Magic behind CIFilter modernization (for swift) on macOS 10.15 uses class_addMethod

CIFilter+CILineOverlay.h

#import <Cocoa/Cocoa.h>
#import <CoreImage/CoreImage.h>

@protocol CILineOverlay <CIFilter>
  @property (nonatomic, retain) CIImage *inputImage;
  @property (nonatomic) float threshold;
@end

@interface CIFilter(CILineOverlay)
+ (CIFilter<CILineOverlay>*) lineOverlayFilter;
@property (nonatomic, retain) CIImage *inputImage;
@property (nonatomic) float threshold; //notice that they dropped input keyword
@end

CIFilter+CILineOverlay.m

#import <CoreImage/CoreImage.h>
#include <objc/runtime.h>

@implementation CIFilter(CILineOverlay)

@dynamic inputImage;
@dynamic threshold;

+ (CIFilter<CILineOverlay>*) lineOverlayFilter
{
    CIFilter<CILineOverlay>*filter = (CIFilter<CILineOverlay>*)[CIFilter filterWithName:@"CILineOverlay"];
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        class_addMethod([self class], @selector(threshold), (IMP)floatGetter, "f@:"); //dynamically add input keyword
        class_addMethod([self class], @selector(setThreshold:), (IMP)floatSetter, "v@:f");
    });
    return filter;
}

static float floatGetter(id self, SEL _cmd) {

    NSString *selector = NSStringFromSelector(_cmd);
    NSString *firstLetter = [[selector substringWithRange:NSMakeRange(0, 1)] uppercaseString];
    NSString *key = [@"input" stringByAppendingString:[selector stringByReplacingCharactersInRange:NSMakeRange(0, 1) withString:firstLetter]];
    /// SEL == threshold
    /// key == inputThreshold
    id value = [self valueForKey:key];
    float number = NAN;
    if (value && [value isKindOfClass:[NSNumber class]]) {
        number = [value floatValue];
    }
    return number;
}

static void floatSetter(id self, SEL _cmd, float value) {
    NSString *selector = NSStringFromSelector(_cmd);
    NSString *key = [selector stringByReplacingCharactersInRange:NSMakeRange(0, 3) withString:@"input"];
    /// SEL == setThreshold
    /// key == inputThreshold
    [self setValue:@(value) forKey:[key substringWithRange:NSMakeRange(0, [key length] - 1)]];
}

@end