全局变量似乎丢失了值(swift)

Question

这是我从Firebase中提取数据的代码。
listofnames(双维(和names(单维(是字符串类型的全局数组。

但是，当我打印listofnames输出窗口中会显示一个空数组，而如果我注释第一行(self.listofnames = [[]](，代码将正常运行。

我已经将引用与Firebase数据库链接在一起

请帮忙。

var databasehandle:FIRDatabaseHandle?
let time = ["8AM - 9AM", "9AM - 10AM","10AM - 11AM", "11AM -12PM", "12PM - 1PM","1PM - 1:30PM","2PM - 3PM","3PM - 4PM","4PM - 5PM","5PM - 6PM","6PM - 7PM","7PM - 7:30PM"]
var ref:FIRDatabaseReference!
var day: String = "Monday"
var users :String = "First years"
func loaddata()
{
self.listofnames = [[]]
for x in 0...((time.count)-1)
{
ref.child(users).child(day).child(time[x]).observeSingleEvent(of: .value, with: {(snapshot) -> Void in
if let namesfromFirebase = snapshot.value as? [String:Any]
{
let y = namesfromFirebase.count
for i in 1...y
{
self.names.append(namesfromFirebase["s"+"(i)"] as! String)
}
}
self.listofnames.append(self.names)
self.names = []
})
}
print(self.listofnames)
self.tableView.reloadData()
}

Answer 1

>observeSingleEvent异步工作，闭包稍后调用。

有一个简单的解决方案：将两行移动到闭合中：

func loaddata()
{
self.listofnames = [[]]
for x in 0..<time.count // no parentheses and math needed when using `..<`
{
ref.child(users).child(day).child(time[x]).observeSingleEvent(of: .value, with: {(snapshot) -> Void in
if let namesfromFirebase = snapshot.value as? [String:Any]
{
let y = namesfromFirebase.count
for i in 1...y
{
self.names.append(namesfromFirebase["s"+"(i)"] as! String)
}
}
self.listofnames.append(self.names)
self.names = []
print(self.listofnames)
self.tableView.reloadData()
})
}
}