如何一次获取Firebase数据库的值(Android) [英] How to get a value once of a firebase database (Android)

问题描述

我刚开始使用firebase，我对复杂的事物有多么简单，以及多么简单的事物感到惊讶.

I just started using firebase and I'm amazed of how simple are complex things, and how complex simple things can be.

我需要检查Firebase数据库中是否存在用户，如果存在，则需要获取其键和值.

I need to check if a user exists in my Firebase database, and if exists, I need to get its key and value.

我有密钥，我尝试通过转到用户子树并寻找具有相同密钥(ID)的孩子来在数据库中找到它

I have the key and I try to find it in the DB by going to my user subtree and loking for a child with the same key (ID)

DatabaseReference root_firebase = FirebaseDatabase.getInstance().getReference();
DatabaseReference users_firebase = root_firebase.child("Users");
DatabaseReference friend_found = users_firebase.child(key_searched_friend);

一旦有了，我就会尝试调用类似的方法

once I have it I try to call some method like

friend_found.getValue(); 

或

friend_found.child("user_name").getValue();

但是它不存在，

这看起来很奇怪，因为我可以两者都做

this seems weird since I can do both

friend_found.getKey();
friend_found.child("user_name").getKey();

我无法使用重写方法onDataChanged()来执行此操作，因为查询此值时数据不会更改.

I can't do this with the overriden method onDataChanged() since the data does not change when I query for this value.

这是我的firebase数据库结构:

This is my firebase database structure:

{
  "RoomNames" : {
    "Room-KM5cof0jcoMN8a4g6vC" : {
      "room_name" : "TESTrOOM"
    },
    "Room-KM5dg_WPRdEOT4_oJ1r" : {
      "room_name" : "testRoom2"
    }
  },
  "Users" : {
    "User-KM5ZaGq0xvjQis05CPF" : {
      "user_name" : "Enrique"
    }
  }
}

推荐答案

您说:我无法使用覆盖方法onDataChanged()来执行此操作，因为查询该值时数据不会更改

有关获取数据的指南说明:

The guide for how to Retrieve Data explains that:

通过将异步侦听器附加到FirebaseDatabase参考.会在一次触发监听器数据的初始状态，并在数据更改时再次显示.

Firebase data is retrieved by attaching an asynchronous listener to a FirebaseDatabase reference. The listener is triggered once for the initial state of the data and again anytime the data changes.

因此，当您将侦听器附加到某个位置时， onDataChanged()触发并为您提供当前值.

So when you attach a listener to a location, onDataChanged() fires and gives you the current value.

在标题为一次读取数据"的部分中，指南指出:

In the section titled Read Data Once, the guide states:

在某些情况下，您可能希望一次调用一次回调立即删除，例如初始化您的UI元素时不要指望改变.您可以使用 addListenerForSingleValueEvent()方法可简化此情况:触发一次，然后不再触发.

In some cases you may want a callback to be called once and then immediately removed, such as when initializing a UI element that you don't expect to change. You can use the addListenerForSingleValueEvent() method to simplify this scenario: it triggers once and then does not trigger again.

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