如何将Firestore查询中的数字保留在变量中以备将来使用? [英] How to keep number from a Firestore Query in a variable for further usage?

问题描述

我想将要从文档中查询的​​数字保留在firestore的集合中，并将该数字保存在有角度的组件的局部变量中，以便在该组件的其他方法中使用.

I would like to keep the number I am querying from a document in a collection in firestore and save that number in a local variable in an angular component to use it within other methods in that component.

我已经尝试实施Ittu提供的解决方案，但这对我不起作用，因为我没有这样做了解在何处使用 Promise.all

I already tried to implement the solution provided by Ittu, but that did not work for me, since I did not understood where to use Promise.all

这是我的代码:

var magNum: number;

docRef.ref.get().then(
  function(doc) {
  console.log("Document data:", doc.get('painPoint'));
  magNum = doc.get('painPoint');
});

console.log('number \n' + magNum);

在控制台中的结果为 undefined ，除非我将其放入以下函数中:

The result in the console is undefined, except if I put it into the function like:

var magNum: number;

docRef.ref.get().then(
  function(doc) {
  console.log("Document data:", doc.get('painPoint'));
  magNum = doc.get('painPoint');
  console.log('number \n' + magNum);
});

然后控制台中的magNum为3

then magNum will be 3 in the console

推荐答案

您必须学习有关回调函数和promise的知识.

You have to learn something about callback functions and promises.

查看您的代码:

var magNum: number;

docRef.ref.get().then(
  function(doc) {
  console.log("Document data:", doc.get('painPoint'));
  magNum = doc.get('painPoint');
});

console.log('number \n' + magNum);

代码流如下所示:

1. 您将变量 magNum 声明为数字
2. 您说:从firestore中获取文档，如果完成，则将 painPoint 号存储到 magNum 变量中"
3. 从firestore中打印出我的 magNum .

1. You declare a variable magNum as number
2. You say: "get the document from firestore and if you done, THEN store the painPoint number to the magNum variable"
3. Print out my magNum from firestore.

但是真正发生的是这件事

But what really happens is this:

1. 代码将新变量 magNum 声明为数字
2. 代码开始从Firestore ASYNC
获取您的文档
3. 该代码会打印出您的 magNum (未定义，因为尚未完成Firestore文档的获取)
4. 加载文档后，变量 magNum 将设置为 painPoint 的值.

1. The code declares a new variable magNum as number
2. The code starts to get your document from firestore ASYNC
3. The code prints out your magNum (Which is undefined, because getting the firestore document isn't done yet)
4. After some times the document is loaded and your variable magNum will be set to the value of painPoint.

安慰: console.log('number \ n'+ magNum); 将在 magNum = doc.get('painPoint'); ，这就是为什么控制台中 magNum 未定义 的原因！

Consolusion: console.log('number \n' + magNum); will be called before the line magNum = doc.get('painPoint'); and this is why magNum is undefined in console!

您可以使用 async/await :

async yourFunction(...){
    var magNum: number;

    var doc = await docRef.ref.get();
    magNum = doc.get('painPoint');

    console.log('number \n' + magNum); // output is the value of painPoint
}

这篇关于如何将Firestore查询中的数字保留在变量中以备将来使用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！