coretext绘制 个人理解为
一个CTFrame有几个CTLine组成，有几行文字就有几行CTLine。一个CTLine有包含多个CTRun，一个CTRun是所有属性都相同的那部分富文本的绘制单元。所以CTRun是CTFrame的基本绘制单元。
资料博客链接地址:http://www.jianshu.com/p/6db3289fb05d
计算绘制的coreText内容的高度
+ (int)getAttributedStringHeightWithString:(NSAttributedString *)string WidthValue:(int)width
{
int total_height = 0;
CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)string); //string 为要计算高度的NSAttributedString
CGRect drawingRect = CGRectMake(0, 0, width, 1000); //这里的高要设置足够大
CGMutablePathRef path = CGPathCreateMutable();
CGPathAddRect(path, NULL, drawingRect);
CTFrameRef textFrame = CTFramesetterCreateFrame(framesetter,CFRangeMake(0,0), path, NULL);
CGPathRelease(path);
CFRelease(framesetter);
NSArray *linesArray = (NSArray *) CTFrameGetLines(textFrame);
CGPoint origins[[linesArray count]];
CTFrameGetLineOrigins(textFrame, CFRangeMake(0, 0), origins);
int line_y = (int) origins[[linesArray count] -1].y; //最后一行line的原点y坐标
CGFloat ascent;
CGFloat descent;
CGFloat leading;
CTLineRef line = (__bridge CTLineRef) [linesArray objectAtIndex:[linesArray count]-1];
CTLineGetTypographicBounds(line, &ascent, &descent, &leading);
total_height = 1000 - line_y + (int) descent +1; //+1为了纠正descent转换成int小数点后舍去的值
CFRelease(textFrame);
return total_height;
}