Randomly change the font size while running your app.

SEPrankster.h

@import UIKit;

@interface SEPrankster: NSObject
@property (class, readonly) SEPrankster *sharedPrankser;
- (void)setUpFontRandomness:(BOOL)run;
- (void)setFontCategory:(UIContentSizeCategory)category;
@end

SEPrankster.m

@import JRSwizzle;
#import "SEPrankster.h"

static UIContentSizeCategory SEPranksterPrankContentSizeCategory = nil;

@interface SEPrankster ()
@property (nonatomic) NSTimer *dynamicTypeTimer;
@end

@interface UIApplication (Prank)
- (UIContentSizeCategory)SE_preferredContentSizeCategory;
@end

@implementation SEPrankster

+ (void)load
{
    [UIApplication jr_swizzleMethod:@selector(preferredContentSizeCategory) withMethod:@selector(SE_preferredContentSizeCategory) error:NULL];
}

+ (instancetype)sharedPrankster
{
    static SEPrankster *prankster;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        prankster = [[self alloc] init];
    });
    return prankster;
}

- (void)setUpFontRandomness:(BOOL)run
{
    if (run && self.dynamicTypeTimer == nil) {
        self.dynamicTypeTimer = [NSTimer scheduledTimerWithTimeInterval:5 target:self selector:@selector(changeSeats:) userInfo:nil repeats:YES];
    } else if (!run && self.dynamicTypeTimer) {
        [self.dynamicTypeTimer invalidate];
        self.dynamicTypeTimer = nil;
        [self setFontCategory:nil];
    }
}

- (void)changeSeats:(id)blahBlahBlah
{
    NSArray<UIContentSizeCategory> *fonts = @[UIContentSizeCategorySmall, UIContentSizeCategoryMedium, UIContentSizeCategoryLarge, UIContentSizeCategoryExtraSmall, UIContentSizeCategoryExtraLarge, UIContentSizeCategoryExtraExtraLarge, UIContentSizeCategoryExtraExtraExtraLarge];
    [self setFontCategory:fonts[arc4random_uniform((u_int32_t)fonts.count)]];
}

- (void)setFontCategory:(UIContentSizeCategory)category
{
    SEPranksterPrankContentSizeCategory = category;
    NSLog(@"Mix up time! Becoming: %@", [[UIApplication sharedApplication] preferredContentSizeCategory]);
    [[NSNotificationCenter defaultCenter] postNotificationName:UIContentSizeCategoryDidChangeNotification object:[UIApplication sharedApplication] userInfo:@{UIContentSizeCategoryNewValueKey: [[UIApplication sharedApplication] preferredContentSizeCategory]}];
}

@end

@implementation UIApplication (Prank)

- (UIContentSizeCategory)SE_preferredContentSizeCategory
{
    return SEPranksterPrankContentSizeCategory ?: [self SE_preferredContentSizeCategory];
}

@end