问题
I am trying to get a list of 'players' from Firebase from the child node 'PlayerInPool' by calling the following functions
func getPlayersInPool(completion: @escaping (_ success: Bool) -> Void) {
self.handle = self.poolPlayersRef.child(self.pID)observe(.value, with: { snapshot in
// Calling the second function
self.loadPlayersInPool(/*items: items,*/ snapshot: snapshot) { (success) -> Void in
if success {
print("Players Array Count = ", self.players.count)
for j in 0 ..< self.players.count {
print(self.players[j].description)
}
//
//self.actionController.reloadData()
//
completion(true)
}
}
}) { (error) in
print(error.localizedDescription)
}
}
Second function called from the above function:
func loadPlayersInPool(/*items: [String],*/ snapshot: DataSnapshot, completion: @escaping (_ success: Bool) -> Void) {
var items: [String] = []
for item in snapshot.children {
let user = (item as! DataSnapshot).key
self.userRef.child(user).child("username").observeSingleEvent(of: .value, with: { (snapshot) in
let username = snapshot.value
items.append(username! as! String)
})
}
self.players = items
// ISSUE HERE THE PLAYER.COUNT IS ALWAYS ZERO (0)!!!!!!!!!
for j in 0 ..< players.count {
print(players[j]description)
}
completion(true)
}
The issue is that the player array count is always zero (0)!? So I'm not sure if I am using the completion handler correctly?
回答1:
The observeSingleEvent call loads data from the server, so happens asynchronously. This means that your items.append(username! as! String) runs after your call to completion(true).
A simple way to get the behavior you want, is to check if you've loaded all user names by using a simple counter:
func loadPlayersInPool(/*items: [String],*/ snapshot: DataSnapshot, completion: @escaping (_ success: Bool) -> Void) {
var items: [String] = []
let loadedCount = 0
for item in snapshot.children {
let user = (item as! DataSnapshot).key
self.userRef.child(user).child("username").observeSingleEvent(of: .value, with: { (snapshot) in
let username = snapshot.value
items.append(username! as! String)
loadedCount++
if loadedCount == snapshot.childrenCount {
self.players = items
for j in 0 ..< players.count {
print(players[j]description)
}
completion(true)
}
})
}
}
来源：https://stackoverflow.com/questions/52972911/swift-completion-handler-with-firebase