一、NSError是系統錯誤信息類

//兩種初始化方法：其中，domain 不能為空 dict可以為空
- (instancetype)initWithDomain:(NSErrorDomain)domain code:(NSInteger)code userInfo:(nullable NSDictionary *)dict;

+ (instancetype)errorWithDomain:(NSErrorDomain)domain code:(NSInteger)code userInfo:(nullable NSDictionary *)dict;

@property (readonly, copy) NSErrorDomain domain;//錯誤域

錯誤域主要有四個，對於Carbon框架的Error，歸於OSStatus domain（NSOSStatusErrorDomain），對於POSIX error，歸於NSPOSIXErrorDomain，而對於我們的iOS開發，一般使用NSCocoaErrorDomain。NSError.h定義了四個domain

{

// Predefined domain for errors from most AppKit and Foundation APIs.
FOUNDATION_EXPORT NSErrorDomain const NSCocoaErrorDomain;

// Other predefined domains; value of "code" will correspond to preexisting values in these domains.
FOUNDATION_EXPORT NSErrorDomain const NSPOSIXErrorDomain;
FOUNDATION_EXPORT NSErrorDomain const NSOSStatusErrorDomain;
FOUNDATION_EXPORT NSErrorDomain const NSMachErrorDomain;

}

@property (readonly) NSInteger code;//錯誤代碼

@property (readonly, copy) NSDictionary *userInfo;//錯誤信息


@property (readonly, copy) NSString *localizedDescription;//獲取本地化描述


@property (nullable, readonly, copy) NSString *localizedFailureReason;//獲取失敗原因


@property (nullable, readonly, copy) NSString *localizedRecoverySuggestion;//獲取恢復建議


@property (nullable, readonly, copy) NSArray *localizedRecoveryOptions;本地恢復建議


@property (nullable, readonly, strong) id recoveryAttempter;


@property (nullable, readonly, copy) NSString *helpAnchor;

預定義的userinfo鍵名

NSString *const NSUnderlyingErrorKey;//推薦的標准方式，通用鍵

NSString *const NSLocalizedDescriptionKey; // 詳細描述鍵
NSString *const NSLocalizedFailureReasonErrorKey; // 失敗原因鍵
NSString *const NSLocalizedRecoverySuggestionErrorKey; //恢復建議鍵
NSString *const NSLocalizedRecoveryOptionsErrorKey; // 恢復選項鍵

//其他鍵

NSString *const NSRecoveryAttempterErrorKey;
NSString *const NSHelpAnchorErrorKey;

NSString *const NSStringEncodingErrorKey ;
NSString *const NSURLErrorKey;
NSString *const NSFilePathErrorKey;

用法示例：

NSDictionary *userInfo1 = [NSDictionary dictionaryWithObjectsAndKeys:@"由於文件不存在，無法打開", NSLocalizedDescriptionKey, @"失敗原因：文件不存在", NSLocalizedFailureReasonErrorKey, @"恢復建議：請創建該文件",NSLocalizedRecoverySuggestionErrorKey,nil];

NSError *error = [[NSError alloc] initWithDomain:NSCocoaErrorDomain code:4 userInfo:userInfo1];//此處code是4，對照下面對照表4代表文件不存在。userinfo傳userinfo1 查看自定義打印。userinfo傳nil，查看本地化描述。

NSLog(@"========%@",[error localizedDescription]);
NSLog(@"========%@",[error localizedFailureReason]);
NSLog(@"========%@",[error localizedRecoverySuggestion]);

二、NSError錯誤code對照表

Foundation Constants Reference
NSError Codes

NSError codes in the Cocoa error domain.

enum {
NSFileNoSuchFileError = 4,
NSFileLockingError = 255,
NSFileReadUnknownError = 256,
NSFileReadNoPermissionError = 257,
NSFileReadInvalidFileNameError = 258,
NSFileReadCorruptFileError = 259,
NSFileReadNoSuchFileError = 260,
NSFileReadInapplicableStringEncodingError = 261,
NSFileReadUnsupportedSchemeError = 262,
NSFileReadTooLargeError = 263,
NSFileReadUnknownStringEncodingError = 264,
NSFileWriteUnknownError = 512,
NSFileWriteNoPermissionError = 513,
NSFileWriteInvalidFileNameError = 514,
NSFileWriteInapplicableStringEncodingError = 517,
NSFileWriteUnsupportedSchemeError = 518,
NSFileWriteOutOfSpaceError = 640,
NSFileWriteVolumeReadOnlyError = 642m
NSKeyValueValidationError = 1024,
NSFormattingError = 2048,
NSUserCancelledError = 3072,

NSFileErrorMinimum = 0,
NSFileErrorMaximum = 1023,
NSValidationErrorMinimum = 1024,
NSValidationErrorMaximum = 2047,
NSFormattingErrorMinimum = 2048,
NSFormattingErrorMaximum = 2559,

NSPropertyListReadCorruptError = 3840,
NSPropertyListReadUnknownVersionError = 3841,
NSPropertyListReadStreamError = 3842,
NSPropertyListWriteStreamError = 3851,
NSPropertyListErrorMinimum = 3840,
NSPropertyListErrorMaximum = 4095

NSExecutableErrorMinimum = 3584,
NSExecutableNotLoadableError = 3584,
NSExecutableArchitectureMismatchError = 3585,
NSExecutableRuntimeMismatchError = 3586,
NSExecutableLoadError = 3587,
NSExecutableLinkError = 3588,
NSExecutableErrorMaximum = 3839,

}
URL Loading System Error Codes

These values are returned as the error code property of an NSError object with the domain “NSURLErrorDomain”.

typedef enum
{
NSURLErrorUnknown = -1,
NSURLErrorCancelled = -999,
NSURLErrorBadURL = -1000,
NSURLErrorTimedOut = -1001,
NSURLErrorUnsupportedURL = -1002,
NSURLErrorCannotFindHost = -1003,
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,
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
}

三、自定義NSError

系統的NSError是可以自己定制的，以下提供代碼來實現並表示如何使用：

YXError.h 與YXError.m

//
//  YXError.h
//  CustomYXError
//
//  Copyright (c) 2014年 Y.X. All rights reserved.
//

#import 

// 自行添加錯誤碼
typedef enum : NSUInteger {
  Crash		= -10000,
  DisConnect,
  Unknow,
} YXErrorCode;

@interface YXError : NSObject

+ (NSError *)errorCode:(YXErrorCode)code userInfo:(NSDictionary *)dic;
+ (NSString *)transformCodeToStringInfo:(YXErrorCode)code;

@end

//
//  YXError.m
//  CustomYXError
//
//  Copyright (c) 2014年 Y.X. All rights reserved.
//

#import "YXError.h"

static NSDictionary *errorDictionary = nil;

@implementation YXError

+ (void)initialize
{
    if (self == [YXError class])
    {
        errorDictionary = \
        @{
          /* code        :        errorWithDomain */
          /* ==================================== */
          
          @(Crash)       :        @"Crash",
          @(DisConnect)  :        @"DisConnect",
          @(Unknow)      :        @"Unknow",
          
          /* ==================================== */
          };
    }
}

+ (NSError *)errorCode:(YXErrorCode)code userInfo:(NSDictionary *)dic
{
    return [NSError errorWithDomain:errorDictionary[@(code)]
                               code:code
                           userInfo:dic];
}

+ (NSString *)transformCodeToStringInfo:(YXErrorCode)code
{
    return errorDictionary[@(code)];
}

@end

以下是使用的代碼：

//
//  AppDelegate.m
//  NSError
//
//  Copyright (c) 2014年 YouXianMing. All rights reserved.
//

#import "AppDelegate.h"
#import "YXError.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // 定制一個NSError
    NSError *error = [YXError errorCode:Crash
      userInfo:nil];
    NSLog(@"%@", error);
    
    // 翻譯一個NSError（將code值翻譯成字符串）
    NSLog(@"%@", [YXError transformCodeToStringInfo:-10000]);
    
    return YES;
}

@end