UIImage(NFExtensions), a category for rescaling UIImage instances to generate thumbnails. Originally distributed as part of the UW PCE iOS course; now available under the MIT License.

UIImage+NFExtensions.h

// Copyright Tim Ekl 2014–2016. Available under MIT License.

#import <UIKit/UIKit.h>

@interface UIImage (NFExtensions)

- (UIImage *)scaledImageConstrainedToSize:(CGSize)size;

@end

UIImage+NFExtensions.m

// Copyright Tim Ekl 2014–2016. Available under MIT License.

#import "UIImage+NFExtensions.h"

@implementation UIImage (NFExtensions)

- (UIImage *)scaledImageConstrainedToSize:(CGSize)maximumSize;
{
    return [self scaledImageConstrainedToSize:maximumSize contentScale:[[UIScreen mainScreen] scale]];
}

- (UIImage *)scaledImageConstrainedToSize:(CGSize)maximumSize contentScale:(CGFloat)contentScale;
{
#if 1 && defined(DEBUG)
    // Leave a giant delay in Debug builds to stress the importance of threading this method
    sleep(2);
#endif

    CGSize currentSize = [self size];
    CGFloat scaleFactor = MIN(maximumSize.width / currentSize.width, maximumSize.height / currentSize.height) * contentScale;

    CGSize desiredSize = CGSizeApplyAffineTransform(currentSize, CGAffineTransformMakeScale(scaleFactor, scaleFactor));

    UIGraphicsBeginImageContext(desiredSize);
    [self drawInRect:CGRectIntegral((CGRect){.size = desiredSize, .origin = CGPointZero})];
    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return [[UIImage alloc] initWithCGImage:[scaledImage CGImage] scale:contentScale orientation:[scaledImage imageOrientation]];
}

@end