iOS之NSError

NSError的組成

NSError主要由3部分內容組成：

1.Error Domains：在文件NSError.h中定義，包含：

NSMachErrorDomain
NSPOSIXErrorDomain
NSOSStatusErrorDomain
NSCocoaErrorDomain

2.Error Codes：在以下三個文件中定義

<Foundation/FoundationErrors.h>    Generic Foundation error codes
<AppKit/AppKitErrors.h>            Generic Application Kit error codes
<CoreData/CoreDataErrors.h>        Core Data error codes

3.User Info：這個用戶信息字典包含了NSError本地化后的信息

Error description：
    User info key: NSLocalizedDescriptionKey
    Method: localizedDescription (never returns nil)
Failure reason：
    User info key: NSLocalizedFailureReasonErrorKey 
    Method:localizedFailureReason (can return nil)
Recovery suggestion：
    User info key: NSLocalizedRecoverySuggestionErrorKey
    Method: localizedRecoverySuggestion (can return nil)
Recovery options：
    User info key: NSLocalizedRecoveryOptionsErrorKey
    Method: localizedRecoveryOptions (if returns nil, implies a single “OK button)

 

使用和創建NSError

許多Cocoa和Cocoa Touch方法調用時需要傳入一個引用的NSError對象，example：

NSError *theError;
BOOL success = [myDoc writeToURL:[self docURL] ofType:@"html" error:&theError];
if (success == NO) {
      // Maybe try to determine cause of error and recover first.
      NSAlert *theAlert = [NSAlert alertWithError:theError];
      [theAlert runModal]; // Ignore return value.
}

顯示NSError信息可以直接使用本地化的NSError信息或是更具Error Domain和Error Code來定制Error信息，example：

- (void)webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error {
      NSString *titleString = @"Error Loading Page";
      NSString *messageString = [error localizedDescription];
      NSString *moreString = [error localizedFailureReason] ?
                          [error localizedFailureReason] :
　　　 NSLocalizedString(@"Try typing the URL again.", nil); 
　　　 messageString = [NSString stringWithFormat:@"%@. %@", messageString, moreString];
      UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:titleString
          　　　　　　　　　　　　　　　　　　　　　　　　　　　　  message:messageString delegate:self
          　　　　　　　　　　　　　　　　　　　　　　　cancelButtonTitle:@"Cancel" otherButtonTitles:nil];
      [alertView show];
  }

or

- (void)webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error {
      NSString *errorMsg;
      if ([[error domain] isEqualToString:NSURLErrorDomain]) {
          switch ([error code]) {
              case NSURLErrorCannotFindHost:
                  errorMsg = NSLocalizedString(@"Cannot find specified host. Retype URL.", nil);
                  break;
              case NSURLErrorCannotConnectToHost:
                  errorMsg = NSLocalizedString(@"Cannot connect to specified host. Server may be down.", nil);
         　　　　　break;
              case NSURLErrorNotConnectedToInternet:
                  errorMsg = NSLocalizedString(@"Cannot connect to the internet.Service may not be available.", nil);
                  break;
              default:
                  errorMsg = [error localizedDescription];
                  break;
　　　　　 }
　　   } else {
          errorMsg = [error localizedDescription];
      }
      UIAlertView *av = [[UIAlertView alloc] initWithTitle:NSLocalizedString(@"Error Loading Page", nil)
          　　　　　　　　　　　　　　　　　　　　　　　　 message:errorMsg delegate:self
          　　　　　　　　　　　　　　　　　　 cancelButtonTitle:@"Cancel" otherButtonTitles:nil];
　　　 [av show]; 
}      

創建返回自定義的NSError：

- (NSString *)fooFromPath:(NSString *)path error:(NSError **)anError {
      const char *fileRep = [path fileSystemRepresentation];
      int fd = open(fileRep, O_RDWR|O_NONBLOCK, 0);
      if (fd == -1) {
          if (anError != NULL) { NSString *description;
              NSDictionary *uDict;
              int errCode;
              if (errno == ENOENT) {
　　　　　　　　　　 description = NSLocalizedString(@"No file or directory at requested location", @"");
                  errCode = MyCustomNoFileError;
              } else if (errno == EIO) {
                  // Continue for each possible POSIX error...
              }
              // Create the underlying error.
              NSError *underlyingError = [[NSError alloc] initWithDomain:NSPOSIXErrorDomain
                  　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　code:errno userInfo:nil];
              // Create and return the custom domain error.
              NSDictionary *errorDictionary = @{ NSLocalizedDescriptionKey : description,
                  　　　　　　　　　　　　　　　　　　　　　 NSUnderlyingErrorKey : underlyingError, 
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 NSFilePathErrorKey : path
              *anError = [[NSError alloc] initWithDomain:MyCustomErrorDomain
                      　　　　　　　　　　　　　　　　　　code:errCode userInfo:errorDictionary];
　　　　}
　　　　return nil; 
}
// ...

 

NSError和NSException的區別

iOS官方文檔原文描述如下

Exceptions (represented by NSException objects) are for programming errors, such as an array index that is out of bounds or an invalid method argument. User-level errors (represented by NSError objects) are for runtime errors, such as when a file cannot be found or a string in a certain encoding cannot be read. Conditions giving rise to exceptions are due to programming errors; you should deal with these errors before you ship a product. Runtime errors can always occur, and you should communicate these (via NSError objects) to the user in as much detail as is required.

原文鏈接：https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSError_Class/Reference/Reference.html