So I Wanted To Get The Height Of A UIImage

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3 min readApr 21, 2019

What, What You Say?

Have you ever wanted to get the size of a UIImage contained within a UIImageView? It’s quite easy really, just use the following line of code:

let size = imageView.image?.size ?? .zero // optional for brevity

However, if you happen to’ve set a contentMode such as .scaleAspectFill on your UIImageView, the size returned upon calling the getter will not be the view’s bounds — it will be the size of the actual image. This can be anywhere between zero and a few trillion gigapixels in size, or however big the image was, which revealed the M87 Black Hole.

Sidebar: Could an iOS device in this day and age render such an image?

Scenario

Given that you download an image from a URL, and set it on a UIImageView with content mode .scaleAspectFill, which is embedded inside a UICollectionViewCell of size CGSize(width: 50, height: 50), it would be ideal if we could find the relative size of the image, after the “scale” has been applied, rather than returning a potential image size equal to that of the planet Saturn.

Baby, Baby Can’t You See? I’ve Got Everything You Need

Much better! I divided the functions between each other, to make the Math clear for the purposes of demonstration. But you could probably adjust it to wind up with one computed variable which covers all four conditions.

Dot The I’s And Cross The T’s

Given a UICollectionViewCell of size CGSize(width: 100, height: 250)
It has a UIImageView of size CGSize(width: 100, height: 100) and takes an image downloaded from a URL, and the content mode is .scaleAspectFill.
Once the image has been set, we want to calculate the relative, scaled down height of the image based on the width. The math for this is standard aspect ratio calculation.
Given a bounds width of 100, we divide 100 by the accessible size of the image itself, which, in my example is 750x750px. 100 / 750 == 0.1333333333. Since my image view frame is square, the same applies for the height calculation.
The width and height are now computable by multiplying the relativeWidth by the image.size.height and the relativeHeight by the image.size.width.
New width value: 0.1333333333 * 750 == 99.999999975 or 100, as you expect. The height is also the same in this case.
Disclaimer: The values will differ if the image holds an aspect ratio other than 1:1!
I don’t calculate the scaled size for other content modes, because when you use those, the image is set to its actual size, and positioned left/right/top/bottom etcetera.
I use ceil because I want whole numbers.
I also use traitCollections well, because you should be anyway but if it’s not, it falls back to UIScreen scale.

Conclusion

You may wonder why in fact that I use an extension as opposed to a separate utility class which could very easily do this for me. Everything is relative! You can use whichever approach you choose, and in this case I chose a common one, which is one that even beginner Swift engineers can make use of.

You do however, need to err on the side of caution with extensions. Wherever possible, you should restrictively extend whatever it is, in order to only apply the extension where you want it. It can be easy to extend something you don’t mean to. If you pick out that there’s a risk of that happening, go the utility class route!

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