Convenience functions for getting the screen scale and rounding metrics relative to it.

UIScale.h

#import <UIKit/UIKit.h>

#define UI_MAIN_SCREEN_SCALE() ([UIScreen instancesRespondToSelector:@selector(scale)] ? [UIScreen mainScreen].scale : 1.0f)

static inline CGFloat CGFloatRoundToScale(CGFloat value, CGFloat scale)
{
    return roundf(value * scale) / scale;
}

static inline CGFloat CGFloatRoundToMainScreen(CGFloat value)
{
    return CGFloatRoundToScale(value, UI_MAIN_SCREEN_SCALE());
}

static inline CGPoint CGPointRoundToScale(CGPoint value, CGFloat scale)
{
    return (CGPoint) { CGFloatRoundToScale(value.x, scale), CGFloatRoundToScale(value.x, scale) };
}

static inline CGPoint CGPointRoundToMainScreen(CGPoint value)
{
    return CGPointRoundToScale(value, UI_MAIN_SCREEN_SCALE());
}

static inline CGSize CGSizeRoundToScale(CGSize value, CGFloat scale)
{
    return (CGSize) { CGFloatRoundToScale(value.width, scale), CGFloatRoundToScale(value.height, scale) };
}

static inline CGSize CGSizeRoundToMainScreen(CGSize value)
{
    return CGSizeRoundToScale(value, UI_MAIN_SCREEN_SCALE());
}

static inline CGRect CGRectRoundToScale(CGRect value, CGFloat scale)
{
    CGRect result;
    result.origin = CGPointRoundToScale(value.origin, scale);
    // Rounds such that the error present in the top and left positioning is
    // taken into account when calculating the right and bottom positioning
    result.size.width = CGFloatRoundToScale(value.origin.x + value.size.width, scale) - result.origin.x;
    result.size.height = CGFloatRoundToScale(value.origin.y + value.size.height, scale) - result.origin.y;
    return result;
}

static inline CGRect CGRectRoundToMainScreen(CGRect value)
{
    return CGRectRoundToScale(value, UI_MAIN_SCREEN_SCALE());
}