等待循环完成，然后再执行下一个功能 [英] wait for for loop to finish before executing next function

问题描述

我需要等待for循环完成，然后才能开始使用'experimentArray'.在继续使用experimentArray之前，我该如何等到完成为止?我已经尝试过Promise，异步等待等

I need to wait for my for loop to be completed before i can start using the 'experimentArray'. How do i wait until this is complete before i move onto using the experimentArray? i've tried promises, async await, etc

 let experimentArray = []
     for (i = 0; i < this.state.gameRoomMembers.length; i++) {
            firebase.firestore().collection('users').doc(this.state.gameRoomMembers[i]).collection('groupChats').get().then(snapshot => {
                 if (!snapshot.empty) {
                      snapshot.forEach(doc => {
                          experimentArray.push(doc.data().key)
                       })
                 }
             })
}
console.log(experimentArray.length) // outputs 0

推荐答案

所见即所得，因为对Firestore的 get()调用是异步的.

What you're seeing is expected, as the get() calls to Firestore are asynchronous.

您不能真正等待.但是，通过 await (甚至是 Promise.all())，您可以非常接近:

You can't really wait for them. But with await (or even Promise.all()) you can get pretty close:

let promises = [];
for (i = 0; i < this.state.gameRoomMembers.length; i++) {
  promises.push(firebase.firestore().collection('users').doc(this.state.gameRoomMembers[i]).collection('groupChats').get());
}
let experimentArray = []
Promise.all(promises).then(snapshots => {
  snapshot.forEach(snapshot => {
     if (!snapshot.empty) {
       snapshot.forEach(doc => {
         experimentArray.push(doc.data().key)
       })
     }
  })
});
console.log(experimentArray.length)

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