How to clear font cache filled with emoji characters?

Question

I am developing keyboard extension for iPhone. There is an emoji screen smilar to Apples own emoji keyboard that shows some 800 emoji characters in UICollectionView

Answer 1

I've been around the houses on this too, and I've come to the following conclusion after numerous tests:

While the font cache does contribute to your extension's memory footprint and the total usage in the Xcode Debug Navigator and Memory Report, it isn't treated in quite the same way as the rest of your budget.

Some people cite 50 MB as the extension limit, and on Apple docs I think I've seen either 30 or 32 MB cited. We see memory warnings at various points between 30 and 40 MB, and this is too inconsistent to be happy with any particular value, but one thing that does seem to be concrete is a memory exception that occurs at 53 MB, which is logged out by Xcode as exactly that number. If I take a blank keyboard extension and populate it with even 40 MB of image views, this is one thing, but if I use 30 MB and then 20 MB of font glyph usage, I find that my keyboard isn't shut down.

From my observations, the font cache looks to get cleaned up, but not as often as you might feel necessary (especially if you're becoming nervous when that unhelpful combined memory value exceeds 30 or 32 MB).

If you budget your own memory usage at, say, 30 MB, you should be safe, provided that you don't introduce a scenario where 23 MB (i.e. 53-30) of font glyphs are all required in one swoop. This will be influenced by how dense your emoji grid is, and possibly even the font size used. It's common understanding here that if you were to scroll from one end of your emoji collection view to the other, you'll have passed through more than 23 MB of font glyphs, but if the rest of your memory footprint is reasonable (i.e. 30 MB or below), the font cache should get a chance to clean up.

In my testing, I attempted to automate bombardment of the extension with far more font glyphs, and I think I was able to beat the font cache cleanup process, resulting in a crash.

Therefore, given the use case of a UICollectionView and how fast it can be scrolled, it may be possible to crash the application if you really pushed the 30 MB memory budget and also scrolled very quickly. You might allow yourself to hit this 53 MB hard limit.

Given all of the above - with a fully fledged keyboard extension, as long as I keep to approximately 30 MB of my own (non-font-glyph) footprint I haven't encountered a crash, even when rapidly changing emoji categories and scrolling fast. I do, however, encounter system memory warnings this way, which is the thing that re-instills doubt for me.

Another problem with this approach versus using UIImage(contentsOfFile) is that it's harder to use the memory report's overall memory footprint to scrutinise your application besides what the font cache is doing. Perhaps there's a way to separate these out, but I don't know of one.