我无法从 Firebase 数据库中检索数据答案

【问题标题】：I am Unable to retrieve data from Firebase database我无法从 Firebase 数据库中检索数据
【发布时间】：2018-04-24 19:13:33
【问题描述】：

我发布这个问题是因为我没有找到任何与我的问题相关的适当解决方案。

我正在尝试从 Firebase 检索我的传感器数据，但我无法做到。请指出我哪里做错了

Try.java

public class Try extends AppCompatActivity {

Button button;

ListView list;
DatabaseReference databaseTry;

List<TryObjectClass> tryObjectList;

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

    databaseTry = 
FirebaseDatabase.getInstance().getReference("sensor_data");

    button = (Button) findViewById(R.id.button);
    list = (ListView) findViewById(R.id.list);
    //list.setVisibility(View.INVISIBLE);

    tryObjectList = new ArrayList<>();

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

        }
    });
}

@Override
protected void onStart() {
    super.onStart();

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

            tryObjectList.clear();

            for (DataSnapshot TrySnapshot : dataSnapshot.getChildren()){
                TryObjectClass tryObjectClass = 
TrySnapshot.getValue(TryObjectClass.class);

                tryObjectList.add(tryObjectClass);
            }

            TryAdapter adapter = new TryAdapter(Try.this, tryObjectList);
            list.setAdapter(adapter);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });


}
}

这是我的对象类

TryObjectClass.java

public class TryObjectClass  {

private String date;
private String time;
private String humidity;
private String motion;
private String distance;
private String temperature;



public TryObjectClass(){

}

public TryObjectClass(String date, String time, String humidity, String 
motion, String distance, String temperature) {
    this.date = date;
    this.time = time;
    this.humidity = humidity;
    this.motion = motion;
    this.distance = distance;
    this.temperature = temperature;
}

public String getDate() {
    return date;
}

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

public String getTime() {
    return time;
}

public void setTime(String time) {
    this.time = time;
}

public String getHumidity() {
    return humidity;
}

public void setHumidity(String humidity) {
    this.humidity = humidity;
}

public String getMotion() {
    return motion;
}

public void setMotion(String motion) {
    this.motion = motion;
}

public String getDistance() {
    return distance;
}

public void setDistance(String distance) {
    this.distance = distance;
}

public String getTemperature() {
    return temperature;
}

public void setTemperature(String temperature) {
    this.temperature = temperature;
}
}

这是我的适配器类

TryAdapter.java

public class TryAdapter extends ArrayAdapter<TryObjectClass> {

private Activity context;
private List<TryObjectClass> objectList;

public TryAdapter(@NonNull Activity context, List<TryObjectClass> 
objectList) {
    super(context, R.layout.new_list,0);
    this.context = context;
    this.objectList = objectList;
}

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull 
ViewGroup parent) {
    LayoutInflater inflater = context.getLayoutInflater();

    View listViewItem = inflater.inflate(R.layout.new_list,null,true);

    TextView date = (TextView) listViewItem.findViewById(R.id.date);
    TextView distance = (TextView) 
listViewItem.findViewById(R.id.distance);
    TextView humidity = (TextView) 
listViewItem.findViewById(R.id.humidity);
    TextView motion = (TextView) listViewItem.findViewById(R.id.motion);
    TextView temperature = (TextView) 
listViewItem.findViewById(R.id.temperature);
    TextView time = (TextView) listViewItem.findViewById(R.id.time);

    TryObjectClass tryObjectClass = objectList.get(position);

    date.setText(tryObjectClass.getDate());
    distance.setText(tryObjectClass.getDistance());
    humidity.setText(tryObjectClass.getHumidity());
    motion.setText(tryObjectClass.getMotion());
    temperature.setText(tryObjectClass.getTemperature());
    time.setText(tryObjectClass.getTime());

    return super.getView(position, convertView, parent);
}
}

try_activity.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=".Try"
android:orientation="vertical">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Your Retrieved data"
    android:textAppearance="@style/Base.TextAppearance.AppCompat.Large"
    android:padding="10dp"
    android:layout_marginLeft="90dp"
    android:layout_marginTop="20dp"/>

<Button
    android:id="@+id/button"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="click here to retrieve the data"/>

<ListView
    android:id="@+id/list"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>

</LinearLayout>

这是我的列表项

new_list.xml

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

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

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Date -"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            android:textColor="#000"
            android:padding="10dp"
            android:layout_marginLeft="20dp"/>
        <TextView
            android:id="@+id/date"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"/>

    </LinearLayout>

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

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Distance -"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            android:textColor="#000"
            android:padding="10dp"
            android:layout_marginLeft="20dp"/>
        <TextView
            android:id="@+id/distance"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"/>

    </LinearLayout>

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

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Humidity -"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            android:textColor="#000"
            android:padding="10dp"
            android:layout_marginLeft="20dp"/>
        <TextView
            android:id="@+id/humidity"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"/>

    </LinearLayout>

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

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Motion -"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            android:textColor="#000"
            android:padding="10dp"
            android:layout_marginLeft="20dp"/>
        <TextView
            android:id="@+id/motion"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"/>

    </LinearLayout>

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

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Temperature -"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            android:textColor="#000"
            android:padding="10dp"
            android:layout_marginLeft="20dp"/>
        <TextView
            android:id="@+id/temperature"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"/>

    </LinearLayout>

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

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Time -"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"
            android:textColor="#000"
            android:padding="10dp"
            android:layout_marginLeft="20dp"/>
        <TextView
            android:id="@+id/time"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"/>

    </LinearLayout>

</LinearLayout>

</ScrollView>

这是我的数据库

smartborder-cef61
sensor_data
    100
        date: 1875
        distance: "50M"
        humidity:  4573
        motion: "yes"
        temperature: "30c"
        time: 7377200
    200
        date: 23111996
        distance: "50M"
        humidity: 45
        motion: "yes"
        temperature: "30C"
        time: 73775

【问题讨论】：

这段代码有什么问题？你有错误吗？
您能具体说明您面临的问题和问题吗？
我无法填充列表视图
应用启动时列表视图为空
首先学会调试你的代码。尝试将 firebase 中的数据打印到您的日志中，并确认您正在获取值。如果您正在获取它们，请通过在日志中打印列表来检查它是否添加到列表中。如果它们都在工作，那么检查列表视图代码。您可以使用Log.e("key", "your data"); 将数据打印到日志中。找出问题出在哪里。

标签： android firebase listview firebase-realtime-database

【解决方案1】：

我发现了一些东西，试着把你的setAdapter()放在你的听众之外。在你的监听器中，你将放置以下代码：

public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot TrySnapshot : dataSnapshot.getChildren()){
                TryObjectClass tryObjectClass = TrySnapshot.getValue(TryObjectClass.class);
                tryObjectList.add(tryObjectClass);
            }
            adapter.notifyDataSetChanged();
}

并尝试将您声明的内容放入onCreate 方法中，例如您的ValueEventListener，这样您就可以在onStart 中添加它并在onDestroy 中删除它，当您的活动不再需要监听您的更改时被摧毁。这是我的片段 ContatsFragment 的一部分，它工作正常：

public class ContatsFragment extends Fragment {

    private ListView listView;
    private ArrayAdapter adapter;
    private ArrayList<Contat> contats;
    private DatabaseReference databaseReference;
    private ValueEventListener valueEventListenerContatos;
    private Context context;

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


    @Override
    public void onStart() {
        super.onStart();
        context = getActivity();
        databaseReference.addValueEventListener(valueEventListenerContatos);
    }

    @Override
    public void onStop() {
        super.onStop();
        databaseReference.removeEventListener(valueEventListenerContatos);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        contats = new ArrayList<>();
        View view = inflater.inflate(R.layout.fragment_contats, container, false);

        listView = (ListView) view.findViewById(R.id.lv_contatos);

        adapter = new ContatAdapter(getActivity(), contats);
        listView.setAdapter(adapter);

        Preferences preferences = new Preferences(getActivity());
        String loggedUserId = preferences.getLoggedUserId();

        databaseReference = FirebaseConfig.getFirebaseReference();
        databaseReference = databaseReference.child("contats")
                .child(loggedUserId);

        valueEventListenerContatos = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                contats.clear();
                for (DataSnapshot dados : dataSnapshot.getChildren()) {
                    Contat contat = dados.getValue(Contat.class);
                    contats.add(contat);
                }
                adapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        };

        return view;
    }
}

【讨论】：

【解决方案2】：

我已经找到了解决方案...我在提供列表对象名称时犯了错误...

【讨论】：