Error协议是用来进行错误处理的。
源码如下：
public protocol Error {
}
extension Error {
//错误的本地化描述
public var localizedDescription: String { get }
}
extension Error where Self.RawValue : SignedInteger {
}
extension Error where Self.RawValue : UnsignedInteger {
}
可以看到，标准库中只提供了一个localizedDescription只读属性，用来描述错误的具体信息。除此之外，并没有其他的方法。
Apple官方推荐使用枚举类型，具体说明如下。
Swift's enumerations are well suited to represent simple errors. Create an enumeration that conforms to the Error protocol with a case for each possible error. If there are additional details about the error that could be helpful for recovery, use associated values to include that information.
关于错误处理，我自己对这方面的了解的也不够深。这里我给大家介绍一下Alamofire的AFError类，间接的来了解一下Error协议的用法。
定义
public enum AFError: Error {
public enum ParameterEncodingFailureReason {
case missingURL
case jsonEncodingFailed(error: Error)
case propertyListEncodingFailed(error: Error)
}
public enum MultipartEncodingFailureReason {
case bodyPartURLInvalid(url: URL)
case bodyPartFilenameInvalid(in: URL)
case bodyPartFileNotReachable(at: URL)
. . .
}
public enum ResponseValidationFailureReason {
case dataFileNil
case dataFileReadFailed(at: URL)
case missingContentType(acceptableContentTypes: [String])
. . .
}
public enum ResponseSerializationFailureReason {
case inputDataNil
case inputDataNilOrZeroLength
case inputFileNil
. . .
}
case invalidURL(url: URLConvertible)
case parameterEncodingFailed(reason: ParameterEncodingFailureReason)
case multipartEncodingFailed(reason: MultipartEncodingFailureReason)
case responseValidationFailed(reason: ResponseValidationFailureReason)
case responseSerializationFailed(reason: ResponseSerializationFailureReason)
}
每个case分别对应一种错误类型，并关联一个Reason。
嵌套4个Reason枚举，来对错误的原因再次进行分类。不得不说代码很有逻辑感，也写的很好看。
错误描述
extension AFError.ParameterEncodingFailureReason {
var localizedDescription: String {
switch self {
case .missingURL:
return "URL request to encode was missing a URL"
case .jsonEncodingFailed(let error):
return "JSON could not be encoded because of error:\n\(error.localizedDescription)"
case .propertyListEncodingFailed(let error):
return "PropertyList could not be encoded because of error:\n\(error.localizedDescription)"
}
}
}
extension AFError.MultipartEncodingFailureReason {
var localizedDescription: String {
switch self {
case .bodyPartURLInvalid(let url):
return "The URL provided is not a file URL: \(url)"
case .bodyPartFilenameInvalid(let url):
return "The URL provided does not have a valid filename: \(url)"
case .bodyPartFileNotReachable(let url):
return "The URL provided is not reachable: \(url)"
. . .
}
}
}
extension AFError.ResponseSerializationFailureReason {
var localizedDescription: String {
switch self {
case .inputDataNil:
return "Response could not be serialized, input data was nil."
case .inputDataNilOrZeroLength:
return "Response could not be serialized, input data was nil or zero length."
case .inputFileNil:
return "Response could not be serialized, input file was nil."
. . .
}
}
}
extension AFError.ResponseValidationFailureReason {
var localizedDescription: String {
switch self {
case .dataFileNil:
return "Response could not be validated, data file was nil."
case .dataFileReadFailed(let url):
return "Response could not be validated, data file could not be read: \(url)."
case .missingContentType(let types):
return (
"Response Content-Type was missing and acceptable content types " +
"(\(types.joined(separator: ","))) do not match \"*/*\"."
)
. . .
}
}
}
使用localizedDescription属性，对每一个Reason的分支，进行详细的描述。
当然了，AFError并不是只有这些内容，还有很多方便我们使用的扩展等，由于它并不是我这篇文章的主要讨论方向，就不做更多介绍了。 有兴趣的可以去看看下面这个参考链接。
http://www.jianshu.com/p/99e6ba32f244
