如何正确地等待函数在swift中完成?
我现在尝试了很多东西,但似乎都不管用 我有一个for循环,它解析一些数据并将坐标转换为ZIP字符串:如何正确地等待函数在swift中完成?,swift,closures,completionhandler,Swift,Closures,Completionhandler,我现在尝试了很多东西,但似乎都不管用 我有一个for循环,它解析一些数据并将坐标转换为ZIP字符串: for i in 0 ... results.count - 1 { result = results[i] self.coordinateToString(lat: result.lat, long: result.long, completion: { (place) in someCell.label.text = place
for i in 0 ... results.count - 1
{
result = results[i]
self.coordinateToString(lat: result.lat, long: result.long, completion: { (place) in
someCell.label.text = place
})
}
func coordinateToString(lat: Double, long: Double, completion: @escaping (String) -> ()) {
let geoCoder = CLGeocoder()
let location = CLLocation(latitude: lat, longitude: long)
var ret = ""
geoCoder.reverseGeocodeLocation(location, completionHandler:
{
placemarks, error -> Void in
guard let placeMark = placemarks?.first else { return }
if let zip = placeMark.postalCode, let town = placeMark.subAdministrativeArea
{
let toAppend = "\(zip)" + " \(town)"
ret = toAppend
}
})
DispatchQueue.main.async {
completion(ret)
}
}
然而,我从来没有在单元格中显示正确的位置,它总是显示空的空间,因为它不会等待完成处理程序完成转换。我在这里做错了什么?发生这种情况是因为
reverseGeocodeLocation
立即返回,其完成处理程序随后运行。这意味着ret
值在放入主队列时可能为空。您应该从回调中分派到main,如下所示:
func coordinateToString(lat: Double, long: Double, completion: @escaping (String) -> ()) {
let geoCoder = CLGeocoder()
let location = CLLocation(latitude: lat, longitude: long)
var ret = ""
geoCoder.reverseGeocodeLocation(location, completionHandler:
{
placemarks, error -> Void in
guard let placeMark = placemarks?.first else { return }
if let zip = placeMark.postalCode, let town = placeMark.subAdministrativeArea
{
let toAppend = "\(zip)" + " \(town)"
ret = toAppend
DispatchQueue.main.async {
completion(ret)
}
}
})
当然,在这种情况下,您需要相应地处理错误案例。更好的方法是使用延迟,这样无论发生什么情况都可以调用completion:
func coordinateToString(lat: Double, long: Double, completion: @escaping (String) -> ()) {
let geoCoder = CLGeocoder()
let location = CLLocation(latitude: lat, longitude: long)
var ret = ""
geoCoder.reverseGeocodeLocation(location, completionHandler:
{
defer {
DispatchQueue.main.async {
completion(ret)
}
}
placemarks, error -> Void in
guard let placeMark = placemarks?.first else { return }
if let zip = placeMark.postalCode, let town = placeMark.subAdministrativeArea
{
let toAppend = "\(zip)" + " \(town)"
ret = toAppend
}
})
您对DispatchQueue.main.async{completion(ret)}
的调用位于错误的位置。哇,请阅读一些有关如何编码的swift约定,这是不可读的