor.getColumnIndex(Constants.DATABASE.PRICE))));
                        flower.setInstructions(cursor.getString(cursor.getColumnIndex(Constants.DATABASE.INSTRUCTIONS)));
                        flower.setCategory(cursor.getString(cursor.getColumnIndex(Constants.DATABASE.CATEGORY)));
                        flower.setProductId(Integer.parseInt(cursor.getString(cursor.getColumnIndex(Constants.DATABASE.PRODUCT_ID))));

                        flowerList.add(flower);
                        publishFlower(flower);

                    } while (cursor.moveToNext());
                }
            }
            Handler handler = new Handler(Looper.getMainLooper());
            handler.post(new Runnable() {
                @Override
                public void run() {
                    mListener.onDeliverAllFlowers(flowerList);
                    mListener.onHideDialog();
                }
            });
        }

        public void publishFlower(final Flower flower) {
            Handler handler = new Handler(Looper.getMainLooper());
            handler.post(new Runnable() {
                @Override
                public void run() {
                    mListener.onDeliverFlower(flower);
                }
            });
        }
    }
}

FlowerAdapter 类是

public class FlowerAdapter extends RecyclerView.Adapter<FlowerAdapter.Holder> {

    private static final String TAG = FlowerAdapter.class.getSimpleName();
    private final FlowerClickListener mListener;
    private List<Flower> mFlowers;

    public FlowerAdapter(FlowerClickListener listener) {
        mFlowers = new ArrayList<>();
        mListener = listener;
    }

    @Override
    public Holder onCreateViewHolder(ViewGroup parent, int viewType) {
        View row = LayoutInflater.from(parent.getContext()).inflate(R.layout.row_item, null, false);
        return new Holder(row);
    }

    @Override
    public void onBindViewHolder(Holder holder, int position) {

        Flower currFlower = mFlowers.get(position);

        holder.mName.setText(currFlower.getName());
        holder.minstruction.setText(currFlower.getInstructions());

    }

    @Override
    public int getItemCount() {
        return mFlowers.size();
    }

    public void addFlower(Flower flower) {
        mFlowers.add(flower);
        notifyDataSetChanged();
    }

    /**
     * @param position
     * @return
     */
    public Flower getSelectedFlower(int position) {
        return mFlowers.get(position);
    }

    public void reset() {
        mFlowers.clear();
        notifyDataSetChanged();
    }

    public class Holder extends RecyclerView.ViewHolder implements View.OnClickListener {

        private TextView mName, minstruction;

        public Holder(View itemView) {
            super(itemView);
            mName = (TextView) itemView.findViewById(R.id.flowerName);
            minstruction = (TextView) itemView.findViewById(R.id.flowerPrice);
            itemView.setOnClickListener(this);
        }

        @Override
        public void onClick(View v) {
            mListener.onClick(getLayoutPosition());
        }
    }

    public interface FlowerClickListener {

        void onClick(int position);
    }
}

我的 fragment 类是我尝试将上面的 Mainactivity 代码转换为 Frament 类

public class nepali extends Fragment {
    private static final String TAG = nepali.class.getSimpleName();
    private RecyclerView mRecyclerView;
    private RestManager mManager;
    private FlowerAdapter mFlowerAdapter;
    private FlowerDatabase mDatabase;
    private Button mReload;
    private ProgressDialog mDialog;
    View view;
   

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.activity_data, container, false);
     configViews();

          mManager = new RestManager();
        mDatabase = new FlowerDatabase(getActivity());

        loadFlowerFeed();

        mReload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                loadFlowerFeed();
            }
        });

        return view;
    }

    private void loadFlowerFeed() {

        mDialog = new ProgressDialog(getActivity());
        mDialog.setMessage("Loading Flower Data...");
        mDialog.setCancelable(true);
        mDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
        mDialog.setIndeterminate(true);

        mFlowerAdapter.reset();

        mDialog.show();

        if (getNetworkAvailability()) {
            getFeed();
        } else {
            getFeedFromDatabase();
        }
    }

 private void getFeedFromDatabase() {
        mDatabase.fetchFlowers(this);
    }
    private void configViews() {
        mReload = (Button) view.findViewById(R.id.reload);
        mRecyclerView = (RecyclerView)  view.findViewById(R.id.recyclerView);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setRecycledViewPool(new RecyclerView.RecycledViewPool());
        mRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity(), LinearLayoutManager.VERTICAL, false));

     mFlowerAdapter = new FlowerAdapter((FlowerAdapter.FlowerClickListener) this);;

        mRecyclerView.setAdapter(mFlowerAdapter);
    }



    public void getFeed() {

        Call<List<Flower>> listCall = mManager.getFlowerService().getAllFlowers();
        listCall.enqueue(new Callback<List<Flower>>() {

            @Override
            public void onResponse(Call<List<Flower>> call, Response<List<Flower>> response) {

                if (response.isSuccessful()) {
                    List<Flower> flowerList = response.body();

                    for (int i = 0; i < flowerList.size(); i++) {
                        Flower flower = flowerList.get(i);

                        SaveIntoDatabase task = new SaveIntoDatabase();
                        task.execute(flower);

                        mFlowerAdapter.addFlower(flower);
                    }
                } else {
                    int sc = response.code();
                    switch (sc) {
                        case 400:
                            Log.e("Error 400", "Bad Request");
                            break;
                        case 404:
                            Log.e("Error 404", "Not Found");
                            break;
                        default:
                            Log.e("Error", "Generic Error");
                    }
                }
                mDialog.dismiss();

            }

            @Override
            public void onFailure(Call<List<Flower>> call, Throwable t) {
                mDialog.dismiss();
                Toast.makeText(getActivity(), t.getMessage(), Toast.LENGTH_SHORT).show();
            }




        });
    }

    public boolean getNetworkAvailability() {
        return Utils.isNetworkAvailable(getActivity());
    }




    public void onDeliverFlower(Flower flower) {
        mFlowerAdapter.addFlower(flower);
    }


    public void onHideDialog() {
        mDialog.dismiss();
    }

    public class SaveIntoDatabase extends AsyncTask<Flower, Void, Void> {


        private final String TAG = SaveIntoDatabase.class.getSimpleName();

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Flower... params) {

            Flower flower = params[0];

            try {


            } catch (Exception e) {
                Log.d(TAG, e.getMessage());
            }

            return null;
        }
    }

}

最后，FlowerService 类是

public interface FlowerService {

    @GET("/routine/first.json")
    Call<List<Flower>> getAllFlowers();
}

而 FlowerFetchListener 类是

public interface FlowerFetchListener {

    void onDeliverAllFlowers(List<Flower> flowers);

    void onDeliverFlower(Flower flower);

    void onHideDialog();
}

请帮忙......并提前致谢。

Answer 1

您不应从 Activity 或 fragment 中调用数据库和网络。尝试学习MVVM架构并使用ViewModel来存储来自数据库或网络的数据。您可能会付出很多努力让您的应用程序正常工作，但它仍然会导致崩溃(特别是在您引入 fragment 并从那里调用数据库和API之后)。您需要在配置更改期间处理数据状态。听这个演讲并开始编写干净的代码 https://m.youtube.com/watch?v=5qlIPTDE274

您收到的错误是因为您将错误的参数传递给适配器构造函数“new FlowerAdapter((FlowerAdapter.FlowerClickListener) this)”。如果要将监听器传递给适配器，则需要传递实现监听器的类:或者 Activity - 然后传递 getActivity()，或者 fragment “this” - 然后使 fragment 实现 FlowerAdapter.FlowerClickListener。请注意，它可以当 fragment 未附加到 Activity 时为 null，例如配置更改。