如果let不解开MKAnnotation的title属性的可选值 [英] if let doesn't unwrap optional value for MKAnnotation's title property

问题描述

我想用if-let语句解开可选值. 我需要获得MKAnnotation的标题.

I want to unwrap optional value with if-let statement. I need to get title of MKAnnotation.

func mapView(_ mapView: MKMapView, didSelect view: MKAnnotationView) {
    if let title = view.annotation?.title {
        print(title) //Optional("Moscow")
    }
}

为什么if-let在这里不起作用?

Why if-let doesn't work here?

推荐答案

MKAnnotation.title的类型为String??，它是嵌套的Optional，因此您需要可选地将其绑定两次.

The type of MKAnnotation.title is String??, it's a nested Optional, so you need to optional bind it twice.

func mapView(_ mapView: MKMapView, didSelect view: MKAnnotationView) {
    if let optionalTitle = view.annotation?.title, let title = optionalTitle {
        print(title)
    }
}

即使根据 MKAnnotation.title 的文档，标题的类型应为String?，因为title被声明为不需要的协议属性:

Even though according to the documentation of MKAnnotation.title, the type of title should be String?, since title is declared as a non-required protocol property:

optional var title: String? { get }

通过MKAnnotation协议类型而不是通过实现协议的具体类型访问

时，它将被包裹在另一个Optional中，这表示title属性甚至可能无法通过具体类型实现来实现协议.因此，当访问MKAnnotation对象的title属性而不是具体类型符合MKAnnotation的对象时，title的类型将为String??.

when accessed through the MKAnnotation protocol type rather than the concrete type implementing the protocol, it becomes wrapped in another Optional, which represents the fact that the title property might not even be implemented by the concrete type implementing the protocol. Hence, when accessing the title property of an MKAnnotation object rather than an object with a concrete type conforming to MKAnnotation, the type of title will be String??.

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