在一次开发过程中，发现带有行间距的富文本计算高度，会有不准确的情况，富文本内容明明很长，但是计算出的高度只有不到20像素，导致整个cell的高度计算异常。

需求上是文字固定宽度，最多显示3行，超过3行尾部打点展示。按照需求设置了尾部打点 paraStyle.lineBreakMode = .byTruncatingTail， 然后计算富文本的大小。

let range = NSRange(location: 0, length: introduce.count)
var att = NSMutableAttributedString(string: introduce)
let paraStyle = NSMutableParagraphStyle()
paraStyle.lineSpacing = 4
att.addAttribute(.font, value: UIFont.systemFont(ofSize: 14), range: range)
// 计算内容高度
let screenWidth = UIScreen.main.bounds.size.width
let maxSzie = CGSizeMake(screenWidth-40, CGFLOAT_MAX)

// 换行模式byClipping
paraStyle.lineBreakMode = .byTruncatingTail
att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
print("使用byTruncatingTail计算size ", size)

这时候算出来的size高度只有17像素，真实显示到屏幕上Label的高度是58像素。导致整体cell的高度计算错误。

后来发现先将换行模式改为paraStyle.lineBreakMode = .byWordWrapping 在计算高度是正确的。于是把所有的换行模式都逐个测试，看看到底是什么情况。

func getTextAttributed(_ introduce: String) -> NSAttributedString {

    let range = NSRange(location: 0, length: introduce.count)
    var att = NSMutableAttributedString(string: introduce)
    let paraStyle = NSMutableParagraphStyle()
    paraStyle.lineSpacing = 4
    att.addAttribute(.font, value: UIFont.systemFont(ofSize: 14), range: range)
    // 计算内容高度
    let screenWidth = UIScreen.main.bounds.size.width
    let maxSzie = CGSizeMake(screenWidth-40, CGFLOAT_MAX)


    // 换行模式byWordWrapping
    paraStyle.lineBreakMode = .byWordWrapping
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    var size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byWordWrapping计算size ", size)


    // 换行模式byCharWrapping
    paraStyle.lineBreakMode = .byCharWrapping
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byCharWrapping计算size ", size)

    // 换行模式byClipping
    paraStyle.lineBreakMode = .byClipping
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byClipping计算size ", size)


    // 换行模式byTruncatingHead
    paraStyle.lineBreakMode = .byTruncatingHead
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byTruncatingHead计算size ", size)


    // 换行模式byClipping
    paraStyle.lineBreakMode = .byTruncatingTail
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byTruncatingTail计算size ", size)


    // 换行模式byClipping
    paraStyle.lineBreakMode = .byTruncatingMiddle
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byTruncatingMiddle计算size ", size)

    return att
}

测试后发现，系统的5个枚举中，只有 byWordWrapping和byCharWrapping 计算准确，带有截断方式的case，计算都是错误的。

• case byWordWrapping = 0 // Wrap at word boundaries, default
• case byCharWrapping = 1 // Wrap at character boundaries
• case byClipping = 2 // Simply clip
• case byTruncatingHead = 3 // Truncate at head of line: "...wxyz"
• case byTruncatingTail = 4 // Truncate at tail of line: "abcd..."
• case byTruncatingMiddle = 5 // Truncate middle of line:  "ab...yz"

好吧，系统这个样子，我们也没有办法，最后采用的方式

1. 先用 byWordWrapping 计算高度
2.  高度计算完成，在修改换行模式，paraStyle.lineBreakMode = .byTruncatingTail 
3. 返回富文本

func getTextAttributed(_ introduce: String) -> NSAttributedString {

    let range = NSRange(location: 0, length: introduce.count)
    let att = NSMutableAttributedString(string: introduce)
    let paraStyle = NSMutableParagraphStyle()
    paraStyle.lineSpacing = 4
    att.addAttribute(.font, value: UIFont.systemFont(ofSize: 14), range: range)
    // 计算内容高度
    let screenWidth = UIScreen.main.bounds.size.width
    let maxSzie = CGSizeMake(screenWidth-40, CGFLOAT_MAX)

    // 换行模式byWordWrapping
    paraStyle.lineBreakMode = .byWordWrapping
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    var size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    print("使用byWordWrapping计算size ", size)

    // 换行模式byTruncatingTail
    paraStyle.lineBreakMode = .byTruncatingTail
    att.addAttribute(.paragraphStyle, value: paraStyle, range: range)
    // size = att.boundingRect(with: maxSzie, options: [.usesLineFragmentOrigin,.usesFontLeading], context: nil).size
    // print("使用byTruncatingTail计算size ", size)

    return att
}