IOS 判断字符串是否为空

//判断字符串是否为空
-(BOOL)isEmpty:(NSString *)str
{
    if(!str)
    {
        return true;
    }
    else
    {
        NSCharacterSet *set=[NSCharacterSet whitespaceAndNewlineCharacterSet];
        NSString *trimedString=[str stringByTrimmingCharactersInSet:set];
        if([trimedString length]==0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
}