How to make a function with a loop asynchronous in Swift? - swift

Question

I am creating an application for a library. I am trying to fetch all the books the user has checked out from Firebase, but my attempts to make the function asynchronous with a DispatchGroup doesn't seem to be working. I suspect this to be because of the for-in loop found inside of the function.
func fetchHistory() {
if items.count > 0 {
items.removeAll()
}
let myGroup = DispatchGroup()
myGroup.enter()
var itemNames = [String]() // this holds the names of the child values of /users/uid/items/ <-- located in Firebase Database
guard let uid = fAuth.currentUser?.uid else {return}
fData.child("users").child(uid).child("items").observe(.value, with: { snapshot in
// make sure there is at least ONE item in the history
if snapshot.childrenCount > 0 {
let values = snapshot.value as! NSDictionary
for i in values.allKeys {
itemNames.append(i as! String)
}
print(itemNames)
let uid = fAuth.currentUser!.uid // get the UID of the user
for item in itemNames {
fData.child("users").child(uid).child("items").child(item).observe(.value, with: { snapshot in
let values = snapshot.value as! NSDictionary
let bookTitle = values["title"] as! String
print(bookTitle)
let bookAuthor = values["author"] as! String
print(bookAuthor)
let bookCoverUrl = values["coverUrl"] as! String
print(bookCoverUrl)
let bookStatus = values["status"] as! String
print(bookStatus)
let bookDueDate = values["dueDate"] as! String
print(bookDueDate)
let book = Book(name: bookTitle, author: bookAuthor, coverUrl: bookCoverUrl, status: bookStatus, dueDate: bookDueDate)
self.items.append(book)
})
}
self.booksTable.isHidden = false
} else {
self.booksTable.isHidden = true
}
})
myGroup.leave()
myGroup.notify(queue: DispatchQueue.main, execute: {
self.booksTable.reloadData()
print("Reloading table")
})
}
Here is the output from the print() statements:
########0
Reloading table
["78DFB90A-DE5B-47DE-ADCA-2DAB9D43B9C8"]
Mockingjay (The Hunger Games, #3)
Suzanne Collins
https://images.gr-assets.com/books/1358275419s/7260188.jpg
Checked
Replace
The first two lines of output should be printed AFTER everything else has printed. I really need some help on this, I have been stuck on this for hours. Thanks!
Edit:
As requested, here is my Firebase structure:
users:
meZGWn5vhzXpk5Gsh92NhSasUPx2:
ID: "12345"
firstname: "Faraaz"
items:
78DFB90A-DE5B-47DE-ADCA-2DAB9D43B9C8
author: "Suzanne Collins"
coverUrl: "https://images.gr assets.com/books/1358275419s/..."
dueDate: "Date"
status: "Checked"
title: "Mockingjay (The Hunger Games, #3)"
type: "regular"

Answer 1

A couple of issues:
The pattern is that leave must be called inside the completion handler of the asynchronous call. You want this to be the last thing performed inside the closure, so you could add it as the the last line within completion handler closure.
Or I prefer to use a defer clause, so that not only do you know it will be the last thing performed in the closure, but also:
you ensure you leave even if you later add any "early exits" inside your closure; and
the enter and leave calls visually appear right next to each other in the code saving you from having to visually hunt down at the bottom of the closure to make sure it was called correctly.
You also, if you want to wait for the asynchronous calls in the for loop, have to add it there, too.
A very minor point, but you might want to not create the group until you successfully unwrapped uid. Why create the DispatchGroup if you could possibly return and not do any of the asynchronous code?
Thus, perhaps:
func fetchHistory() {
if items.count > 0 {
items.removeAll()
}
var itemNames = [String]()
guard let uid = fAuth.currentUser?.uid else {return}
let group = DispatchGroup()
group.enter()
fData.child("users").child(uid).child("items").observe(.value, with: { snapshot in
defer { group.leave() } // in case you add any early exits, this will safely capture
if snapshot.childrenCount > 0 {
...
for item in itemNames {
group.enter() // also enter before we do this secondary async call
fData.child("users").child(uid).child("items").child(item).observe(.value, with: { snapshot in
defer { group.leave() } // and, again, defer the `leave`
...
})
}
...
} else {
...
}
})
group.notify(queue: .main) {
self.booksTable.reloadData()
print("Reloading table")
}
}

Answer 2

While there is a brilliant answer from Rob, I would approach a solution from a different direction.
A book can only ever had one person check it out (at a time), but a borrower can have multiple books. Because of that relationship, simply combine who has the book with the book itself:
Here's a proposed users structure
users
uid_0
name: "Rob"
uid_1
name: "Bill"
and then the books node
books
78DFB90A-DE5B-47DE-ADCA-2DAB9D43B9C8
author: "Suzanne Collins"
coverUrl: "https://images.gr assets.com/books/1358275419s/..."
dueDate: "Date"
status: "Checked"
title: "Mockingjay (The Hunger Games, #3)"
checked_out_by: "uid_0"
check_date: "20180118"
Then to get ALL of the books that Rob has checked out and use those results to populate an array and display it in a tableview becomes super simple:
//var bookArray = [Book]() //defined as a class var
let booksRef = self.ref.child("books")
let query = booksRef.queryOrdered(byChild: "checked_out_by").queryEqual(toValue: "uid_0")
booksRef.observeSingleEvent(of: .value, with: { snapshot in
for child in snapshot.children {
let snap = child as! DataSnapshot
let book = Book(initWithSnap: snap) //take the fields from the snapshot and populate the book
self.bookArray.append(book)
}
self.tableView.reloadData()
})
But then you ask yourself, "self, what if I want a record of who checked out the book?"
If you need that functionality, just a slight change to the books node so we can leverage a deep query;
books
78DFB90A-DE5B-47DE-ADCA-2DAB9D43B9C8
author: "Suzanne Collins"
coverUrl: "https://images.gr assets.com/books/1358275419s/..."
dueDate: "Date"
status: "Checked"
title: "Mockingjay (The Hunger Games, #3)"
check_out_history
"uid_0" : true
"uid_1" : true
and move the check out dates to the users node. Then you can query for any user of any book and have history of who checked out that book as well. (there would need to be logic to determine who has the book currently so this is just a starting point)
Or if you want another option, keep a separate book history node
book_history
78DFB90A-DE5B-47DE-ADCA-2DAB9D43B9C8
-9j9jasd9jasjd4 //key is created with childByAutoId
uid: "uid_0"
check_out_date: "20180118"
check_in_date: "20180122"
condition: "excellent"
-Yuhuasijdijiji //key is created with childByAutoId
uid: "uid_1"
check_out_date: "20180123"
check_in_date: "20180125"
condition: "good"
The concept is to let Firebase do the work for you instead of iterating over arrays repeatedly and having to issue dozens of calls to get the data you need. Adjusting the structure makes it much simpler to maintain and expand in the future as well - and it avoids all of the issues with asynchronous code as it's all within the closure; nice and tidy.