iOS中自定义UILabel控件，实现一些常用的效果

1、单行左右对齐

方法：通过计算字符串占用的宽度来给定label控件的字间距

-(NSMutableAttributedString*)returnAttributedString:(NSString*)string
{
    //添加特殊属性字符串
    NSMutableAttributedString *attString = [[NSMutableAttributedString alloc]initWithString:string];
    
    //计算宽度
    CGSize size = [string boundingRectWithSize:CGSizeMake(self.frame.size.width, self.font.pointSize) options:NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName:self.font} context:nil].size;
    
    //需要添加字间距使两边对齐
    if (size.width