(CGSize)sizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size lineBreakMode:(NSLineBreakMode)lineBreakMode

UIFont *font = [UIFont systemFontOfSize:12];

CGSize size = [text sizeWithFont:font constrainedToSize:CGSizeMake(150.0f, 1000.0f) lineBreakMode:UILineBreakModeCharacterWrap];

获取字符串在指定的size内(宽度超过150,则换行)所需的的实际高度和宽度. 
将获取的宽度和高度用于UILabel 
UILabel *bubbleText = [[UILabel alloc] initWithFrame:CGRectMake(21.0f, 14.0f, size.width+10, size.height+10)];

bubbleText.backgroundColor = [UIColor clearColor];

bubbleText.font = font;

bubbleText.numberOfLines = 0;

bubbleText.lineBreakMode = UILineBreakModeCharacterWrap; 

bubbleText.text = text;