UITextField validation

Question

I am creating a text field.

Whenever the value in text field is greater than 100 I need to display an alert saying "value must be less than 100", then the text field's value should be cleared.

How can I do this?

Answer 1

You can have your view controller conform to the UITextFieldDelegate protocol and implement the -textField:shouldChangeCharactersInRange:replacementString: method:

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    if (range.location == 0 && string.length == 0) {
        return YES;
    }

    // Build up the resulting string…
    NSMutableString *fullString = [[NSMutableString alloc] init];

    [fullString appendString:[textField.text substringWithRange:NSMakeRange(0, range.location)]];
    [fullString appendString:string];

    // Set up number formatter…
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    NSNumber *replaceNumber = [formatter numberFromString:fullString];

    [fullString release];
    [formatter release];

    return !(replaceNumber == nil || [replaceNumber intValue] > 100);
}

Answer 2

To build the full string you must take into account edits in the middle of the text field too. Made some changes to the code posted above.

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    BOOL result = NO;

    // Build the final string...   
    NSMutableString *fullString = [[NSMutableString alloc] init];
    NSUInteger firstChunkLength = range.location;
    NSUInteger secondChunkStart = range.location + range.length;
    NSUInteger secondChunkLength = [textField.text length] - secondChunkStart;

    [fullString appendString:[textField.text substringWithRange:NSMakeRange( 0, firstChunkLength )]];
    [fullString appendString:string];
    [fullString appendString:[textField.text substringWithRange:NSMakeRange( secondChunkStart, secondChunkLength )]];

    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
    NSNumber *replaceNumber = [formatter numberFromString:fullString];
    NSNumber *originalNumber = [formatter numberFromString:textField.text];

    result = ( replaceNumber != nil && replaceNumber != originalNumber );

    [fullString release];
    [formatter release];

    return result;
}