LETextField.m

@import ObjectiveC.runtime;
#import "LETextField.h"

@interface LETextField () <UITextFieldDelegate>
@end

@implementation LETextField {
    __weak id<UITextFieldDelegate> _realDelegate;
}

- (void)_init
{
    [super setDelegate:self];
}
- (instancetype)initWithFrame:(CGRect const)aFrame
{
    if((self = [super initWithFrame:aFrame]))
        [self _init];
    return self;
}
- (instancetype)initWithCoder:(NSCoder * const)aDecoder
{
    if((self = [super initWithCoder:aDecoder]))
        [self _init];
    return self;
}

- (void)setDelegate:(id<UITextFieldDelegate> const)aDelegate
{
    _realDelegate = aDelegate;
}
- (id<UITextFieldDelegate>)delegate
{
    return _realDelegate;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    BOOL isDelegateMethod =
           protocol_getMethodDescription(@protocol(UITextFieldDelegate), anInvocation.selector, NO,  YES).name
        || protocol_getMethodDescription(@protocol(UITextFieldDelegate), anInvocation.selector, YES, YES).name;
    if(isDelegateMethod)
        anInvocation.target = _realDelegate;
    [anInvocation invoke];
}

- (BOOL)             textField:(UITextField *)aTextField
 shouldChangeCharactersInRange:(NSRange)aRange
             replacementString:(NSString *)aReplacementString
{
    if(_formatter) {
        NSString *newString = [self.text stringByReplacingCharactersInRange:aRange withString:aReplacementString];
        NSRange const oldSelection = {
            [self offsetFromPosition:self.beginningOfDocument toPosition:self.selectedTextRange.start],
            [self offsetFromPosition:self.selectedTextRange.start toPosition:self.selectedTextRange.end]
        };
        NSRange newSelectionRange = oldSelection;
        BOOL const valid = [_formatter isPartialStringValid:&newString
                                      proposedSelectedRange:&newSelectionRange
                                             originalString:self.text
                                      originalSelectedRange:oldSelection
                                           errorDescription:NULL];
        if(!valid) {
            if(![_realDelegate respondsToSelector:_cmd]
                || [_realDelegate textField:self
                    shouldChangeCharactersInRange:aRange
                           replacementString:newString]) {
                [super setText:newString];
                UITextPosition * const start = [self positionFromPosition:self.beginningOfDocument
                                                                   offset:newSelectionRange.location];
                self.selectedTextRange = [self textRangeFromPosition:start
                                                          toPosition:[self positionFromPosition:start
                                                                                         offset:newSelectionRange.length]];
            }
            return NO;
        }
    }
    
    if([_realDelegate respondsToSelector:_cmd]) {
        return [_realDelegate textField:self
          shouldChangeCharactersInRange:aRange
                      replacementString:aReplacementString];
    } else
        return YES;
}
@end