How to clear font cache filled with emoji characte

问题:

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.

When this emoji UIScrollView is scrolled the memory usage increases and does not drop down. I am reusing cells correctly and when testing with single emoji character displayed 800 times the memory does not increase during scrolling.

Using instruments I found that there is no memory leak in my code but it seems that the emoji glyphs are cached and can take around 10-30MB of memory depending on font size (reseach shows they are actually PNGs). Keyboard extensions can use little memory before they are killed. Is there a way to clear that font cache?

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Edit

Adding code example to reproduce the problem:

let data = Array("