How to disable alertview's button in iPhone?

UPDATE 2: For Swift 5.1

<#your alert controller#>.addTextField {(tf) in

                        //... set your tf characteristics i.e .keyboardType here

                        NotificationCenter.default.addObserver(forName: UITextField.textDidChangeNotification,
                                                               object: tf,
                                                               queue: OperationQueue.main) { _ in

                                                                //enable or disable the selected action depending on whether the textField text is empty
                                                                <#your alert controller#>.actions[0].isEnabled = !tf.text!.isEmpty

                        }

                    }

posting this to update the response since ios 5 :

- (BOOL)alertViewShouldEnableFirstOtherButton:(UIAlertView *)alertView
{
  UITextField *textField = [alertView textFieldAtIndex:0];
  if ([textField.text length] == 0)
  {
    return NO;
  }
  return YES;
}

UPDATE:iOS 8 Since Apple have deprecated the UIAlertView in favour of the UIAlertController. There is no longer a delegate call to alertViewShouldEnableFirstOtherButton:

So instead you would set the buttons enabled property via the UITextFieldTextDidChangeNotification Add a textView to the alert with

• (void)addTextFieldWithConfigurationHandler:(void (^)(UITextField *textField))configurationHandler

[<#your alert#> addTextFieldWithConfigurationHandler:^(UITextField *textField) {
textField.delegate = self;
textField.tag = 0; //set a tag to 0 though better to use a #define
}];

Then implement the delegate method

• (void)textFieldDidBeginEditing:(UITextField *)textField

- (void)textFieldDidBeginEditing:(UITextField *)textField{
//in here we want to listen for the "UITextFieldTextDidChangeNotification"

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(textFieldHasText:)
                                         name:UITextFieldTextDidChangeNotification
                                       object:textField];

}

When the text in textField changes it will invoke a call to "textFieldHasText:" and pass along a NSNotification*

-(void)textFieldHasText:(NSNotification*)notification{
//inside the notification is the object property which is the textField
//we cast the object to a UITextField*
if([[(UITextField*)notification.object text] length] == 0){
//The UIAlertController has actions which are its buttons.
//You can get all the actions "buttons" from the `actions` array
//we have just one so its at index 0

[<#your alert#>.actions[0] setEnabled:NO];
}
else{

[<#your alert#>.actions[0] setEnabled:YES];
}
}

Don't forget to remove your observer when done

I wanted to extend the answer by Ryan Forsyth by adding this. If you add a Default styled UIAlertView, you can get an out of range exception if you try to access a textfield as none exist, so check your view style first.

-(BOOL)alertViewShouldEnableFirstOtherButton:(UIAlertView*)alertView
{
    if(alertView.alertViewStyle == UIAlertViewStyleLoginAndPasswordInput ||
       alertView.alertViewStyle == UIAlertViewStylePlainTextInput ||
       alertView.alertViewStyle == UIAlertViewStyleSecureTextInput)
    {
        NSString* text = [[alertView textFieldAtIndex:0] text];
        return ([text length] > 0);
    }
    else if (alertView.alertViewStyle == UIAlertViewStyleDefault)
        return true;
    else
        return false;
}