NSInvalidArgumentException'，原因：'应用程序试图在目标上呈现一个 nil 模态视图控制器

Question

我使用 Xcode 7.3 beta 3 创建了一封电子邮件。我从 appcoda 获取代码

- (IBAction)showEmail:(id)sender {
    // Email Subject
    NSString *emailTitle = @"Test Email";
    // Email Content
    NSString *messageBody = @"iOS programming is so fun!";
    // To address
    NSArray *toRecipents = [NSArray arrayWithObject:@"support@appcoda.com"];

    MFMailComposeViewController *mc = [[MFMailComposeViewController alloc] init];
    mc.mailComposeDelegate = self;
    [mc setSubject:emailTitle];
    [mc setMessageBody:messageBody isHTML:NO];
    [mc setToRecipients:toRecipents];

    // Present mail view controller on screen
    [self presentViewController:mc animated:YES completion:NULL];

}

- (void) mailComposeController:(MFMailComposeViewController *)controller didFinishWithResult:(MFMailComposeResult)result error:(NSError *)error
{
    switch (result)
    {
        case MFMailComposeResultCancelled:
            NSLog(@"Mail cancelled");
            break;
        case MFMailComposeResultSaved:
            NSLog(@"Mail saved");
            break;
        case MFMailComposeResultSent:
            NSLog(@"Mail sent");
            break;
        case MFMailComposeResultFailed:
            NSLog(@"Mail sent failure: %@", [error localizedDescription]);
            break;
        default:
            break;
    }

    // Close the Mail Interface
    [self dismissViewControllerAnimated:YES completion:NULL];
}

但在 iPhone 上运行应用程序后，它给了我一个错误（如下所示）

* 由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因：“应用程序试图在目标上呈现 nil 模态视图控制器。”* 首先抛出调用堆栈：()libc++abi。 dylib：以 NSException 类型的未捕获异常终止

Answer 1

NSInvalidArgumentException'，原因：'应用程序试图在目标上呈现一个 nil 模态视图控制器——这意味着你尝试在目标上执行一次 nil objcet 检查它是否为 nil

MFMailComposeViewController *mc = [[MFMailComposeViewController alloc] init];
if (!mc) {  
  //When the device has not added mailViewController mail account is empty , the following present view controller causes the program to crash here
NSLog(@"no email accounts are set up in your device");  
return;  
} 

样本tutorial

Answer 2

斯威夫特

如果让则使用

if let mc: MFMailComposeViewController = MFMailComposeViewController() {
    mc.mailComposeDelegate = self
    mc.setSubject(emailTitle)
    mc.setToRecipients(toRecipents)
    self.presentViewController(mc, animated: true, completion: nil)
}