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android - recylerView 未显示在应用程序中

问题描述

嘿，我是使用 Android 的新手，我用这个应用程序来制作我正在学习的教程，当我尝试在 Fragment 中工作时，教程从 MainActivity.java 读取和写入数据。

虽然没有错误，但应用程序没有显示任何内容，但 Firebase 实时数据库的 JSON 数据未按预期显示在 RecylerView 中。

这是我的文件

MainActivity.java

package com.example.plutus;

import android.os.Bundle;
import android.view.MenuItem;
import android.widget.FrameLayout;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import com.google.android.material.bottomnavigation.BottomNavigationView;

public class MainActivity extends AppCompatActivity {

    private TaskFragment taskFragment;
    private TimeFragment timeFragment;
    private HabitFragment habitFragment;
    private HourFragment hourFragment;
    private BottomNavigationView mMainNav;
    private FrameLayout mMainFrame;

    @Override
    protected void onCreate(Bundle savedInstanceState) {


        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        mMainNav = (BottomNavigationView) findViewById(R.id.main_nav);
        mMainFrame = (FrameLayout) findViewById(R.id.main_frame);


        taskFragment = new TaskFragment();
        timeFragment = new TimeFragment();
        habitFragment = new HabitFragment();
        hourFragment = new HourFragment();

        setFragment(taskFragment);


        mMainNav.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                switch (item.getItemId()) {
                    case R.id.nav_task:
                        setFragment(taskFragment);
                        return true;


                    case R.id.nav_time:
                        setFragment(timeFragment);
                        return true;


                    case R.id.nav_habit:
                        setFragment(habitFragment);
                        return true;


                    case R.id.nav_hour:
                        setFragment(hourFragment);
                        return true;

                    default:
                        return false;
                }

            }
        });

    }
    private void setFragment(Fragment fragment) {
        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.main_frame, fragment);
        fragmentTransaction.commit();
    }


}

任务适配器.java

package com.example.plutus;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;

public class TaskAdapter extends RecyclerView.Adapter<TaskAdapter.MyViewHolder> {

    ValueEventListener context;
    ArrayList<MyTasks> myTasks;

    public TaskAdapter(ValueEventListener c, ArrayList<MyTasks> p) {
        context =  c;
        myTasks = p;
    }


    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new MyViewHolder(LayoutInflater.from((Context) context).inflate(R.layout.task_item, parent, false));
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        holder.tasktitle.setText(myTasks.get(position).getTasktitle());

    }

    @Override
    public int getItemCount() {

        return myTasks.size();
    }

    class MyViewHolder extends RecyclerView.ViewHolder {

        TextView tasktitle;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            tasktitle = (TextView) itemView.findViewById(R.id.tasktitletext);
        }
    }

}

任务片段.java

package com.example.plutus;


import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;


/**
 * A simple {@link Fragment} subclass.
 */
public class TaskFragment extends Fragment {

    //Firebase Json integration
    RecyclerView ourTasks;
    ArrayList<MyTasks> list;
    TaskAdapter taskAdapter;

    public TaskFragment() {

    }

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

    View rootView = inflater.inflate(R.layout.fragment_task, container, false);

        ourTasks = rootView.findViewById(R.id.ourTasks);
        ourTasks.setLayoutManager(new LinearLayoutManager(getActivity()));
        list = new ArrayList<MyTasks>();

        // Inflate the layout for this fragment

        DatabaseReference taskDatabaseReference = FirebaseDatabase.getInstance().getReference();
        DatabaseReference taskuser = taskDatabaseReference.child("Task1").child("tasktitle");

        taskuser.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                for (DataSnapshot dataSnapshot1: dataSnapshot.getChildren()){
                    MyTasks p = dataSnapshot1.getValue(MyTasks.class);
                    list.add(p);
                }

                taskAdapter = new TaskAdapter(this, list);
                ourTasks.setAdapter(taskAdapter);
                taskAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Toast.makeText(getContext(), "I'm fetching this", Toast.LENGTH_SHORT).show();
            }
        });

        return rootView;
    };

}

MyTasks.java

package com.example.plutus;

public class MyTasks {
    String tasktitle;

    public MyTasks() {

    }

    public MyTasks(String tasktitle) {

        this.tasktitle = tasktitle;
    }

    public String getTasktitle() {

        return tasktitle;
    }

    public void setTasktitle(String tasktitle) {

        this.tasktitle = tasktitle;
    }
}

我不明白真正的问题在哪里。

编辑：包含的 JSON 文件。

{
    "Plutus":{
        "Task1":{
            "tasktitle":"Create an awesome app"
        },

        "Task2":{
            "tasktitle":"Learn More Android"
        },

        "Task3":{
            "tasktitle":"Learn Android Pentesting too"
        },

        "Task4":{
            "tasktitle":"Oi don't forget Bug Hunting"
        }
    }
}

标签： androidfirebaseandroid-fragmentsfirebase-realtime-databaseandroid-recyclerview

解决方案

您的代码中存在一些问题：

  • 您没有Plutus在代码中处理 JSON 的级别。
  • 您的行为好像快照中可以有多个节点，但实际上您正在加载一个非常特定的节点。所以你不需要循环getChildren()。
  • 由于您title仅加载节点，因此结果不是MyTasks.

要仅加载一项任务的标题，您可以执行以下操作：

DatabaseReference taskDatabaseReference = FirebaseDatabase.getInstance().getReference();
DatabaseReference taskuser = taskDatabaseReference.child("Plutus/Task1/tasktitle");

taskuser.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        Log.i("Firebase", dataSnapshot.getValue(String.class));
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        throw databaseError.toException(); // make sure to show the programming error
    }
});

要加载单个任务，但要加载整个任务，它会是这样的：

DatabaseReference taskDatabaseReference = FirebaseDatabase.getInstance().getReference();
DatabaseReference taskuser = taskDatabaseReference.child("Plutus/Task1");

taskuser.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        MyTasks p = dataSnapshot.getValue(MyTasks.class);
        Log.i("Firebase", p.toString());
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        throw databaseError.toException(); // make sure to show the programming error
    }
});

并加载所有任务，并将它们添加到适配器/视图，它会是这样的：

taskAdapter = new TaskAdapter(this, list);
ourTasks.setAdapter(taskAdapter);

DatabaseReference taskDatabaseReference = FirebaseDatabase.getInstance().getReference();
DatabaseReference taskuser = taskDatabaseReference.child("Plutus");

taskuser.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        for (DataSnapshot taskSnapshot: dataSnapshot.getChildren()){
            MyTasks p = taskSnapshot.getValue(MyTasks.class);
            list.add(p);
        }
        taskAdapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        throw databaseError.toException(); // make sure to show the programming error
    }
});

你会注意到我在这里把适配器从监听器中拉了出来。您不需要在每次数据更改时都重新构建一个新的适配器，而是可以只更新其中的数据，list然后通知适配器，以便它更新 UI。

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