Android从Firebase数据快照中提取数据

Question

我正在尝试从firebase的DataSnapshot中获取数据。我遵循了firebase开发人员示例中的说明。当我尝试拉取数据时，它显示为空，尽管我在Android Studio中运行调试器时可以看到数据。下面列出了我正在使用的代码。

    Query mQueryRef = mDatabase.child("users").child(mUserId).child("bullets");
    // This type of listener is not one time, and you need to cancel it to stop
    // receiving updates.
    mQueryRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot snapshot, String previousChild) {
            // This will fire for each matching child node.
            Bullet bullet = snapshot.getValue(Bullet.class);
            String date = bullet.getDate();
            String title = bullet.getTitle();
            mEmailBody + "\n" + title + " " + date;

这是Bullet类：

public class Bullet {

// Bullet basics
private String title;
private String orderDate;
private String date;
private String action;
private String result;
private String impact;

// Default constructor required by Firebase
public Bullet (){

}

//Constructor
public Bullet (String title, String orderDate, String date, String action, String result, String impact) {
    this.title = title;
    this.orderDate = orderDate;
    this.date = date;
    this.action = action;
    this.result = result;
    this.impact = impact;

}

//Getters
public String getTitle() {return title;}
public String getOrderDate() {return orderDate;}
public String getDate() {return date;}
public String getAction() {return action;}
public String getResult() {return result;}
public String getImpact() {return impact;}

//Setters
public void setTitle(String title){
    this.title = title;
}
public void setOrderDate(String orderDate){
    this.orderDate = orderDate;
}
public void setDate(String date){
    this.date = date;
}
public void setAction(String action){
    this.action = action;
}
public void setResult(String result){
    this.result = result;
}
public void setImpact(String impact){
    this.impact = impact;
}

}

这是来自firebase的json数据表：

- appname-1234b
        - users
            - WRQK8Fo3TPUnZyPXYnQ9gHVfFms2
                - bullets
                    - Kfc8jCy5f2bb1aN-o0C
                        - bullet
                          action: "did some stuff"
                          date: "19 Mar 2017"
                          impact: "some stuff was impacted"
                          orderDate: "2017078"
                          result: "a result happened"
                          title: "new one for the date"
                    - KfcMg-7-xp98Kwq-PUR
                        - bullet 
                          action: "did some stuff"
                          date: "19 Mar 2017"
                          impact: "some stuff was impacted"
                          orderDate: "2017078"
                          result: "a result happened"
                          title: "new one for the date"

当我运行调试器时，当它到达Bullet bullet = snapshot.getValue(Bullet.class);行时，我可以看到所有看起来像Bullet: snapshot {key: Kfc8jCy5f2bb1aN-o0C title:"new one for the date" date:"19 Mar 2017"......}的数据点，但是当它到达下两行时

String date = bullet.getDate();
String title = bullet.getTitle();

调试器将值显示为date = null bullet:3248b和title = null bullet:3248b

我尝试了许多不同的方法来提取数据：

String date = snapshot.getValue(Bullet.Class).getDate();
String textBullet = snapshot.getValue(Bullet.Class).toString();

但每次出现时，它都是空的，或者具有无法提取的全部项目符号数据。

Answer 1

您在每个推流ID下都有一个名为bullet的子项，但是您的监听器中没有考虑到这一点。

最简单的解决方案是跳过代码中的bullet节点：

Query mQueryRef = mDatabase.child("users").child(mUserId).child("bullets");
// This type of listener is not one time, and you need to cancel it to stop
// receiving updates.
mQueryRef.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot snapshot, String previousChild) {
        Bullet bullet = snapshot.child("bullet").getValue(Bullet.class);
        //                      ^^^^^^^^^^^^^^^^
        String date = bullet.getDate();
        String title = bullet.getTitle();

但更好的解决方案是更改数据结构，使其不具有bullet级别。如果你没有在你的代码中使用它，那么在你的数据库中就没有用了。

Answer 2

您的数据库引用指向Bullet，它的值为HashMap，而不是直接指向Bullet对象。您需要一个Hashmap来检索数据。

private static HashMap<String, Bullet> bullets = new HashMap<>(); 

Query mQueryRef = mDatabase.child("users").child(mUserId).child("bullets");
    // This type of listener is not one time, and you need to cancel it to stop
    // receiving updates.
    mQueryRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                Log.d(TAG, "onDataChange():" + dataSnapshot.toString());

                for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
                    Bullet bullet = snapshot.getValue(Bullet.class);
                    bullets.put(snapshot.getKey(), bullet);
                }

               //Iterate and Print your bullets hashmap here.
        System.out.println(Arrays.asList(bullets)); 
            }

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

            }
        });
    }

Answer 3

根据DataSnapshot文档：

将DataSnapshots传递给使用addValueEventListener(ValueEventListener)、addChildEventListener(ChildEventListener)或addListenerForSingleValueEvent(ValueEventListener)附加的侦听器中的方法。