如何在ios 7中加载到webview之前计算html字符串的高度？

NSAttributedString *text = [[NSAttributedString alloc] initWithData:[html dataUsingEncoding:NSUTF8StringEncoding]
                                                            options:@{NSDocumentTypeDocumentAttribute : NSHTMLTextDocumentType,NSCharacterEncodingDocumentAttribute : @(NSUTF8StringEncoding)}
                                                 documentAttributes:nil
                                                              error:nil]; // make sure to check for error in a real app

// Then get the bounding rect based on a kNown width
CGRect textFrame = [text boundingRectWithSize:CGSizeMake(someKNownWidth,CGFLOAT_MAX)
                                      options:NsstringDrawingUsesLineFragmentOrigin | NsstringDrawingUsesFontLeading
                                      context:nil];

CGFloat height = textFrame.size.height;

我没有尝试过各种HTML,所以YMMV.