无法将类型 'ViewController?' 的值转换为预期的参数类型“_OptionalNilComparisonType”

Question

我正在我的应用程序中实施 MapKit 我已经能够显示用户当前位置。现在，我正在尝试将当前街道名称和街道地址显示到故事板上的标签上，但出现此错误::

Cannot convert value of type 'ViewController?' to expected argument type '_OptionalNilComparisonType' on this line of code

guard let self = self else { return }

下面是我的示例代码：

extension ViewController: MKMapViewDelegate {
func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool) {
    let center = getCenterLocation(for: mapView)
    let geoCoder = CLGeocoder()

    guard let previousLocation = self.previousLocation else { return }
    guard center.distance(from: previousLocation) > 50 else { return }
    self.previousLocation = center

    geoCoder.reverseGeocodeLocation(center) { [weak self] (placemarks, error) in
        guard let self = self else { return }

        if let _ = error {
            //TODO show alert informing the user
            return
        }

        guard let placemark = placemarks?.first else {
            //TODO Show alert informing the user
            return
        }

        let streetNumber = placemark.subThoroughfare ?? ""
        let streetName = placemark.thoroughfare ?? ""

        DispatchQueue.main.async {
            self?.addressLabel.text = "\(streetNumber) \(streetName)"
        }
    }
}
}

Answer 1

更新到 Xcode 10 和 Swift 4.2。那是 Swift 的最早版本，其中表达式

guard let self = self else { return }

是合法的。

如果做不到，请在等号左侧使用其他变量名。例如：

guard let strongself = self else { return }

无法将类型 'ViewController?' 的值转换为预期的参数类型“_OptionalNilComparisonType”

Cannot convert value of type 'ViewController?' to expected argument type '_OptionalNilComparisonType'

ios

swift4.1