首先并且最重要的是计算文本高度，然后再设置label的高度，
计算文本高度：
假设文本：
NSString*strtest =@"
新华网北京12月16日电据新华社“新华国际”客户端报道，“一日之计在于晨”。克强总理15日的紧张日程从一顿早饭开始。天刚蒙蒙亮，哈萨克斯坦总理马西莫夫就来到克强总理下榻的酒店——北京大厦，邀请克强总理一起吃早餐。这是一顿简单的早餐，只有一张圆桌，两把椅子，一些简单的中西餐点。这也是一顿丰盛的早餐，两位总理在餐桌上就中哈产能合作达成的共识内容丰富，覆盖广泛。这更是一顿登高望远的早餐，这项合作不仅将惠及中哈两国人民，更将为区域内各国带来长远的共同利益。站在大厦23层的旋转餐厅，阿斯塔纳城尽收眼底，这里适合登高远眺。";
事实上ios 有两种方法：
ios7.0之前用：
[strtestsizeWithFont:ContentFontconstrainedToSize:CGSizeMake(ScreenWeight
-20, 1000) lineBreakMode:NSLineBreakByWordWrapping];
之后用下面的方法比较好：
CGRect frame = [strtestboundingRectWithSize:CGSizeMake(220,1000)
options:NSStringDrawingUsesLineFragmentOriginattributes:@{NSFontAttributeName:[UIFontsystemFontOfSize:10]}context:nil];
attributes参数有：
NSFontAttributeName // UIFont, default Helvetica(Neue) 12
NSParagraphStyleAttributeName // NSParagraphStyle, default defaultParagraphStyle
NSForegroundColorAttributeName // UIColor, default blackColor
NSBackgroundColorAttributeName // UIColor, default nil: no background
NSLigatureAttributeName // NSNumber containing integer, default 1: default ligatures, 0: no ligatures
NSKernAttributeName // NSNumber containing floating point value, in points; amount to modify default kerning. 0 means kerning is disabled.
NSStrikethroughStyleAttributeName ; // NSNumber containing integer, default 0: no strikethrough
NSUnderlineStyleAttributeName // NSNumber containing integer, default 0: no underline
NSStrokeColorAttributeName // UIColor, default nil: same as foreground color
NSStrokeWidthAttributeName // NSNumber containing floating point value, in percent of font point size, default 0: no stroke; positive for stroke alone, negative for stroke
and fill (a typical value for outlined text would be 3.0)
NSShadowAttributeName ; // NSShadow, default nil: no shadow
NSTextEffectAttributeName // NSString, default nil: no text effect
NSAttachmentAttributeName // NSTextAttachment, default nil
NSLinkAttributeName // NSURL (preferred) or NSString
NSBaselineOffsetAttributeName // NSNumber containing floating point value, in points; offset from baseline, default 0
NSUnderlineColorAttributeName // UIColor, default nil: same as foreground color
NSStrikethroughColorAttributeName // UIColor, default nil: same as foreground color
NSObliquenessAttributeName // NSNumber containing floating point value; skew to be applied to glyphs, default 0: no skew
NSExpansionAttributeName // NSNumber containing floating point value; log of expansion factor to be applied to glyphs, default 0: no expansion
建立label的时候注意：
① label的font，必须和计算时用的font一样
② label的numberOfLines要设置为零
③ label的宽度设置成计算时设置的宽度一致