IOS Error Domain=NSURLErrorDomain Code=-999 "(null)"

Printing description of error: Error Domain=NSURLErrorDomain Code=-999 "(null)" 

出现错误的原因是因为webview在之前的请求还没有加载完成，下一个请求开始发起了，因此webview会取消掉之前的请求，便会回调到didFailLoadWithError。

- (void)webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error
{
    // 被取消
    if([error code] == NSURLErrorCancelled)  {
        return;
    }
}

/*!
    @enum NSURL-related Error Codes
    @abstract Constants used by NSError to indicate errors in the NSURL domain
*/
NS_ENUM(NSInteger)
{
    NSURLErrorUnknown = 			-1,
    NSURLErrorCancelled = 			-999,
    NSURLErrorBadURL = 				-1000,
    NSURLErrorTimedOut = 			-1001,
    NSURLErrorUnsupportedURL = 			-1002,
    NSURLErrorCannotFindHost = 			-1003,
    NSURLErrorCannotConnectToHost = 		-1004,
    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,
    NSURLErrorAppTransportSecurityRequiresSecureConnection NS_ENUM_AVAILABLE(10_11, 9_0) = -1022,
    NSURLErrorFileDoesNotExist = 		-1100,
    NSURLErrorFileIsDirectory = 		-1101,
    NSURLErrorNoPermissionsToReadFile = 	-1102,
    NSURLErrorDataLengthExceedsMaximum NS_ENUM_AVAILABLE(10_5, 2_0) =	-1103,
    
    // SSL errors
    NSURLErrorSecureConnectionFailed = 		-1200,
    NSURLErrorServerCertificateHasBadDate = 	-1201,
    NSURLErrorServerCertificateUntrusted = 	-1202,
    NSURLErrorServerCertificateHasUnknownRoot = -1203,
    NSURLErrorServerCertificateNotYetValid = 	-1204,
    NSURLErrorClientCertificateRejected = 	-1205,
    NSURLErrorClientCertificateRequired =	-1206,
    NSURLErrorCannotLoadFromNetwork = 		-2000,
    
    // Download and file I/O errors
    NSURLErrorCannotCreateFile = 		-3000,
    NSURLErrorCannotOpenFile = 			-3001,
    NSURLErrorCannotCloseFile = 		-3002,
    NSURLErrorCannotWriteToFile = 		-3003,
    NSURLErrorCannotRemoveFile = 		-3004,
    NSURLErrorCannotMoveFile = 			-3005,
    NSURLErrorDownloadDecodingFailedMidStream = -3006,
    NSURLErrorDownloadDecodingFailedToComplete =-3007,

    NSURLErrorInternationalRoamingOff NS_ENUM_AVAILABLE(10_7, 3_0) =         -1018,
    NSURLErrorCallIsActive NS_ENUM_AVAILABLE(10_7, 3_0) =                    -1019,
    NSURLErrorDataNotAllowed NS_ENUM_AVAILABLE(10_7, 3_0) =                  -1020,
    NSURLErrorRequestBodyStreamExhausted NS_ENUM_AVAILABLE(10_7, 3_0) =      -1021,
    
    NSURLErrorBackgroundSessionRequiresSharedContainer NS_ENUM_AVAILABLE(10_10, 8_0) = -995,
    NSURLErrorBackgroundSessionInUseByAnotherProcess NS_ENUM_AVAILABLE(10_10, 8_0) = -996,
    NSURLErrorBackgroundSessionWasDisconnected NS_ENUM_AVAILABLE(10_10, 8_0)= -997,
};