How does control flow work when retrieving Information from Firebase? - ios

Question

var ergebnisBluetezeit = Set<String>()
let refBluetezeit = rootRef.child("Pflanzen").child("Eigenschaften").child("Blütezeit")
refBluetezeit.child("Februar").observeSingleEvent(of: .value, with: { snapshot in
for plant in snapshot.children {
self.ergebnisBluetezeit.insert((plant as AnyObject).value)
}
})
print(ergebnisBluetezeit)
I want to retrieve Data from my Firebase Database. The Retrieving Process does work already, but the following confuses me: the current output from the print is an empty set, but when i use the var ergebnisBluetezeit elsewhere (for example setup a button, which action is to print ergebnisBluetezeit), it is filled. When i put the print in the for loop, it does print the right output, too.
I seem to not have understood the control flow here, so my Question:
How can i use the Set where the print statement is at the moment?
Thanks for your help.

Answer 1

It's the logic of asynchronous calls
print("1") // empty
refBluetezeit.child("Februar").observeSingleEvent(of: .value, with: { snapshot in
print("3") // empty
for plant in snapshot.children {
self.ergebnisBluetezeit.insert((plant as AnyObject).value)
}
print(ergebnisBluetezeit) // not empty
})
print("2") // empty
the value is empty until the request finishes regardless of where in code ordering you run the print , as the numbering above in order 1 , 2 , 3 to know when it finishes you can use completions like
func getData(completion:#escaping() -> ()) {
let refBluetezeit = rootRef.child("Pflanzen").child("Eigenschaften").child("Blütezeit")
refBluetezeit.child("Februar").observeSingleEvent(of: .value, with: { snapshot in
for plant in snapshot.children {
self.ergebnisBluetezeit.insert((plant as AnyObject).value)
}
completion()
})
}
And call
getData {
print(ergebnisBluetezeit)
}