如何在Java / Android中按降序获取Firebase项目

Question

Let's say I have some data as follow:

"teilnehmer" : {
      "ATYFdfNRThzrEC83" : {
        "score" : 2500,
        "teamname" : "Was ist mit Karsten los?"
      },
      "VMT9hp65Tui55J2" : {
        "score" : 12500,
        "teamname" : "Team Zombie.de"
      },
      "dPkdf233dbMjCA2" : {
        "score" : 21635,
        "teamname" : "Bielefeld zwei"
      },
      "U1xTtKJa0wdZ13"  : {
        "score" : 500,
        "teamname"  : "Max der Eugen"
      }.............

And this Java Code:

    mDatabase.child("rallye").child("teilnehmer").orderByChild("score").addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                // teamname, Platzierung, Punkte
                Teilnehmer teilnehmer = new Teilnehmer(((String) dataSnapshot.child("teamname").getValue()), "Platz x", (dataSnapshot.child("score").getValue())+" Punkte");
                teilnehmerList.add(teilnehmer);     // add to recyclerview
                mAdapter.notifyDataSetChanged();    // save
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) { }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                //adapter.remove((String) dataSnapshot.child("teamname").getValue());
                //teilnehmerList.remove();
                mAdapter.notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) { }

            @Override
            public void onCancelled(DatabaseError databaseError) {}

        });

This doesn't work

.orderByChild("score")

Screenshot here : i.stack.imgur.com/Y8NlW.png

I get the teamname and the teamscore in a Rallygame. But the order is ascending, not descending :/

Output:
Teamname ----------- Score
Team Zombie            50
Team xyz               1500
Team foo              16000

Wrong order :/

---

And then I have a second question, how can I output the current Position in Ranking?

For example:

My List show something like this:

Rank     Teamname      Points
1        "Team 1"       25.000
2        "Team red"     12.000
3        "Team google"   9.500
4        "Team yellow"   1.250

My Firebase contain:

"ATYFdfNRThzrEC83" : {
            "score" : 2500,
            "teamname" : "Was ist mit Karsten los?"
          },

I have only 2 columns, score and teamname

In my first Question i solve the problem with the Order (most Points -> first place)

My layout file:

<TextView
        android:id="@+id/teamName"
        android:textColor="@color/teamTitle"
        android:textSize="16dp"
        android:textStyle="bold"
        android:layout_alignParentTop="true"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/teamRank"
        android:layout_below="@id/title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/teamScore"
        android:textColor="@color/teamScore"
        android:layout_width="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_height="wrap_content" />

3 Output TextViews, but only 2 Datacolumns in Firebase. The Team with the Most points will be on Rank / Place 1 (i dont know the correct english word for my German "Platz n"). Than Rank 2, Rank 3....

My first Try:

int i = 1;
while(dataSnapshot.child("teamname").getValue()) {

Teilnehmer teilnehmer = new Teilnehmer(((String) dataSnapshot.child("teamname").getValue()), "Platz "+i, (dataSnapshot.child("score").getValue())+" Punkte");
                teilnehmerList.add(0,   teilnehmer);     // add to recyclerview: add(0, teilnehmer) gebe in desc aus von 0 zum n. eintrag
                mAdapter.notifyDataSetChanged();    // save
i++;
}

I would like three ouputs in one row. Teamname, Rank in Ranking and the current score in the Game.

but my while Loop does not work.

Answer 1

In this function:

@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
    // teamname, Platzierung, Punkte
    Teilnehmer teilnehmer = new Teilnehmer(((String) dataSnapshot.child("teamname").getValue()), "Platz x", (dataSnapshot.child("score").getValue())+" Punkte");
    teilnehmerList.add(teilnehmer);     // add to recyclerview
    mAdapter.notifyDataSetChanged();    // save
}

I assume you are adding to an array list. Instead of adding to the end, add to the beginning because orderByChild is working, its in ascending order. Firebase does not support descending order.

teilnehmerList.add(0, teilnehmer); // <--- add 0 as 1st parameter

Your second question is lacking detail. Create another question.