如何检查FireBase(Android)中是否存在特定的子值 [英] How do I check if specific child value exists in FireBase (Android)

问题描述

尝试检查用户信息是否已存储在FireBase数据库中时遇到一些麻烦. 基本上，我正在尝试做这样的愚蠢的事情: 从用户中选择user_name，其中user_id ="+ userID +" 如果昵称存在，则应使布尔值var isFirstTime = false，否则应保持为true.然后，它应该显示或不显示注册框.

I have some trouble trying to check if user information is stored already in the FireBase database. Basically I'm trying to do something stupid like this: "select user_name from user where user_id="+userID+" And if the nickname exists it should make the boolean var isFirstTime = false and if it doesn't it should stay true. And after that it should show register box or not.

这是我的数据库: Firebase 这是我在onCreate方法中的代码:

This is my db: Firebase And this is my code in onCreate method:

databaseReference = FirebaseDatabase.getInstance().getReference();
    DatabaseReference dbRefFirstTimeCheck = databaseReference.child("User").child(user.getUid()).child("Nickname");

    isFirstTime = true;
    dbRefFirstTimeCheck.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if(dataSnapshot.getValue() != null) {
                isFirstTime=false;
            }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });

    if(isFirstTime) {
        showNewUserBox();
    }
    else {

    }

无论我做什么，都会调用methor showNewUserBox().如何获取所需的数据并检查是否存在?

No matter what I do, the methor showNewUserBox() is being called. How do I get the data i need and check if it's there?

推荐答案

正如其他人所评论的那样，数据是从Firebase异步加载的.到您检查isFirstTime时，尚未加载数据，onDataChange尚未运行，因此ifFirstTime将具有其默认值(布尔值为false).

As others have commented, data is loaded from Firebase asynchronously. By the time you check isFirstTime, the data hasn't been loaded yet, onDataChange hasn't been run yet, so ifFirstTime will have its default value (false for a boolean).

所有需要从数据库获取数据的代码都应在onDataChange内部(或在其中调用).您的代码最简单的解决方法是:

All code that requires data from the database should be inside onDataChange (or invoked from within there). The simplest fix for your code is:

databaseReference = FirebaseDatabase.getInstance().getReference();
DatabaseReference dbRefFirstTimeCheck = databaseReference.child("User").child(user.getUid()).child("Nickname");

dbRefFirstTimeCheck.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if(dataSnapshot.exists()) {
            showNewUserBox();
        }
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException(); // don't ignore errors
    }
});

另请参阅许多有关从Firebase异步加载的问题，例如 getContactsFromFirebase()方法返回空列表(或这个非常古老的经典:设置Singleton属性值在Firebase侦听器中).

Also see some of the many questions about asynchronous loading from Firebase, such as getContactsFromFirebase() method return an empty list (or this quite old classic: Setting Singleton property value in Firebase Listener).

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