iOS:UITextView与自定义NSTextStorage崩溃

我正在Swift中编写一个iOS 9应用程序.我有一个托管UITextView的视图控制器,我正在为它分配一个自定义的NSTextStorage对象.NSTextStorage现在是基本的.一切正常,文本显示在UITextView中,但是当我点击文本进行编辑时,应用程序崩溃时出现以下异常:

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

如果我从UITextView中删除我的自定义NSTextStorage对象,它的工作原理.

我的视图控制器使用故事板中的模态segue加载.我加载文本内容并在viewDidLoad方法中配置UITextView和NSTextStorage对象:

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并且MainQueue是包含对相应调度队列的引用的帮助程序内容.

我的自定义NSTextStorage类看起来像这样:

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()
  }
}

有任何想法吗？提前感谢您的协助.