sizeWithFont:constrainedToSize:lineBreakMode的使用

	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;