如何从 Firebase 数据库中检索数据 [ANDROID STUDIO]

Question

I am very new to Android Studio. I've made an app with a bottom navigation so users can access different screens (using fragments). On one screen I've made a fragment which is meant to show users their upcoming tasks. I've also made an activity so that users can create a new task, and when done they are redirected back to the Tasks screen. I've connected my app to a Firebase database, so when users input their data it gets stored into the database (THAT WORKS AND SHOWS ON DATABASE). However, when the user saves the task and is redirected back to the tasks screen their data is not shown and I am not sure why it doesn't show.

I've tried to watch loads of videos to try their solutions, but it doesn't work for me, and I am really not sure on how to do it. How do I get the data to show up??

I have been using FIREBASE DATABASE for my database (not firestore).

[Sorry for showing lots of code in advance - I'm just trying to be thorough]

[CHECK IMAGES THEY RELATE TO PROBLEM]

https://i.stack.imgur.com/thNWo.png - Tasks screen and button

https://i.stack.imgur.com/d1Xg9.png - Add Tasks screen

https://i.stack.imgur.com/BgLcC.png - Data being added into database after pressing 'save task'

https://i.stack.imgur.com/gTNpb.png - No data being shown on tasks screen???

.Javas

MainActivity.java:

public class MainActivity extends AppCompatActivity {

TextView title, description, date;
LinearLayoutManager linearLayoutManager;
FirebaseDatabase firebaseDatabase;
DatabaseReference reference;

FirebaseRecyclerOptions<TaskItem> options;
FirebaseRecyclerAdapter<TaskItem, MyViewHolder> adapter;
RecyclerView recyclerView;


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

    HomeFragment fragment1 = new HomeFragment();
    FragmentManager homemanager = getSupportFragmentManager();
    homemanager.beginTransaction().add(R.id.fragment_container, fragment1).commit();

    BottomNavigationView bottomNav = findViewById(R.id.bottom_navigation);
    bottomNav.setOnNavigationItemSelectedListener(navListener);

    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
            new HomeFragment()).commit();

}

private BottomNavigationView.OnNavigationItemSelectedListener navListener =
        new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                Fragment selectedFragment = null;

                switch (item.getItemId()) {
                    case R.id.nav_home:
                        selectedFragment = new HomeFragment();
                        break;
                    case R.id.nav_calendar:
                        selectedFragment = new CalendarFragment();
                        break;
                    case R.id.nav_school:
                        selectedFragment = new SchoolFragment();
                        break;
                }

                FragmentManager navmanager = getSupportFragmentManager();
                navmanager.beginTransaction().replace(R.id.fragment_container,
                        selectedFragment).commit();

                return true;
            }
        };

}

NewTaskHome.java [for adding new tasks]

public class NewTaskHome extends AppCompatActivity {

TextView title;
EditText titleName;
TextView description;
EditText descName;
TextView date;
EditText dateName;

Button saveTask;
Button cancelTask;

DatabaseReference database;

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

    database = FirebaseDatabase.getInstance().getReference().child("TASKS");


    titleName = findViewById(R.id.addTitleName);
    descName = findViewById(R.id.addDescName);
    dateName = findViewById(R.id.addDateName);

    saveTask = findViewById(R.id.btnSaveTask);
    cancelTask = findViewById(R.id.btnCancel);

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

            submitTask();


        }
    });

    cancelTask.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(NewTaskHome.this,MainActivity.class);
            startActivity(i);
        }
    });
}

private void submitTask() {

    String title_val = titleName.getText().toString().trim();
    String desc_val = descName.getText().toString().trim();
    String date_val = dateName.getText().toString().trim();

    DatabaseReference newTask = database.push();

    newTask.child("title").setValue(title_val);
    newTask.child("description").setValue(desc_val);
    newTask.child("date").setValue(date_val);

    Intent i = new Intent(NewTaskHome.this,MainActivity.class);
    startActivity(i);


}

}

HomeFragment.java [for Tasks screen]:

public class HomeFragment extends Fragment {

Button addNewBtn;

public HomeFragment(){
    //Required empty constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_home, container, false);

    addNewBtn = v.findViewById(R.id.btnAddNew);
    addNewBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            startActivity(new Intent(getActivity(),NewTaskHome.class));
        }
    });

    return v;
}

}

I also made a TaskItem class with getter and setter methods when I was trying out some solutions (that I couldn't make work)?

public class TaskItem {
String title, description, date;

public TaskItem(){
}

public TaskItem(String title, String description, String date) {
    this.title = title;
    this.description = description;
    this.date = date;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public String getDescription() {
    return description;
}

public void setDescription(String description) {
    this.description = description;
}

public String getDate() {
    return date;
}

public void setDate(String date) {
    this.date = date;
}

XMLs:

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"
android:background="@android:color/holo_blue_dark" >

    <LinearLayout
        android:id="@+id/fragment_container"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:orientation="vertical"
        app:layout_constraintBottom_toTopOf="@+id/bottom_navigation"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottom_navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:menu="@menu/bottom_navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

fragment_home.xml:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     tools:context=".MainActivity"
     android:background="#F4F4F4"
     android:orientation="vertical">

     <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:background="@color/colorPrimaryDark">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal">

            <LinearLayout
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:orientation="vertical"
                android:layout_marginLeft="16dp">

                <TextView
                    android:id="@+id/heading"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_marginTop="20dp"
                    android:text="TASKS"
                    android:textColor="#FFF"
                    android:textSize="32sp" />
                    <!-- android:fontFamily="@font/montserrat2" -->

                <TextView
                    android:id="@+id/subheading"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_marginTop="8dp"
                    android:text="To-do list:"
                    android:textSize="18sp"
                    android:textColor="#4A4E6A"/>

            </LinearLayout>

            <Button
                android:id="@+id/btnAddNew"
                android:layout_width="55dp"
                android:layout_height="55dp"
                android:layout_marginTop="20dp"
                android:layout_marginLeft="230dp"
                android:background="@drawable/bg_btn_new"
                android:text="+"
                android:textSize="38sp"
                android:textColor="#fff"
                android:textAlignment="center"/>

        </LinearLayout>

        <View
            android:layout_width="match_parent"
            android:layout_height="1dp"
            android:layout_marginTop="1dp"
            android:background="#131E69"/>

     </LinearLayout>

     <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/taskList"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:layout_marginRight="16dp"
        android:layout_marginLeft="16dp">

     </androidx.recyclerview.widget.RecyclerView>

     <TextView
        android:id="@+id/footer"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="No More To Do"
        android:textAlignment="center"
        android:textColor="#9A9A9A"
        android:textSize="16dp">

     </TextView>


</LinearLayout>

task_item.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp">
    <LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:background="@drawable/bg_do_item"
        android:layout_marginBottom="20dp">

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="220dp"
            android:layout_height="match_parent"
            android:layout_marginLeft="16dp">

            <TextView
                android:id="@+id/taskTitle"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginTop="19dp"
                android:text="Title name"
                android:textSize="22sp"
                android:textColor="#000C5D"
                android:textStyle="bold"/>

            <TextView
                android:id="@+id/taskDesc"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginTop="8dp"
                android:text="Description"
                android:textSize="18sp"
                android:textColor="#ADADAD"/>

        </LinearLayout>

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="45dp"
            android:layout_marginTop="30dp">

            <TextView
                android:id="@+id/taskDate"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginTop="8dp"
                android:text="Date"
                android:textSize="18sp"
                android:textColor="#F63EA5"/>

        </LinearLayout>



    </LinearLayout>
</androidx.cardview.widget.CardView>

Again sorry for the amount of code, and for it not being so organised well.. i struggled to put my code into the question.

Please help me.. If you need more code for explanation then I'm happy to show it.

Answer 1

You have to add a listener, like this:

// Read from the database
myRef.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        // This method is called once with the initial value and again
        // whenever data at this location is updated.
        String value = dataSnapshot.getValue(String.class);
        Log.d(TAG, "Value is: " + value);
    }

    @Override
    public void onCancelled(DatabaseError error) {
        // Failed to read value
        Log.w(TAG, "Failed to read value.", error.toException());
    }
});