如何正确等待 swift 中的功能完成？

Question

我现在已经尝试了很多方法，但 none 似乎有效。

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
                    })
}

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)
    }
}

但是我从来没有设法在单元格中显示正确的位置，它总是显示为空 space 因为它不知何故不等待完成处理程序完成转换。我在这里做错了什么？

Answer 1

发生这种情况是因为 reverseGeocodeLocation returns 立即并且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，鉴于这种情况，您需要相应地处理错误情况。更好的是，使用 defer，这样无论发生什么，都会调用完成：

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
    }
})

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