Validating UITextField input and displaying error - iphone

Question

What's the best practice for validating user input in a UITextField and displaying error? I tried performing checks inside textFieldShouldReturn and textFieldShouldEndEditing and popping up an error box when userever inputted invalid content.
However, during testing, the popup from textFieldShouldEndEditing is getting invoked multiple times. My simple test was to input invalid text in TextFieldA and navigate directly to TextFieldB. I observed 3 error popups, all generated by textFieldShouldReturn

Answer 1

It might be that by showing the alert, the edit field loses focus and thereby issues another textFieldShouldReturn or textFieldShouldEndEditing.
Try postponing the alert, by putting the code to show the alert into a separate method, and invoke that method from textFieldShouldEndEditing with [self performSelector:#selector(yourAlertMethod) withObject:nil afterDelay:0]
Actually, I found that I had a similar problem a while ago. The trick is to make sure that the text field doesn't have the focus any more for the duration that you show the alert.
Here's my solution:
// declare this in your interface as part of your ivars:
UITextField *currTextField;
// here comes the code to check for bad input and show an alert:
#pragma mark -
#pragma mark UITextFieldDelegate
- (BOOL)textFieldShouldReturn:(UITextField *)textField
{
if ([self acceptsEntry:textField.text]) { // this checks if the text is valid
// We're done here
return YES;
} else {
// Setting the delegate to nil will prevent the textField to listen for Return key events.
textField.delegate = nil;
// Removing the observer will prevent the user from typing into the textfield using an external keyboard
[textField resignFirstResponder];
currTextField = textField;
UIAlertView *av = [[UIAlertView alloc] initWithTitle:NSLocalizedString(#"invalidEntry", #"")
message:NSLocalizedString(#"invalidEntryMessage", #"")
delegate:self
cancelButtonTitle:NSLocalizedString(#"OK", nil) otherButtonTitles:nil];
[av show];
[av release];
}
return NO;
}
#pragma mark -
#pragma mark UIAlertViewDelegate
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex
{
// gets called when the user dismisses the alert view
currTextField.text = #""; // erase bad entry
currTextField.delegate = self; // We want to listen for return events again
[currTextField becomeFirstResponder];
currTextField = nil;
}