自定义 NSError 代码

Question

我正在构建一个 Cocoa Touch 框架，它使用自定义 NSError 对象将错误传达给调用者。我需要声明一个定义自定义错误代码的枚举。我去了这个Apple 文档，其中列出了如下所示的 NSURLErrorDomain 错误代码：

enum {    
   NSURLErrorUnknown = -1,   
   NSURLErrorCancelled = -999,   
   NSURLErrorBadURL = -1000,    
   NSURLErrorTimedOut = -1001,   
   NSURLErrorUnsupportedURL = -1002,
   NSURLErrorUnsupportedURL = -1002,
   NSURLErrorCannotConnectToHost = -1004,   
   NSURLErrorDataLengthExceedsMaximum = -1103,   
   NSURLErrorNetworkConnectionLost = -1005,    
   NSURLErrorDNSLookupFailed = -1006,   
   NSURLErrorHTTPTooManyRedirects = -1007,    
   NSURLErrorResourceUnavailable = -1008,   
   NSURLErrorNotConnectedToInternet = -1009,   
   NSURLErrorRedirectToNonExistentLocation = -1010,   
   NSURLErrorBadServerResponse = -1011,   
   NSURLErrorUserCancelledAuthentication = -1012,   
   NSURLErrorUserAuthenticationRequired = -1013,   
   NSURLErrorZeroByteResource = -1014,   
   NSURLErrorCannotDecodeRawData = -1015,    
   NSURLErrorCannotDecodeContentData = -1016,    
   NSURLErrorCannotParseResponse = -1017,   
   NSURLErrorInternationalRoamingOff = -1018,    
   NSURLErrorCallIsActive = -1019,    
   NSURLErrorDataNotAllowed = -1020,    
   NSURLErrorRequestBodyStreamExhausted = -1021,   
   NSURLErrorFileDoesNotExist = -1100,    
   NSURLErrorFileIsDirectory = -1101,    
   NSURLErrorNoPermissionsToReadFile = -1102,    
   NSURLErrorSecureConnectionFailed = -1200,   
   NSURLErrorServerCertificateHasBadDate = -1201,   
   NSURLErrorServerCertificateUntrusted = -1202,   
   NSURLErrorServerCertificateHasUnknownRoot = -1203,   
   NSURLErrorServerCertificateNotYetValid = -1204,  
   NSURLErrorClientCertificateRejected = -1205,   
   NSURLErrorClientCertificateRequired = -1206,   
   NSURLErrorCannotLoadFromNetwork = -2000,    
   NSURLErrorCannotCreateFile= -3000,   
   NSURLErrorCannotOpenFile = -3001,   
   NSURLErrorCannotCloseFile = -3002,    
   NSURLErrorCannotWriteToFile = -3003,    
   NSURLErrorCannotRemoveFile = -3004,    
   NSURLErrorCannotMoveFile = -3005,   
   NSURLErrorDownloadDecodingFailedMidStream = -3006,   
   NSURLErrorDownloadDecodingFailedToComplete = -3007 
   }

Q1：为什么 Apple 对错误代码使用负值？有什么具体原因吗？

Q2：Apple 是否遵循任何模式来随机化错误代码？

Answer 1

1. 因为根据返回码，0 通常是可以的。这留下了正整数和负整数。它是一个通用的 C 偏好，使用正整数作为成功代码（带有附加信息）和负值作为错误代码。
2. 如您所见，错误代码已分组。Unknown 和 cancelled 是特殊的，1000s 是 HTTP 错误，1100s 是访问错误，1200s 是连接和证书错误，2000 网络，3000s 文件和流问题。

根据问题区域对错误进行分组，并将否定用于返回代码目的（因为返回否定通常会被解释为错误）。