let label:UILabel = UILabel(frame: CGRectMake(x, y, width, height))
label.numberOfLines = 4
label.lineBreakMode = NSLineBreakMode.ByWordWrapping
let font = UIFont(name: "Helvetica", size: 20.0)
label.font = font
label.text = "Whatever you want the text enter here"
label.sizeToFit()

如果要根据文本内容设置numberOfLines，请提供最大行数.这在这里非常重要.

If you want to set numberOfLines according to the content of text,give your maximum lines.That is very important here.