Swift Completement Handler For Loop将执行一次，而不是由于循环而执行10次

Question

我有一个循环，其中包含一个重复10次的firestore查询。我需要在所有10个查询完成后调用(completion:block(；这里我有我的代码，这样它就可以执行每个查询(completion:block(，但这对服务器和用户的手机来说太重了。我如何更改以下内容以完成我刚才描述的内容？

static func getSearchedProducts(fetchingNumberToStart: Int, sortedProducts: [Int : [String : Int]], handler: @escaping (_ products: [Product], _ lastFetchedNumber: Int?) -> Void) {
var lastFetchedNumber:Int = 0
var searchedProducts:[Product] = []
let db = Firestore.firestore()
let block : FIRQuerySnapshotBlock = ({ (snap, error) in
guard error == nil, let snapshot = snap else {
debugPrint(error?.localizedDescription)
return
}
var products = snapshot.documents.map { Product(data: $0.data()) }
if !UserService.current.isGuest {
db.collection(DatabaseRef.Users).document(Auth.auth().currentUser?.uid ?? "").collection(DatabaseRef.Cart).getDocuments { (cartSnapshot, error) in
guard error == nil, let cartSnapshot = cartSnapshot else {
return
}
cartSnapshot.documents.forEach { document in
var product = Product(data: document.data())
guard let index = products.firstIndex(of: product) else { return }
let cartCount: Int = document.exists ? document.get(DatabaseRef.cartCount) as? Int ?? 0 : 0
product.cartCount = cartCount
products[index] = product
}
handler(products, lastFetchedNumber)
}
}
else {
handler(products, lastFetchedNumber)
}
})
if lastFetchedNumber == fetchingNumberToStart {
for _ in 0 ..< 10 {
//change the fetching number each time in the loop
lastFetchedNumber = lastFetchedNumber + 1
let productId = sortedProducts[lastFetchedNumber]?.keys.first ?? ""
if productId != "" {
db.collection(DatabaseRef.products).whereField(DatabaseRef.id, isEqualTo: productId).getDocuments(completion: block)
}
}
}
}

正如你在最后看到的，由于for _ in 0 ..< 10:，我为这个查询循环了10次

if productId != "" {
db.collection(DatabaseRef.products).whereField(DatabaseRef.id, isEqualTo: productId).getDocuments(completion: block)
}

因此，我需要使completion: block处理程序只被调用一次，而不是在这里调用10次。

Answer 1

使用DispatchGroup。您可以在每次调用异步代码时进入调度组，然后在每次调用完成后离开。然后，当一切完成时，它将调用notify块，您可以调用处理程序。下面是一个快速的例子：

let dispatchGroup = DispatchGroup()
let array = []
for i in array {
dispatchGroup.enter()
somethingAsync() {
dispatchGroup.leave()
}
}
dispatchGroup.notify(queue: .main) {
handler()
}