How do a set an opaque 1px high shadow image for a UINavigationBar?

Question

In my application delegate's didFinishLaunchingWithOptions function, I try to customize the appearance of my navigation bar.

[UINavigationBar appearance].translucent = NO;
[[UINavigationBar appearance] 
    setBackgroundImage:[UIImage 
        imageWithColor:[UIColor whiteColor] 
        size:CGSizeMake(1.0f, 1.0f)
    ] 
    forBarMetrics:UIBarMetricsDefault
];
[UINavigationBar appearance].shadowImage = [UIImage 
    imageWithColor:[UIColor redColor] 
    size:CGSizeMake(0.5f, 0.5f)
];

I expect a 1px tall opaque red shadow. Instead I am given a 2px tall translucent red shadow. How can make it appear exactly as I want it to? I've done the analogous appearance settings to UITabBar. It, on the other hand, behaves nicely.

The category function that creates dynamic images is defined as so:

+ (UIImage*)imageWithColor:(UIColor *)color size:(CGSize)size
{
    CGRect rect = CGRectMake(0.0f, 0.0f, size.width, size.height);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

Answer 1

You need to create a graphics context that is retina-aware :

let pixelScale = UIGraphicsBeginImageContextWithOptions(fillRect.size, false, UIScreen.main.scale)

After that, your shadow image will become 1 physical pixel tall.

Here's full code:

extension UIImage {

    static func imageWithColor(color: UIColor) -> UIImage {
        let pixelScale = UIScreen.main.scale
        let pixelSize = 1 / pixelScale
        let fillSize = CGSize(width: pixelSize, height: pixelSize)
        let fillRect = CGRect(origin: CGPoint.zero, size: fillSize)
        UIGraphicsBeginImageContextWithOptions(fillRect.size, false, pixelScale)
        let graphicsContext = UIGraphicsGetCurrentContext()
        CGContextSetFillColorWithColor(graphicsContext, color.CGColor)
        CGContextFillRect(graphicsContext, fillRect)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }

}

On GitHub:

https://github.com/salutis/swift-image-with-color