iOS: UITextView with custom NSTextStorage crashes

Question

I am writing an iOS 9 application in Swift. I have a view controller that hosts a UITextView and I am assigning a custom NSTextStorage object to it. The NSTextStorage is just basic right now. Everything works correctly and the text shows up in the UITextView, but when I tap on the text to edit it, the application crashes with the following exception:

2016-01-10 11:24:32.931 PagesWriter[23750:6939530] requesting caretRectForPosition: with a position beyond the NSTextStorage (529)
2016-01-10 11:24:33.040 PagesWriter[23750:6939530] requesting caretRectForPosition: with a position beyond the NSTextStorage (529)
2016-01-10 11:24:33.168 PagesWriter[23750:6939530] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSCFString _getBlockStart:end:contentsEnd:forRange:stopAtLineSeparators:]: Range {529, 0} out of bounds; string length 245'
*** First throw call stack:
(0x181ea5900 0x181513f80 0x181ea5848 0x18278a02c 0x1827e963c 0x186cc6124 0x186b1c818 0x186cc5fbc 0x186d59e68 0x186bdeb60 0x186bde454 0x186fac1f8 0x187501f64 0x186bc2e84 0x186bc4f40 0x186fa65ac 0x186faea5c 0x186bc5c18 0x186bbf1f8 0x186bbed2c 0x186c2047c 0x186c20828 0x186d5811c 0x186d57f94 0x186d57448 0x187118dbc 0x186d3c5b8 0x186bca9b0 0x18711a3bc 0x186b89b58 0x186b868dc 0x186bc8820 0x186bc7e1c 0x186b984cc 0x186b96794 0x181e5cefc 0x181e5c990 0x181e5a690 0x181d89680 0x183298088 0x186c00d90 0x1000615ec 0x18192a8b8)
libc++abi.dylib: terminating with uncaught exception of type NSException

If I remove my custom NSTextStorage object from the UITextView, it works.

My view controller gets loaded using a modal segue in the storyboard. I load the text content and configure the UITextView and NSTextStorage objects in the viewDidLoad method:

class TextEditorViewController: UIViewController {
  @IBOutlet weak var textView: UITextView!

  // Set by parent view controller during transition
  var URL: NSURL?

  var textStorage: NSTextStorage?

  override func viewDidLoad() {
    super.viewDidLoad()

    textStorage = CustomTextStorage()
    textView.textStorage.removeLayoutManager(textView.layoutManager)
    textStorage!.addLayoutManager(textView.layoutManager)
    dispatch_async(UtilityQueue) {
      do {
        let text = try NSString(contentsOfURL: self.URL!, encoding: NSUTF8StringEncoding)
        dispatch_async(MainQueue) {
          self.textStorage!.replaceCharactersInRange(NSRange(location: 0, length: 0), withString: text as String)
        }
      } catch { /* handle error */ }
    }
  }
}

UtilityQueue and MainQueue are helper contents that hold a reference to the corresponding dispatch queue.

My custom NSTextStorage class looks like this:

class CustomTextStorage: NSTextStorage {
  let backingStore = NSMutableAttributedString()

  override var string: String {
    return backingStore.string
  }

  override func attributedAtIndex(location: Int, effectiveRange range: NSRangePointer) -> [String: AnyObject] {
    return backingStore.attributesAtIndex(location, effectiveRange: range)
  }

  override func replaceCharactersInRange(range: NSRange, withString str: String) {
    backingStore.replaceCharactersInRange(range, withString: str)
    edited(.EditedCharacters, range: range, changeInLength: str.characters.count - range.length)
  }

  override func setAttributes(attires: [String: AnyObject]?, range: NSRange) {
    backingStore.setAttributes(attires, range: range)
    edited(.EditedAttributes, range: range, changeInLength: 0)
  }

  override func processEditing() {
    /* Placeholder; will be adding code here in the future. */
    super.processEditing()
  }
}

Any ideas? Thank you in advance for your assistance.

Answer 1

I don't know if it is still relevant, but you are almost right.

You need to call beginEditing() and endEditing() respectively.

Cast String to NSString in length calculation for editing callback (otherwise emoji and some other cases will not work properly).

Also please use NSTextStorage as a backingStore because of performance issues.

You will get something like this:

final class ExampleStorage : NSTextStorage {

    private let container = NSTextStorage()

    override var string: String {
        return container.string
    }

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

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

    override func setAttributes(_ attrs: [NSAttributedStringKey : Any]?, range: NSRange) {
        beginEditing()
        container.setAttributes(attrs, range: range)
        edited([.editedAttributes], range: range, changeInLength: 0)
        endEditing()
    }
}