Ios boundingRectForGlyphRange始终返回字体大小为12的rect

我正在尝试使用

boundingrectforglyprange

方法计算单个glyph的矩形。一切正常，只是它总是返回rect，就好像字体大小是12一样。事件，但我在属性字符串中将其设置为20

let font = UIFont.systemFontOfSize(20)
var attributedString = NSMutableAttributedString(string: text)
let range = NSRange(location: 0, length: countElements(text))
attributedString.addAttribute(NSFontAttributeName, value: font, range: range)


let textContainer = NSTextContainer()
textContainer.size = CGSize(width: 300, height: 100)
let layoutManager = NSLayoutManager()
let textStorage = NSTextStorage(string: attributedString.mutableString)

layoutManager.textStorage = textStorage
layoutManager.addTextContainer(textContainer)

let rect = layoutManager.boundingRectForGlyphRange(NSRange(location: 0, length: 1), inTextContainer: textContainer)

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如何使文本大小为20而不是12时返回rect？

只需使用

attributedString:

初始化

NSTextStorage

let textStorage = NSTextStorage(attributedString: attributedString)

而不是

let textStorage = NSTextStorage(string: attributedString.mutableString)