如何处理 NSFetchedResultsController 获取错误？

Question

这是来自 Apple 示例代码：

if (![fetchedResultsController_ performFetch:&error]) {
     /*
      Replace this implementation with code to handle the error appropriately.
      ...
      If it is not possible to recover from the error, ...
      */
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

我想知道是否真的有必要总是终止应用程序？你怎么能“用代码替换这个实现以适当地处理错误”？您将如何“从错误中恢复”？

任何建议都将不胜感激，费边

Answer 1

好吧，显然没有人有其他（更好的？）解决方案，所以这是我的方法：

在我的 AppController 中，我添加了一个实例变量 errorString 和这个方法：

- (void)presentCoreDataError:(NSError *)error
                    withText:(NSString *)text
{
    NSMutableString *localErrorString = [[NSMutableString alloc] init];

    [localErrorString appendFormat:@"Failed to %@: %@", text, [error localizedDescription]];

    NSArray* detailedErrors = [[error userInfo] objectForKey:NSDetailedErrorsKey];
    if(detailedErrors != nil && [detailedErrors count] > 0) {
        for(NSError* detailedError in detailedErrors) {
            [localErrorString appendFormat:@"- Detail: %@", [detailedError userInfo]];
        }
    } else {
        [localErrorString appendFormat:@"- %@", [error userInfo]];
    }

    UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:[NSString stringWithFormat:@"Failed to %@", text]
                                                     message:@"Please send a report to the developer."
                                                    delegate:self
                                           cancelButtonTitle:@"Cancel"
                                           otherButtonTitles:@"Send Report", nil] autorelease];
    [alert show];

    self.errorString = localErrorString;
    [localErrorString release];
}

如果轻按“发送报告”，UIAlertView 代表将显示带有errorString 的MFMailComposeViewController 和酷信使字体:)。否则它会调用abort():

- (void) alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    if (buttonIndex == 1) {     // Send Report
        MFMailComposeViewController *picker = [[MFMailComposeViewController alloc] init];
        picker.mailComposeDelegate = self;
        NSArray *toRecipients = [NSArray arrayWithObject:@"first@example.com"];
        [picker setToRecipients:toRecipients];
        [picker setSubject:@"Error Report"];
        [picker setMessageBody:[NSString stringWithFormat:@"The application crashed with the following error:<br><br><FONT FACE=%@> %@ </FONT>", 
                                @"courier", errorString] 
                        isHTML:YES];

        [navigationController presentModalViewController:picker animated:YES];
        [picker release];
    } else {
        abort();
    }
}

而MFMailComposeViewControllerDelegate 显示第二个UIAlertView，只有一个按钮（显然该按钮的索引为0，因此它将调用abort()）：

- (void)mailComposeController:(MFMailComposeViewController *)controller
          didFinishWithResult:(MFMailComposeResult)result
                        error:(NSError *)error
{
    [navigationController dismissModalViewControllerAnimated:YES];

    NSMutableString *messageString = [[NSMutableString alloc] init];

    if (result == MFMailComposeResultSent) {
        [messageString appendFormat:@"Thanks! "];
    }

    [messageString appendFormat:@"The application has to quit now."];
    UIAlertView *abortAlert = [[[UIAlertView alloc] initWithTitle:nil
                                                          message:messageString
                                                         delegate:self
                                                cancelButtonTitle:@"OK"
                                                otherButtonTitles:nil] autorelease];

    [abortAlert show];

    [messageString release];
}