{React Native} Async \ Await无法与setSate一起正常使用 [英] {React Native} Async\Await not working properly with setSate

问题描述

有人可以帮助我了解我做错了什么吗? 考虑这个简单的代码

Can someone help me understand what I am not doing correctly? Consider this simple code

 var images = []; 
 const [funImage, setFunImage] = useState([]);


//Some function that does this below
firebase.firestore().collection('PostedFunActivities').where("location", "==" , place).get().then((querySnapshot) =>{
        querySnapshot.forEach(async(doc) =>{ 
            const ref = firebase.storage().ref('images/'+ doc.data().image)
            const result = await ref.getDownloadURL();
            images.push(result);                                                                   
           })
           setFunImage(images);
       });

我不明白为什么setFunImage(images);在images.push(result);完成之前将所有结果推入数组之前被执行.我以为等待会阻塞下面的其余代码 基本上，我要尝试执行的操作背后的概念是将所有结果推送到images，然后调用setFunImage(images);.

I am not understanding why setFunImage(images); gets executed before images.push(result); finishes to push all the results into the array. I thought await would block the remainder of the code below it Basically the concept behind what I am trying to do is to have all my results pushed to images and THEN call setFunImage(images);.

我该如何实现?甚至有可能吗?

How can I achieve that? Is it even possible?

编辑

我更改了代码，希望找到解决方案，这是到目前为止的目标:

I changed my code in hope to find a solution to this and this is where I got to so far:

firebase.firestore().collection('PostedFunActivities').where("location", "==" , place).get().then((querySnapshot) => {
   querySnapshot.forEach(async(doc) => {
     const ref = firebase.storage().ref('images/' + doc.data().image)
     const result = await ref.getDownloadURL();
     images.push(result);
     setFunImage(...funImage,images);
     }) 
});

有趣的是，执行此函数时，funImage填充了1张图像，但是当我刷新时，它填充了我在Firebase中拥有的其余图像.

Interestingly enough, when this function executes funImage is populated with 1 image, but then when I refresh it gets populated with the rest of my images that I have in my firebase.

看看这个我正在运行的应用的GIF以及setState的问题

推荐答案

该代码不起作用，因为您的forEach正在运行异步代码.这意味着它将在您设置图像后完成运行. 这是一个修正了注释的解释-

The code doesn't work because your forEach is running async code. Which means it will finish running after you set your images. Here's a fix with some explanations in comments -

// No need for images array outside
const [funImage, setFunImage] = useState([]);

...

firebase.firestore().collection('PostedFunActivities').where("location", "==" , place).get().then(async (querySnapshot) =>{
    // instead of foreach, using map to aggregate the created promises into one array
    // Promise.all takes an array of promises and resolves after all of them completed running
    // returns an array with the promise results
    const images = await Promise.all(querySnapshot.map(async(doc) =>{ 
        const ref = firebase.storage().ref('images/'+ doc.data().image)
        const result = await ref.getDownloadURL();
        return result;                                         
    }));
    setFunImage(images);
});

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