检索Firebase数据并将其返回的最佳方式，或者其他方式 [英] Best way to retrieve Firebase data and return it, or an alternative way

问题描述

我正在使用Firebase进行一些项目，并且很喜欢这个平台。

我只是偶然发现了一件我不能做的事情，想知道是否有办法实现它，或者有另一种方法来实现它。 p>

我试图做的是创建一个函数来从我的Firebase数据库中检索最后X个条目并将数据返回给调用函数。 b
$ b

类似于这样：

$ p $ function getTwoLatestLocations（）
{
var twoLocations;
myFirebaseRef.limitToLast（2）.once（'value'，function（dataSnapshot）{
twoLocations = dataSnapshot.val（）;
}，function（errorObject）{
/ / code来处理读取错误
console.log（The read failed：+ errorObject.code）;
}）;

返回twoLocations;
}


然后调用它：

 函数someCalcutationsWithTheTwoLastLocations（）
 {
 var twoLocation = getTwoLatestLocations（）; 
 //做一些计算
} 
  

但是正如您可能已经猜到调用数据库引用是异步的，所以返回的对象将永远是未定义的。

有没有办法做到这一点，所以我可以保持方法分开？

PS目前我所有的代码都只是在调用数据库的时候。丑陋的。

解决方案

要将代码分离出来，您需要传递一个回调：

  function getTwoLatestLocations（callback）
 {
 var twoLocations; 
 myFirebaseRef.limitToLast（2）.once（'value'，function（dataSnapshot）{
 twoLocations = dataSnapshot.val（）; 
 callback（twoLocations）; 
}，function （errorObject）{
 //处理读取错误的代码
 console.log（读取失败：+ errorObject.code）; 
}）; 
 
 
 
 
 
 
 
然后像这样调用它：
 函数someCalcutationsWithTheTwoLastLocations（）
 {
 getTwoLatestLocations（function（twoLocations）{
 //做一些计算
}）; 
 
 
 
 $ b $ p 
 $ b 
要记住的重要一点是你不能等待加载的东西异步。  
 
 
已经涵盖了很多次，包括从相关问题列表的顶部答案：在函数中处理异步调用（Firebase） 
 
I am using Firebase for some projects and am liking the platform a lot. 

I just stumbled upon a thing that I could not do and was wondering if there was any way to achieve it or if there was an alternative way of doing it.

What I am trying to do is to create a function to retrieve the last X number of entries from my Firebase database and returning the data to the calling function.

Something like this:
function getTwoLatestLocations()
{
    var twoLocations;
    myFirebaseRef.limitToLast(2).once('value', function (dataSnapshot) {
        twoLocations = dataSnapshot.val();
    }, function (errorObject) {
        // code to handle read error
        console.log("The read failed: " + errorObject.code);
    });

    return twoLocations;
}
And then calling it like this:
function someCalcutationsWithTheTwoLastLocations()
{
    var twoLastLocations = getTwoLatestLocations();
    // Do some calculations
}
But as you might have guessed the call to the database reference is asynchronous  so the returning object will always be undefined.

Is there a way to do this elegantly so I can keep the methods separated?

P.S. Currently all my code is simply inside the call to the database. Ugly.
解决方案
To separate the code out, you'd pass in a callback:
function getTwoLatestLocations(callback)
{
    var twoLocations;
    myFirebaseRef.limitToLast(2).once('value', function (dataSnapshot) {
        twoLocations = dataSnapshot.val();
        callback(twoLocations);
    }, function (errorObject) {
        // code to handle read error
        console.log("The read failed: " + errorObject.code);
    });

}
And then calling it like this:
function someCalcutationsWithTheTwoLastLocations()
{
    getTwoLatestLocations(function(twoLocations) {
        // Do some calculations
    });
}
The important thing to always remember is that you cannot wait for something that is loaded asynchronously. 

This has been covered quite a few times, including in this top answer from the list of related questions: Handling Asynchronous Calls (Firebase) in functions

                        
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