IOS计算文字高度

   NSString* str = @"你好";

　const float kFontSize = 18.f;

    NSStringDrawingOptions options =  NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading;

    CGRect rect = [str boundingRectWithSize:CGSizeMake(MAXFLOAT,MAXFLOAT) options:options attributes:@{NSFontAttributeName:[UIFont systemFontOfSize:kFontSize]} context:nil];

    CGFloat realHeight = ceilf(rect.size.height);