NSLayoutManager drawGlyphs endless loop when UITextView isScrollingEnabled is false

Question

I'm having a custom NSLayoutManager with these two methods overwritten:

override func drawGlyphs(forGlyphRange glyphsToShow: NSRange, at origin: CGPoint) {
    super.drawGlyphs(forGlyphRange: glyphsToShow, at: origin)

    let characterRange = self.characterRange(forGlyphRange: glyphsToShow, actualGlyphRange: nil)
    textStorage?.enumerateAttribute(.blur, in: characterRange, options: .longestEffectiveRangeNotRequired, using: { (value, subrange, _) in
        guard let key = value as? String, !key.isEmpty else { return }
        let blurGlyphRange = glyphRange(forCharacterRange: subrange, actualCharacterRange: nil)
        drawBlur(forGlyphRange: blurGlyphRange)
        textStorage?.addAttributes([NSAttributedString.Key.foregroundColor: UIColor.clear], range: blurGlyphRange)
    })
}

private func drawBlur(forGlyphRange tokenGlypeRange: NSRange) {
    guard let textContainer = textContainer(forGlyphAt: tokenGlypeRange.location, effectiveRange: nil) else { return }
    let withinRange = NSRange(location: NSNotFound, length: 0)
    enumerateEnclosingRects(forGlyphRange: tokenGlypeRange, withinSelectedGlyphRange: withinRange, in: textContainer) { (rect, _) in
        let blurRect = rect.offsetBy(dx: self.textContainerOriginOffset.width, dy: self.textContainerOriginOffset.height)
        UIColor.red.setFill()
        UIBezierPath(roundedRect: blurRect, cornerRadius: 4).fill()
    }

Everything works fine except when I set the UITextView isScrollingEnabled on false I enter an endless loop caused by the textStorage enumerateAttribute method in drawGlyphs.

I don't understand why this happens and also I don't know how to prevent this. Someone who knows more about this?

EDIT

If I remove the textStorage addAttributes with the foregroundColor then it works. So that's causing the loop for some reason.

Sorry if it was not correct. But for me this is another problem. This works only the problem is with the endless loop when isScrollingEnabled is false. I thought if I edit again and again it will become polluted. — user1007522, Mar 16 '20 at 20:23
I understand but we were working out an answer to your question and you just deleted. Instead of letting me turn my comment into an answer and completing the Q&A cycle, you _used_ what my comment suggested but deleted the whole question so that it's impossible to get credit for it. It feels like theft. :) — matt, Mar 16 '20 at 20:36

score 1 · Answer 1 · answered Mar 19 '20 at 09:22

I found the problem why it comes in and endless loop. The textstorage updates/add the attribute and then notifies the layoutmanager again.

Solution is to create your own textstorage like this:

class CustomTextStorage: NSTextStorage {
  private let backingStore = NSMutableAttributedString()

  override var string: String {
    return backingStore.string
  }

  override init() {
    super.init()
  }

  required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
  }

  override func attributes(at location: Int, effectiveRange range: NSRangePointer?) -> [NSAttributedString.Key: Any] {
      return backingStore.attributes(at: location, effectiveRange: range)
  }

  override func replaceCharacters(in range: NSRange, with str: String) {
    beginEditing()
    backingStore.replaceCharacters(in: range, with:str)
    edited(.editedCharacters, range: range, changeInLength: (str as NSString).length - range.length)
    endEditing()
  }

  override func setAttributes(_ attrs: [NSAttributedString.Key: Any]?, range: NSRange) {
    beginEditing()
    backingStore.setAttributes(attrs, range: range)
    if let attrs = attrs, let _ = attrs[.blur] {
        backingStore.addAttribute(.foregroundColor, value: UIColor.clear, range: range)
    }
    edited(.editedAttributes, range: range, changeInLength: 0)
    endEditing()
  }
}

and remove the line under the draw method in the layoutmanager:

textStorage?.addAttributes([NSAttributedString.Key.foregroundColor: UIColor.clear], range: blurGlyphRange)

NSLayoutManager drawGlyphs endless loop when UITextView isScrollingEnabled is false

1 Answers1