Tutorial :How to limit NSTextField text length and keep it always upper case?


Need to have an NSTextField with a text limit of 4 characters maximum and show always in upper case but can't figure out a good way of achieving that. I've tried to do it through a binding with a validation method but the validation only gets called when the control loses first responder and that's no good.

Temporarly I made it work by observing the notification NSControlTextDidChangeNotification on the text field and having it call the method:

- (void)textDidChange:(NSNotification*)notification {    NSTextField* textField = [notification object];    NSString* value = [textField stringValue];    if ([value length] > 4) {      [textField setStringValue:[[value uppercaseString] substringWithRange:NSMakeRange(0, 4)]];    } else {      [textField setStringValue:[value uppercaseString]];    }  }  

But this surely isn't the best way of doing it. Any better suggestion?


I did as Graham Lee suggested and it works fine, here's the custom formatter code:

UPDATED: Added fix reported by Dave Gallagher. Thanks!

@interface CustomTextFieldFormatter : NSFormatter {    int maxLength;  }  - (void)setMaximumLength:(int)len;  - (int)maximumLength;    @end    @implementation CustomTextFieldFormatter    - (id)init {       if(self = [super init]){          maxLength = INT_MAX;     }      return self;  }    - (void)setMaximumLength:(int)len {    maxLength = len;  }    - (int)maximumLength {    return maxLength;  }    - (NSString *)stringForObjectValue:(id)object {    return (NSString *)object;  }    - (BOOL)getObjectValue:(id *)object forString:(NSString *)string errorDescription:(NSString **)error {    *object = string;    return YES;  }    - (BOOL)isPartialStringValid:(NSString **)partialStringPtr     proposedSelectedRange:(NSRangePointer)proposedSelRangePtr            originalString:(NSString *)origString     originalSelectedRange:(NSRange)origSelRange          errorDescription:(NSString **)error {      if ([*partialStringPtr length] > maxLength) {          return NO;      }        if (![*partialStringPtr isEqual:[*partialStringPtr uppercaseString]]) {        *partialStringPtr = [*partialStringPtr uppercaseString];        return NO;      }        return YES;  }    - (NSAttributedString *)attributedStringForObjectValue:(id)anObject withDefaultAttributes:(NSDictionary *)attributes {    return nil;  }    @end  


Have you tried attaching a custom NSFormatter subclass?


In the above example where I commented, this is bad:

// Don't use:  - (BOOL)isPartialStringValid:(NSString *)partialString              newEditingString:(NSString **)newString              errorDescription:(NSString **)error  {      if ((int)[partialString length] > maxLength)      {          *newString = nil;          return NO;      }  }  

Use this (or something like it) instead:

// Good to use:  - (BOOL)isPartialStringValid:(NSString **)partialStringPtr         proposedSelectedRange:(NSRangePointer)proposedSelRangePtr                originalString:(NSString *)origString         originalSelectedRange:(NSRange)origSelRange              errorDescription:(NSString **)error  {      int size = [*partialStringPtr length];      if ( size > maxLength )      {          return NO;      }      return YES;  }  

Both are NSFormatter methods. The first one has an issue. Say you limit text-entry to 10 characters. If you type characters in one-by-one into an NSTextField, it'll work fine and prevent users from going beyond 10 characters.

However, if a user was to paste a string of, say, 25 characters into the Text Field, what'll happen is something like this:

1) User will paste into TextField

2) TextField will accept the string of characters

3) TextField will apply the formatter to the "last" character in the 25-length string

4) Formatter does stuff to the "last" character in the 25-length string, ignoring the rest

5) TextField will end up with 25 characters in it, even though it's limited to 10.

This is because, I believe, the first method only applies to the "very last character" typed into an NSTextField. The second method shown above applies to "all characters" typed into the NSTextField. So it's immune to the "paste" exploit.

I discovered this just now trying to break my application, and am not an expert on NSFormatter, so please correct me if I'm wrong. And very much thanks to you carlosb for posting that example. It helped a LOT! :)


This implementation adopts several of the suggestions commented on above. Notably it works correctly with continuously updating bindings.

In addition:

  1. It implements paste correctly.

  2. It includes some notes on how to use the class effectively in a nib without further subclassing.

The code:

@interface BPPlainTextFormatter : NSFormatter {      NSInteger _maxLength;  }      /*     Set the maximum string length.      Note that to use this class within a Nib:   1. Add an NSFormatter as a Custom Formatter.   2. In the Identity inspector set the Class to BPPlainTextFormatter   3. In user defined attributes add Key Path: maxLength Type: Number Value: 30     Note that rather than attaching formatter instances to individual cells they   can be positioned in the nib Objects section and referenced by numerous controls.   A name, such as Plain Text Formatter 100, can  be used to identify the formatters max length.     */  @property NSInteger maxLength;    @end      @implementation BPPlainTextFormatter  @synthesize maxLength = _maxLength;    - (id)init  {      if(self = [super init]){          self.maxLength = INT_MAX;      }        return self;  }    - (id)initWithCoder:(NSCoder *)aDecoder  {      // support Nib based initialisation      self = [super initWithCoder:aDecoder];      if (self) {          self.maxLength = INT_MAX;      }        return self;  }    #pragma mark -  #pragma mark Textual Representation of Cell Content    - (NSString *)stringForObjectValue:(id)object  {      NSString *stringValue = nil;      if ([object isKindOfClass:[NSString class]]) {            // A new NSString is perhaps not required here          // but generically a new object would be generated          stringValue = [NSString stringWithString:object];      }        return stringValue;  }    #pragma mark -  #pragma mark Object Equivalent to Textual Representation    - (BOOL)getObjectValue:(id *)object forString:(NSString *)string errorDescription:(NSString **)error  {      BOOL valid = YES;        // Be sure to generate a new object here or binding woe ensues      // when continuously updating bindings are enabled.      *object = [NSString stringWithString:string];        return valid;  }    #pragma mark -  #pragma mark Dynamic Cell Editing    - (BOOL)isPartialStringValid:(NSString **)partialStringPtr         proposedSelectedRange:(NSRangePointer)proposedSelRangePtr                originalString:(NSString *)origString         originalSelectedRange:(NSRange)origSelRange              errorDescription:(NSString **)error  {      BOOL valid = YES;        NSString *proposedString = *partialStringPtr;      if ([proposedString length] > self.maxLength) {            // The original string has been modified by one or more characters (via pasting).          // Either way compute how much of the proposed string can be accommodated.          NSInteger origLength = origString.length;          NSInteger insertLength = self.maxLength - origLength;            // If a range is selected then characters in that range will be removed          // so adjust the insert length accordingly          insertLength += origSelRange.length;            // Get the string components          NSString *prefix = [origString substringToIndex:origSelRange.location];          NSString *suffix = [origString substringFromIndex:origSelRange.location + origSelRange.length];          NSString *insert = [proposedString substringWithRange:NSMakeRange(origSelRange.location, insertLength)];    #ifdef _TRACE            NSLog(@"Original string: %@", origString);          NSLog(@"Original selection location: %u length %u", origSelRange.location, origSelRange.length);            NSLog(@"Proposed string: %@", proposedString);          NSLog(@"Proposed selection location: %u length %u", proposedSelRangePtr->location, proposedSelRangePtr->length);            NSLog(@"Prefix: %@", prefix);          NSLog(@"Suffix: %@", suffix);          NSLog(@"Insert: %@", insert);  

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