Android studio FirebaseDatabase：RemoveEventListener无效

Question

我的应用中存在问题。我正在调用valueEventListener，我希望当用户关闭活动或启动另一个活动时停止监听，所以我调用removeEventListener但它无法正常工作

//Firebase
private DatabaseReference mDatabase;
private FirebaseUser mUser;


//Firebase
    mUser = FirebaseAuth.getInstance().getCurrentUser();
    user_id = mUser.getUid();
    mDatabase = FirebaseDatabase.getInstance().getReference();
    mDatabase.keepSynced(true);
    listener =mDatabase.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            url = dataSnapshot.child(CHILD_PIC).child(pic).getValue().toString();
            player = dataSnapshot.child(CHILD_PLAYERS).child(players).getValue().toString();
            Img_check = dataSnapshot.child(CHILD_PIC).child(CHILD_CHECK).getValue().toString();
            balls = dataSnapshot.child("Users").child(user_id).child("Balls").getValue().toString();
            number_miss = dataSnapshot.child("Users").child(user_id).child("Mission").getValue().toString();
            Isso = dataSnapshot.child("Users").child(user_id).child("Isso").getValue().toString();
            Tx = dataSnapshot.child("Users").child(user_id).child("Tx").getValue().toString();
            Three = dataSnapshot.child("Users").child(user_id).child("Three").getValue().toString();
            //verify = dataSnapshot.child(check).child(user_id).child(CHILD_CHECK).getValue().toString();
           // Toast.makeText(PlayActivity.this, verify, Toast.LENGTH_SHORT).show();

            dialog_mi(number_miss);setimg(url);checkverify(verify,player,Img_check);putfloat(Isso,Tx,Three);
            //miss(number_miss);
            //getballs(balls);
            //checkplayer(player);
            //checkvefify(verify,player);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(PlayActivity.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });


@Override
public void onBackPressed() {
    super.onBackPressed();
    mDatabase.removeEventListener(listener);
    Intent intent = new Intent(getApplicationContext(), MissionActivity.class);
    intent.putExtra("level", mission);
    startActivity(intent);

}