How to optimize the memory when displaying same image multiple times?

Question

I have an instance of UIImage with an image with size of 200KB, then I create 5 instances of UIImageView that reference to same this UIImage.

I wonder how much memory allocated in this case - only 200KB (of one UIImage instance) or 1MB (for 5 cloned UIImage instances)? In the case of wasting memory occured, is there effective way to solve it?

Answer 1

A couple of thoughts:

1. UIImage is a reference type, so when you reference the same image five times, you generally will have one image object in memory. It depends a little upon how you do this. For example, if you use UIImage(data:) each time, or something like that, it's possible to instantiate a new object each time, but if you instantiate only one UIImage and then proceed to use if five times, then you won't see duplicative memory consumption taking place.

As an aside:

2. You say the image has a size of 200kb. Is that the size of the original asset, or have you figured out that this is how much memory it will take at run time?

   The reason I ask is that JPG and PNG files are generally compressed, but when you use it in an image view, it will be uncompressed. The amount of memory that an image takes has little to do with the file size of the original asset, but rather corresponds to the dimensions (in pixels) of the image. So a random PNG that is 676 kb that is 2560 x 1440 pixels may actually require 14mb of memory (four bytes per pixel).

   Note, this memory consumption corresponds to the dimension of the image in question, not the dimensions of the image view to which you added it. If you're concerned about memory usage and if the image dimensions exceed the size of the image view (times the device scale), then you might want to consider resizing the image.

3. In the future, you can answer these questions empirically using Instruments. For example, in the following timeline, at the green signpost, I loaded a UIImage with the 676kb asset with modest memory impact, I set the image view image to use this asset at the purple signpost with a significant memory impact as it uncompressed this 2560 x 1440 px image, and I loaded five more image views with the same image at the orange signpost with negligible further memory impact.