添加一个简单的 UIAlertView

UIAlertView *alert = [[UIAlertView alloc]
initWithTitle:@"Title"
message:@"Message"
delegate:nil //or self
cancelButtonTitle:@"OK"
otherButtonTitles:nil];


[alert show];
[alert autorelease];

When you want the alert to show, do this:

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"ROFL"
message:@"Dee dee doo doo."
delegate:self
cancelButtonTitle:@"OK"
otherButtonTitles:nil];
[alert show];


// If you're not using ARC, you will need to release the alert view.
// [alert release];

If you want to do something when the button is clicked, implement this delegate method:

- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex {
// the user clicked OK
if (buttonIndex == 0) {
// do something here...
}
}

And make sure your delegate conforms to UIAlertViewDelegate protocol:

@interface YourViewController : UIViewController <UIAlertViewDelegate>

As a supplementary to the two previous answers (of user "sudo rm -rf" and "Evan Mulawski"), if you don't want to do anything when your alert view is clicked, you can just allocate, show and release it. You don't have to declare the delegate protocol.

UIAlertView *myAlert = [[UIAlertView alloc]
initWithTitle:@"Title"
message:@"Message"
delegate:self
cancelButtonTitle:@"Cancel"
otherButtonTitles:@"Ok",nil];
[myAlert show];

Here is a complete method that only has one button, an 'ok', to close the UIAlert:

- (void) myAlert: (NSString*)errorMessage
{
UIAlertView *myAlert = [[UIAlertView alloc]
initWithTitle:errorMessage
message:@""
delegate:self
cancelButtonTitle:nil
otherButtonTitles:@"ok", nil];
myAlert.cancelButtonIndex = -1;
[myAlert setTag:1000];
[myAlert show];
}

Other answers already provide information for iOS 7 and older, however UIAlertView is deprecated in iOS 8.

In iOS 8+ you should use UIAlertController. It is a replacement for both UIAlertView and UIActionSheet. Documentation: UIAlertController Class Reference. And a nice article on NSHipster.

To create a simple Alert View you can do the following:

UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Title"
message:@"Message"
preferredStyle:UIAlertControllerStyleAlert];
//We add buttons to the alert controller by creating UIAlertActions:
UIAlertAction *actionOk = [UIAlertAction actionWithTitle:@"Ok"
style:UIAlertActionStyleDefault
handler:nil]; //You can use a block here to handle a press on this button
[alertController addAction:actionOk];
[self presentViewController:alertController animated:YES completion:nil];

Swift 3 / 4 / 5:

let alertController = UIAlertController(title: "Title", message: "Message", preferredStyle: .alert)
//We add buttons to the alert controller by creating UIAlertActions:
let actionOk = UIAlertAction(title: "OK",
style: .default,
handler: nil) //You can use a block here to handle a press on this button


alertController.addAction(actionOk)


self.present(alertController, animated: true, completion: nil)

Note, that, since it was added in iOS 8, this code won't work on iOS 7 and older. So, sadly, for now we have to use version checks like so:

NSString *alertTitle = @"Title";
NSString *alertMessage = @"Message";
NSString *alertOkButtonText = @"Ok";


if (@available(iOS 8, *)) {
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:alertTitle
message:alertMessage
delegate:nil
cancelButtonTitle:nil
otherButtonTitles:alertOkButtonText, nil];
[alertView show];
}
else {
UIAlertController *alertController = [UIAlertController alertControllerWithTitle:alertTitle
message:alertMessage
preferredStyle:UIAlertControllerStyleAlert];
//We add buttons to the alert controller by creating UIAlertActions:
UIAlertAction *actionOk = [UIAlertAction actionWithTitle:alertOkButtonText
style:UIAlertActionStyleDefault
handler:nil]; //You can use a block here to handle a press on this button
[alertController addAction:actionOk];
[self presentViewController:alertController animated:YES completion:nil];
}

Swift 3 / 4 / 5:

let alertTitle = "Title"
let alertMessage = "Message"
let alertOkButtonText = "Ok"


if #available(iOS 8, *) {
let alertController = UIAlertController(title: alertTitle, message: alertMessage, preferredStyle: .alert)
//We add buttons to the alert controller by creating UIAlertActions:
let actionOk = UIAlertAction(title: alertOkButtonText,
style: .default,
handler: nil) //You can use a block here to handle a press on this button


alertController.addAction(actionOk)
self.present(alertController, animated: true, completion: nil)
}
else {
let alertView = UIAlertView(title: alertTitle, message: alertMessage, delegate: nil, cancelButtonTitle: nil, otherButtonTitles: alertOkButtonText)
alertView.show()
}

UPD: updated for Swift 5. Replaced outdated class presence check with availability check in Obj-C.

This page shows how to add an UIAlertController if you are using Swift.

UIAlertView is deprecated on iOS 8. Therefore, to create an alert on iOS 8 and above, it is recommended to use UIAlertController:

UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Title" message:@"Alert Message" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *defaultAction = [UIAlertAction actionWithTitle:@"Ok" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action){


// Enter code here
}];
[alert addAction:defaultAction];


// Present action where needed
[self presentViewController:alert animated:YES completion:nil];

This is how I have implemented it.

Simple alert with array data:

NSString *name = [[YourArray objectAtIndex:indexPath.row ]valueForKey:@"Name"];


NSString *msg = [[YourArray objectAtIndex:indexPath.row ]valueForKey:@"message"];


UIAlertView *alert = [[UIAlertView alloc] initWithTitle:name
message:msg
delegate:self
cancelButtonTitle:@"OK"
otherButtonTitles:nil];
[alert show];