What's the best way to validate currency input in UITextField?

Question

My application allows the user to enter a numeric value (currency) in a UITextField control, but the keyboard layout that I wish was available is unfortunately not one of the built-in options, so I had to choose the "Numbers & Punctuation" option in Interface Builder. Here's the corresponding dialog window in IB:

So when my application asks the user for the input, it is displaying the following:

Which is perfectly fine, but look at all of the extra keys available to the user! One could easily enter "12;56!" in the text field, and I assume I have to validate that somehow.

So my question is: how do I validate currency values entered into a UITextField?

Answer 1

I have the urge to answer because this was the first entry I saw when I googled and the highest ranked answer wouldn't allow me to enter a currency.
I'm german, and in germany (and in many other countries) we use , as the decimalseparator.

I just wrote a similar method and this is what I have right now.

- (BOOL) textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    static NSString *numbers = @"0123456789";
    static NSString *numbersPeriod = @"01234567890.";
    static NSString *numbersComma = @"0123456789,";

    //NSLog(@"%d %d %@", range.location, range.length, string);
    if (range.length > 0 && [string length] == 0) {
        // enable delete
        return YES;
    }

    NSString *symbol = [[NSLocale currentLocale] objectForKey:NSLocaleDecimalSeparator];
    if (range.location == 0 && [string isEqualToString:symbol]) {
        // decimalseparator should not be first
        return NO;
    }
    NSCharacterSet *characterSet;
    NSRange separatorRange = [textField.text rangeOfString:symbol];
    if (separatorRange.location == NSNotFound) {
        if ([symbol isEqualToString:@"."]) {
            characterSet = [[NSCharacterSet characterSetWithCharactersInString:numbersPeriod] invertedSet];
        }
        else {
            characterSet = [[NSCharacterSet characterSetWithCharactersInString:numbersComma] invertedSet];              
        }
    }
    else {
        // allow 2 characters after the decimal separator
        if (range.location > (separatorRange.location + 2)) {
            return NO;
        }
        characterSet = [[NSCharacterSet characterSetWithCharactersInString:numbers] invertedSet];               
    }
    return ([[string stringByTrimmingCharactersInSet:characterSet] length] > 0);
}

Answer 2

While you can putz around in the NSNumberFormatter, I found it easier to just screen out anything but 0-9 and .

This is working well for me:

- (BOOL) textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSCharacterSet *nonNumberSet = [[NSCharacterSet characterSetWithCharactersInString:@"0123456789."] invertedSet];

return ([string stringByTrimmingCharactersInSet:nonNumberSet].length > 0);


}

Answer 3

I found this to be a relatively clean approach. I haven't tested it with non-US currencies but since it uses the NSNumberFormatter's properties I believe it should handle them correctly.

Start by setting up a formatter some place:

    formatter = [NSNumberFormatter new];
    [formatter setNumberStyle: NSNumberFormatterCurrencyStyle];
    [formatter setLenient:YES];
    [formatter setGeneratesDecimalNumbers:YES];

Use the formatter to parse and reformat their input. It also handles shifting the number when digits are added and removed.

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSString *replaced = [textField.text stringByReplacingCharactersInRange:range withString:string];
    NSDecimalNumber *amount = (NSDecimalNumber*) [formatter numberFromString:replaced];
    if (amount == nil) {
        // Something screwed up the parsing. Probably an alpha character.
        return NO;
    }
    // If the field is empty (the initial case) the number should be shifted to
    // start in the right most decimal place.
    short powerOf10 = 0;
    if ([textField.text isEqualToString:@""]) {
        powerOf10 = -formatter.maximumFractionDigits;
    }
    // If the edit point is to the right of the decimal point we need to do
    // some shifting.
    else if (range.location + formatter.maximumFractionDigits >= textField.text.length) {
        // If there's a range of text selected, it'll delete part of the number
        // so shift it back to the right.
        if (range.length) {
            powerOf10 = -range.length;
        }
        // Otherwise they're adding this many characters so shift left.
        else {
            powerOf10 = [string length];
        }
    }
    amount = [amount decimalNumberByMultiplyingByPowerOf10:powerOf10];

    // Replace the value and then cancel this change.
    textField.text = [formatter stringFromNumber:amount];
    return NO;
}

Answer 4

Perhaps you could attach a UITextFieldDelegate on the control and have it implement textField:shouldChangeCharactersInRange:replacementString: That way, if it sees any characters that you don't want in the field, it can reject them.

Answer 5

In conjunction with the textField:shouldChangeCharactersInRange:replacementString: suggestion made by Marc, you should pass the text through an NSNumberFormatter using an NSNumberFormatterCurrencyStyle. This will handle the quirks of currency formatting and handle locale specific options.

There's a "Data Formatting Programming Guide for Cocoa" section in the iPhone documentation if you search for it. Sadly, most of the UI information here is Mac OS X specific (doesnt work on iPhone) but it'll show you how to use the formatter classes.

Answer 6

I found that the shouldChangeCharactersInRange screws up the pop-up keyboard, backspace and "Done" button as well. I found if I handled 0 length strings and allowed control characters though, it worked fine.

I don't like using NSNumberFormatter because it insists that the number is well-formed at all stages while the user is editing and that can be infuriating if you, say, want to have two decimal points in the number for a moment until you delete the one that's in the wrong spot.

Here's the code I used:

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if ([string length] < 1)    // non-visible characters are okay
        return YES;
    if ([string stringByTrimmingCharactersInSet:[NSCharacterSet controlCharacterSet]].length == 0)
        return YES;
    return ([string stringByTrimmingCharactersInSet:[self.characterSet invertedSet]].length > 0);
}

Where self.characterSet holds the characters that are acceptable, I used this method to create it for a currency:

- (NSCharacterSet *)createCurrencyCharacterSet
{
    NSLocale *locale = [NSLocale currentLocale];
    NSMutableCharacterSet *currencySet = [NSMutableCharacterSet decimalDigitCharacterSet];

    [currencySet addCharactersInString:@"-"];       // negative symbol, can't find a localised version
    [currencySet addCharactersInString:[locale objectForKey:NSLocaleCurrencySymbol]];
    [currencySet addCharactersInString:[locale objectForKey:NSLocaleGroupingSeparator]];
    [currencySet addCharactersInString:[locale objectForKey:NSLocaleDecimalSeparator]];

    return [[currencySet copy] autorelease];
}

The somewhat unhappy code [[currencySet copy] autorelease] returns an immutable NSCharacterSet.

Using [NSCharacterSet decimalDigitCharacterSet] also includes the Indic and Arabic equivalent characters which hopefully means that people use those languages can use their alphabet's digits to enter numbers.

It's still necessary to check that NSNumberFormatter can parse the user's input and alert if it can't; nonetheless, it makes a nicer experience when only legit characters can be entered.

Answer 7

I keep seeing that people don't know how to properly determine the resulting string when implementing shouldChangeCharactersInRange.

Here's how you get the new text that would be entered if you returned YES:

NSString *newText = [textField.text stringByReplacingCharactersInRange:range withString:string];

Once you have this new text, it's easy to check if that text is a valid number or not, and return YES or NO accordingly.

Keep in mind that all other solutions, such as those shown here, where one checks the length of "string", and such, may not work properly if the user tries to edit the string using a bluetooth keyboard with cursor keys or using the advanced editing features (select, cut, paste).

Answer 8

If you want them to only be able to enter numbers - you might also consider the number keypad

Answer 9

After finding a quick solution on stack overflow to handle US currency, I rewrote the function to safely handle international currencies as well. This solution will dynamically validate user input from a UITextField, correcting as they type.

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {      

   NSNumberFormatter *nf = [[NSNumberFormatter alloc] init];

   [numberFormatter setFormatterBehavior:NSNumberFormatterBehavior10_4];
   [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
   [numberFormatter setMaximumFractionDigits:2];
   [numberFormatter setMinimumFractionDigits:0];

    // Grab the contents of the text field
    NSString *text = [textField text];  

    // the appropriate decimalSeperator and currencySymbol for the current locale
    // can be found with help of the
    // NSNumberFormatter and NSLocale classes.
    NSString *decimalSeperator = numberFormatter.decimalSeparator;  
    NSString *currencySymbol = numberFormatter.currencySymbol;

    NSString *replacementText = [text stringByReplacingCharactersInRange:range withString:string];
    NSMutableString *newReplacement = [[ NSMutableString alloc ] initWithString:replacementText];

    // whenever a decimalSeperator or currencySymobol is entered, we'll just update the textField.
    // whenever other chars are entered, we'll calculate the new number and update the textField accordingly.
    // If the number can't be computed, we ignore the new input.
    NSRange decimalRange = [text rangeOfString:decimalSeperator];
    if ([string isEqualToString:decimalSeperator] == YES && 
        [text rangeOfString:decimalSeperator].length == 0) {
        [textField setText:newReplacement];
    } else if([string isEqualToString:currencySymbol] == YES&& 
              [text rangeOfString:currencySymbol].length == 0) {
        [textField setText:newReplacement];        
    } else if([newReplacement isEqualToString:currencySymbol] == YES) {
        return YES;
    }else {

        NSString *currencyGroupingSeparator = numberFormatter.currencyGroupingSeparator;

        [newReplacement replaceOccurrencesOfString:currencyGroupingSeparator withString:@"" options:NSBackwardsSearch range:NSMakeRange(0, [newReplacement length])];        
        [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];

        NSNumber *number = [numberFormatter numberFromString:newReplacement];

        if([newReplacement length] == 1) {
            [numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
            number = [numberFormatter numberFromString:newReplacement];
        }

        if (number == nil) { 
            [newReplacement release];
            return NO;
        }
        [numberFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];    
        text = [numberFormatter stringFromNumber:number];
        [textField setText:text];       
    }

    [newReplacement release];

    return NO; // we return NO because we have manually edited the textField contents.
}

Answer 10

Using an NSNumberFormatter is the correct answer. It will handle validation and converting the string to and from the correct object type.

Answer 11

You can also use Gamma-Point solution and evaluate if *aNumber is Nil, if its Nil, then it means a character that wasnt 0-9 . or , was entered, this way you can validate just numbers, it will nil the variable if any no-number char is entered.

Answer 12

I Couldn't find the appropriate implementation for currency validate. Here is my variant:

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {

   NSString* proposedString = [textField.text stringByReplacingCharactersInRange:range withString:string];

   //if there is empty string return YES
   if([proposedString length] == 0) {
       return YES;
   }

   //create inverted set for appripriate symbols
   NSCharacterSet *nonNumberSet = [[NSCharacterSet characterSetWithCharactersInString:@"0123456789.,"] invertedSet];

   //if side symbols is trimmed by nonNumberSet - return NO
   if([proposedString stringByTrimmingCharactersInSet:nonNumberSet].length != [proposedString length]) {
       return NO;
   }

   //if there is more than 1 symbol of '.' or ',' return NO
   if([[proposedString componentsSeparatedByCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@".,"]] count] > 2) {
      return NO;
   }

   //finally check is ok, return YES
   return YES;
}

Answer 13

Using shouldChangeCharactersInRange screws up the pop-up key board as the backspace button doesn't work.

Number formatter is the way to go. Here's a sample I used to find positive decimals. I call it during the validation check- for e.g. when the user clicks on the save button.

    -(BOOL) isPositiveNumber: (NSString *) numberString {

    NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
    [numberFormatter setNumberStyle:NSNumberFormatterDecimalStyle];

    NSNumber *aNumber  =  [numberFormatter numberFromString:numberString];
    [numberFormatter release];

    if ([aNumber floatValue] > 0) {
        NSLog( @"Found positive number %4.2f",[aNumber floatValue] );

        return YES;
    }
    else {
        return NO;
    }
}