What is the purpose of the Custom Formatter on Interface builder?

Question

Interface builder has this Custom Formatter that can be dragged to text fields.

But this thing has no properties at all and the documentation, as typical with Apple, is non-existent.

I need to create a formatter than can accept numbers, texts from the alphanumeric set and underscore and reject everything else.

I suspect this Custom Formatter is what I need but how do I use that? Or is that possible to do what I need using regular formatters existent on interface builder?

Can you give an example using interface builder?

Thanks.

Answer 1

The NSFormatter class is an abstract class, so you need to subclass it. On that you need to implement the following methods.

- (BOOL)isPartialStringValid:(NSString *)partialString newEditingString:(NSString **)newString errorDescription:(NSString **)error;
- (NSString *)stringForObjectValue:(id)obj;
- (BOOL)getObjectValue:(out id *)obj forString:(NSString *)string errorDescription:(out NSString **)error;

Create a subclass like:

.h

@interface MyFormatter : NSFormatter

@end

.m

@implementation MyFormatter

- (BOOL)isPartialStringValid:(NSString *)partialString newEditingString:(NSString **)newString errorDescription:(NSString **)error
{
    // In this method you need to write the validation
    // Here I'm checking whether the first character entered in the textfield is 'a' if yes, It's invalid in my case.
    if ([partialString isEqualToString:@"a"])
    {
        NSLog(@"not valid");
        return false;
    }
    return YES;
}

- (NSString *)stringForObjectValue:(id)obj
{
    // Here you return the initial value for the object
    return @"Midhun";
}

- (BOOL)getObjectValue:(out id *)obj forString:(NSString *)string errorDescription:(out NSString **)error
{
    // In this method we can parse the string and pass it's value (Currently all built in formatters won't support so they just return NO, so we are doing the same here. If you are interested to do any parsing on the string you can do that here and pass YES after a successful parsing
    // You can read More on that here: https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSFormatter_Class/index.html#//apple_ref/occ/instm/NSFormatter/getObjectValue:forString:errorDescription:
    return NO;
}
@end

Answer 2

That object is mostly useful for OS X. On OS X, you can attach formatters to controls/cells. In iOS, I suppose you can add a formatter to a NIB or storyboard and connect an outlet to it, but there's not much advantage to that over creating it programmatically.

In particular, the generic Custom Formatter is for when you want to add an instance of a custom subclass of NSFormatter . You drag in the Custom Formatter to the relevant control/cell, and then set its class in the Identity inspector.

If that object weren't available in the object library, you could only drag in instances of specific formatter classes (eg NSNumberFormatter ). You would be able to set the class of the resulting instance, but only to a subclass of that specific formatter class.

If you need to learn how to write a custom formatter class, consult the class reference for NSFormatter and the Data Formatting Guide .