ios - Swift 从嵌套在函数中的闭包中抛出

Question

我有一个抛出错误的函数，在这个函数中，我有一个 inside a 闭包，我需要从它的完成处理程序中抛出错误。这可能吗？

到目前为止，这是我的代码。

enum CalendarEventError: ErrorType {
    case UnAuthorized
    case AccessDenied
    case Failed
}

func insertEventToDefaultCalendar(event :EKEvent) throws {
    let eventStore = EKEventStore()
    switch EKEventStore.authorizationStatusForEntityType(.Event) {
    case .Authorized:
        do {
            try insertEvent(eventStore, event: event)
        } catch {
            throw CalendarEventError.Failed
        }

    case .Denied:
        throw CalendarEventError.AccessDenied

    case .NotDetermined:
        eventStore.requestAccessToEntityType(EKEntityType.Event, completion: { (granted, error) -> Void in
            if granted {
                //insertEvent(eventStore)
            } else {
                //throw CalendarEventError.AccessDenied
            }
        })
    default:
    }
}

Answer 1

当你定义抛出的闭包时:

enum MyError: ErrorType {
    case Failed
}

let closure = {
    throw MyError.Failed
}

那么这个闭包的类型是() throws -> () 并且以这个闭包为参数的函数必须有相同的参数类型:

func myFunction(completion: () throws -> ()) {
}

这个函数你可以同步调用completion闭包:

func myFunction(completion: () throws -> ()) throws {
    completion() 
}

并且您必须将 throws 关键字添加到函数签名或使用 try! 完成调用:

func myFunction(completion: () throws -> ()) {
    try! completion() 
}

或异步:

func myFunction(completion: () throws -> ()) {
    dispatch_async(dispatch_get_main_queue(), { try! completion() })
}

在最后一种情况下，您将无法捕获错误。

因此，如果 eventStore.requestAccessToEntityType 方法中的 completion 闭包，并且该方法本身的签名中没有 throws 或者如果 completion 被异步调用，那么你不能从这个闭包中throw。

我建议您将错误传递给回调而不是抛出错误的函数的以下实现:

func insertEventToDefaultCalendar(event: EKEvent, completion: CalendarEventError? -> ()) {
    let eventStore = EKEventStore()
    switch EKEventStore.authorizationStatusForEntityType(.Event) {
    case .Authorized:
        do {
            try insertEvent(eventStore, event: event)
        } catch {
            completion(CalendarEventError.Failed)
        }

    case .Denied:
        completion(CalendarEventError.AccessDenied)

    case .NotDetermined:
        eventStore.requestAccessToEntityType(EKEntityType.Event, completion: { (granted, error) -> Void in
            if granted {
                //insertEvent(eventStore)
            } else {
                completion(CalendarEventError.AccessDenied)
            }
        })
    default:
    }
}