ios - 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.

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("????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????").map {String($0)}

class CollectionViewTestController: UICollectionViewController {
    override func viewDidLoad() {
        collectionView?.registerClass(Cell.self, forCellWithReuseIdentifier: cellId)
    }

    override func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return data.count
    }

    override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(cellId, forIndexPath: indexPath) as! Cell
        if cell.label.superview == nil {
            cell.label.frame = cell.contentView.bounds
            cell.contentView.addSubview(cell.label)
            cell.label.font = UIFont.systemFontOfSize(34)
        }
        cell.label.text = data[indexPath.item]
        return cell
    }

    override func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        return 1
    }
}

class Cell: UICollectionViewCell {
    private let label = UILabel()
}

After running and scrolling the UICollectionView I get memory usage graph like this:

Answer 1

I ran into the same issue and fixed it by dumping the .png from /System/Library/Fonts/Apple Color Emoji.ttf and using UIImage(contentsOfFile: String) instead of a String.

I used https://github.com/github/gemoji to extract the .png files, renamed the files with @3x suffix.

func emojiToHex(emoji: String) -> String {
    let data = emoji.dataUsingEncoding(NSUTF32LittleEndianStringEncoding)
    var unicode: UInt32 = 0
    data!.getBytes(&unicode, length:sizeof(UInt32))
    return NSString(format: "%x", unicode) as! String
}

let path = NSBundle.mainBundle().pathForResource(emojiToHex(char) + "@3x", ofType: "png")
UIImage(contentsOfFile: path!)

UIImage(contentsOfFile: path!) is properly released so the memory should stay at a low level. So far my keyboard extension hasn't crashed yet.

If the UIScrollView contains a lot of emoji, consider using UICollectionView that retains only 3 or 4 pages in cache and releases the other unseen pages.