如何找到一种方法可能在Swift中抛出并捕获的错误 [英] How to find the kind of errors a method may throw and catch them in Swift

问题描述

我正在使用 NSFileManager.contentsOfDirectoryAtPath 来获取目录中的文件名数组。我想使用新的 do-try-catch 语法来处理错误：

I am using NSFileManager.contentsOfDirectoryAtPath to get an array of file names in a directory. I want to use the new do-try-catch syntax to handle the errors:

do {

    let docsArray = try fileManager.contentsOfDirectoryAtPath(docsPath)

} catch {

    // handle errors
    print(error) // this is the best I can currently do

}

我可以成像一个错误可能是 docsPath 不存在，但我不知道如何捕获这个错误。我不知道可能发生什么其他错误。

I can imaging that an error might be that the docsPath doesn't exist, but I don't know how to catch this error. And I don't know what other possible errors might occur.

错误处理文档具有一个这样的例子

enum VendingMachineError: ErrorType {
    case InvalidSelection
    case InsufficientFunds(centsNeeded: Int)
    case OutOfStock
}

和

do {
    try vend(itemNamed: "Candy Bar")
    // Enjoy delicious snack
} catch VendingMachineError.InvalidSelection {
    print("Invalid Selection.")
} catch VendingMachineError.OutOfStock {
    print("Out of Stock.")
} catch VendingMachineError.InsufficientFunds(let amountNeeded) {
    print("Insufficient funds. Please insert an additional \(amountNeeded) cents.")
}

但我不知道如何做类似的事情用于捕捉使用 throws 关键字的方法的标准Swift类型的错误。

but I don't know how to do something similar for catching the errors of the standard Swift types that have methods using the throws keyword.

NSFileManager contentsOfDirectoryAtPath 的类引用不会说可能返回什么样的错误。所以我不知道要收到什么错误或如何处理它们。

The NSFileManager class reference for contentsOfDirectoryAtPath doesn't say what kind of errors might be returned. So I don't know what errors to catch or how to handle them if I get them.

我想做这样的事情：

do {
    let docsArray = try fileManager.contentsOfDirectoryAtPath(docsPath)
} catch FileManagerError.PathNotFound {
    print("The path you selected does not exist.")
} catch FileManagerError.PermissionDenied {
    print("You do not have permission to access this directory.")
} catch ErrorType {
    print("An error occured.")
}

推荐答案

NSError 自动桥接到 ErrorType 域变为类型（例如 NSCocoaErrorDomain 变为 NSCocoaError ），错误代码变为值（ NSFileReadNoSuchFileError 成为 .FileReadNoSuchFileError ）

NSError automatically bridges to ErrorType where the domain becomes the type (e.g. NSCocoaErrorDomain becomes NSCocoaError) and the error code becomes the value (NSFileReadNoSuchFileError becomes .FileReadNoSuchFileError)

import Foundation

let docsPath = "/file/not/found"
let fileManager = NSFileManager()

do {
    let docsArray = try fileManager.contentsOfDirectoryAtPath(docsPath)
} catch NSCocoaError.FileReadNoSuchFileError {
    print("No such file")
} catch {
    // other errors
    print(error)
}

至于知道特定调用可以返回哪个错误，只有文档可以帮助。几乎所有的基础错误都是 NSCocoaError 域的一部分，可以在 FoundationErrors.h 中找到（尽管有一些罕见的基金会有时也可能在 NSPOSIXErrorDomain 下返回POSIX错误的错误），但这些错误可能未完全桥接，所以您必须回到管理他们的 NSError 级别。

As for knowing which error can be returned by a specific call, only the documentation can help. Almost all Foundation errors are part of the NSCocoaError domain and can be found in FoundationErrors.h (though there are some rare bugs where Foundation can also sometimes return POSIX errors under NSPOSIXErrorDomain) but these ones might not have been fully bridged so you will have to fall back on managing them at the NSError level.

更多信息可以在«使用Swift和Cocoa和Objective-C（Swift 2.2）»

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