准确计算CoreText高度的方法：

- (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 = (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;

}