Struggling with currency in Cocoa

Question

I'm trying to do something I'd think would be fairly simple: Let a user input a dollar amount, store that amount in an NSNumber (NSDecimalNumber?), then display that amount formatted as currency again at some later time.

My trouble is not so much with the setNumberStyle:NSNumberFormatterCurrencyStyle and displaying floats as currency. The trouble is more with how said numberFormatter works with this UITextField. I can find few examples. This thread from November and this one give me some ideas but leaves me with more questions.

I am using the UIKeyboardTypeNumberPad keyboard and understand that I should probably show $0.00 (or whatever local currency format is) in the field upon display then as a user enters numerals to shift the decimal place along:

• Begin with display $0.00
• Tap 2 key: display $0.02
• Tap 5 key: display $0.25
• Tap 4 key: display $2.54
• Tap 3 key: display $25.43

Then [numberFormatter numberFromString:textField.text] should give me a value I can store in my NSNumber variable.

Sadly I'm still struggling: Is this really the best/easiest way? If so then maybe someone can help me with the implementation? I feel UITextField may need a delegate responding to every keypress but not sure what, where and how to implement it?! Any sample code? I'd greatly appreciate it! I've searched high and low...

Edit1: So I'm looking into NSFormatter's stringForObjectValue: and the closest thing I can find to what benzado recommends: UITextViewTextDidChangeNotification. Having really tough time finding sample code on either of them...so let me know if you know where to look?

Answer 1

My solution:

- (BOOL)textField:(UITextField *)textField
    shouldChangeCharactersInRange:(NSRange)range 
    replacementString:(NSString *)string
{
  // Clear all characters that are not numbers
  // (like currency symbols or dividers)
  NSString *cleanCentString = [[textField.text
    componentsSeparatedByCharactersInSet:
    [[NSCharacterSet decimalDigitCharacterSet] invertedSet]]
      componentsJoinedByString:@""];
  // Parse final integer value
  NSInteger centAmount = cleanCentString.integerValue;
  // Check the user input
  if (string.length > 0)
  {
    // Digit added
    centAmount = centAmount * 10 + string.integerValue;
  }
  else
  {
    // Digit deleted
    centAmount = centAmount / 10;
  }
  // Update call amount value
  [_amount release];
  _amount = [[NSNumber alloc] initWithFloat:(float)centAmount / 100.0f];
  // Write amount with currency symbols to the textfield
  NSNumberFormatter *_currencyFormatter = [[NSNumberFormatter alloc] init];
  [_currencyFormatter setNumberStyle:NSNumberFormatterCurrencyStyle];
  [_currencyFormatter setCurrencyCode:@"USD"];
  [_currencyFormatter setNegativeFormat:@"-¤#,##0.00"];
  textField.text = [_currencyFormatter stringFromNumber:_amount];
  [_currencyFormatter release]
  // Since we already wrote our changes to the textfield
  // we don't want to change the textfield again
  return NO;
}

Answer 2

Here's the rough plan of attack I'd use if I had to write that now. The trick will be typing into a hidden UITextField and updating a UILabel with the formatted value as the user types.

1. Create a UITextField, make it hidden, assign it a delegate, and then make it the first responder to summon the keyboard.
2. In your delegate, respond to the textDidChange: message (too lazy to look up the exact name) by taking the text field's new value and converting it to a number. Make sure empty string converts to zero.
3. Run the number through your formatter, and update a UILabel with that formatted currency value.

On every key press, the label will be updated, so the user will feel as though she is editing the formatted value, when she is really editing the hidden text field. How sneaky!