Firebase isn't completing before calling next method - swift

Question

I am making a call to a firebase database with the following:
let myRef1 = Database.database().reference(withPath: "/tests/\ .
(myTest)")
print(myTest)
myRef1.observe(.value, with: {
snapshot in
print(snapshot)
var newItems: [qItem] = []
for item in snapshot.children {
let mItem = qItem(snapshot: item as! DataSnapshot)
newItems.append(mItem)
print(newItems)
}
self.myArray = newItems.shuffled()
print(self.myArray)
})
loadNext()
...
However it never completes the completion handler before it moves on to the next method call, which is dependent on the results from this.
Tried making it a separate method, etc. but nothing seems to work.

Answer 1

You need to add the method loadNext() inside the observe to be able to call it after retrieving the data. When you retrieve data it is happening asynchronously which means the compiler wont wait until all the data is retrieved, it will call the method loadNext() first and then after finishing retrieving the data it will execute print(newItems), so you need to do the following:
myRef1.observe(.value, with: {
snapshot in
print(snapshot)
var newItems: [qItem] = []
for item in snapshot.children {
let mItem = qItem(snapshot: item as! DataSnapshot)
newItems.append(mItem)
print(newItems)
loadNext()
}
self.myArray = newItems.shuffled()
print(self.myArray)
})

Answer 2

Firebase data is handled asynchronously. Doug Stevenson wrote a great blog about why that is and what it means for developers. I also wrote a blog where I show how to use closures in your functions to handle asynchronous data. While you definitely can call loadNext() from inside the observe closure, you may find that you end up with a long sequence of closures inside the same function. I go into this a little more in the blog post. In your specific case, you could do something like this:
func getArray(completion: #escaping ([qtItem]) -> Void) {
let myRef1 = Database.database().reference(withPath: "/tests/\ .
(myTest)")
print(myTest)
myRef1.observe(.value, with: {
snapshot in
print(snapshot)
var newItems: [qItem] = []
for item in snapshot.children {
let mItem = qItem(snapshot: item as! DataSnapshot)
newItems.append(mItem)
print(newItems)
}
self.myArray = newItems.shuffled()
print(self.myArray)
completion(self.myArray)
})
}
Then when you call getArray(), you can handle data from within the closure:
getArray() { newItems in
// do something with newItems, like pass to `loadNext()` if needed
self.loadNext()
}