I've been driving myself crazy for hours trying to figure this one out, and I'm not moving anywhere with it.
I'm creating a checkout till for a cashier, specifically I need to sum the items, then apply the promotional discounts. I'm trying to do it without violating any design principles (impossible, I know, I can let things slide when it makes sense).
Promotional discounts could be anything, from a black friday deal flat discount, to 'Orders over £100 save 10%' to '3 for 2 on these items', or 'Buy at least two cans of coke, and the price for them all drops to £0.50!'
I cannot see how to fit the promotional deals in. Each may require a different set of data from different locations. For instance one of the big problems being the '3 for 2' deal. Getting access to the items in the Checkout has been a plague on my mind.
So far, my best approach has been to use the Decorator pattern. Wrap the checkout up in a bunch of promotional deals when the price is calculated, as each decorator holds an instance of the checkout, we'll have access to the orginal checkout with the list of items.
In the future, the only thing I'd need to do is write the new rule, add it to the factory, and update any DB data which is perfect, the minimal change.
This kind of works. I can justify it in my head that it's still a checkout, and therefore all the rules being able to access the checkout make sense, gives me a nice way to chain the discount. But there is a problem, in that I'm sure I shouldn't be using it the way I'm suggesting. For instance, if one of the promotions wears off, you shouldn't really 'unwrap' it, and realistically while it's nice to be able to add promotions dynamically to extend an instance, it's not necessary.
I've read through more design patterns but can't seem to find anything that applies. I saw the following article: https://levelup.gitconnected.com/rules-design-pattern-in-c-6c62f0e20ee0
This is basically what I want to do, but the implementation feels clumsy to me.
public bool IsValid(FileInfo fileInfo)
{
var rules = new List<IFileValidationRule> { new FileExtensionRule(new string[] { "txt", "html" }) };
if (AdminConfig.CheckFileSize)
{
rules.Add(new FileSizeRule("txt", 5 * 1024 * 1024));
rules.Add(new FileSizeRule("html", 10 * 1024 * 1024));
}
if (User.Status != UserStatus.Premium)
{
rules.Add(new MaxFileLengthRule(50));
}
bool isValid = rules.All(rule => rule.IsValid(fileInfo));
return isValid;
}
Specifically this part, seems to violate a few key principles, Open-Closed principle, Dependency Inversion, etc.
The other big problem I can't wrap my head arround is as below: Imagine for the above example, a new rule needs to be added that reads the file data, check if there are any bad characters in there, doesn't matter what. Implementing this is easy, you inject the file or the file data, whatever into the 'FileValidator' class, you then instantiate your rule and pass the file data into it. You then run the rule and return the success, great! But is this ok?
Reading this says no: http://wiki.c2.com/?TellDontAsk
"Tell, don't ask" - It is okay to use accessors to get the state of an object, as long as you don't use the result to make decisions outside the object.
That would be exactly what the code is doing! I guess the alternative to this is to update the 'FileData' object to essentially take a list of bad characters, check the file data, return false to the rule, which then fails the whole process, but this would start to throw a bunch more rules out the door. You're now breaking the Open-Closed principle, Single Responsibility principle, and it feels like you're building a rod for your own back, adding these custom methods for singular rules, bloating your object. (The link does discuss how you can pass a function into the method, which is pretty nice, but still not perfect, at the end of the day, aren't you just indirectly handing control to the caller?)
The above alone wouldn't be enough to stop me, but I'm struggling to justify making a private set of items public so one rule out of the bunch can make use of that data.
I'm in OOP recursion, tumbling towards a stack overflow. Can anyone pull me out and help me consilidate my thoughts? None of the design patterns seem to work but I'm sure this is a basic problem solved many times in the past. What am I missing?
