Monday, 9 July 2018

Reasoning with SOLID Principles: Single Responsibility

The SOLID Principles are a great tool to help you learn object oriented principles, but after trying to apply them for quite some time I think there are definite boundaries to when and where they should be applied.

I'll break this into parts! here is part 1!

Single Responsibility Principle

Single responsibility is a great tool for quickly noticing when you've got too much stuff in your stuff, if its obvious that there are two very different responsibilities in an object it can be worth separating them to make things clearer. The issues you hit with this really as you have to be careful about thinking what abstraction level you are reasoning about when considering this.

Say I have an Order object, my order object contains things that are order related, perhaps and ID for the order and some methods to update the order and send the order. But if someone ended up adding a method that draws an alert to the UI, this perhaps would stand out.

get_order_id()
update_order()
get_order_items()
draw_ui_alert()
I wouldn't normally name like this, just trying to make it clear that all the methods apart from the draw_ui_alert clearly relate to the order.

Perhaps I add a method to print the order, to start this method is small and just outputs the order id, this makes sense within the SRP right? The responsibility of the object is to manage the order, the responsibility of the print function is to print the order. We can see that if we were to make the print function also add a new item to the order that this could be a violation of the principle. But what about when the print method grows very large because it also includes a lot of code that describes how printing works, not necessarily related to the printing of the order.

Is there two responsibilities there sounds like it, but when that code was smaller it didn't feel like there was, so surely the principle has to be used in conjunction with balancing the size of things. So now we look at our print method the first line initialises a printer object... well thats a single responsibility depending on the abstraction level we are thinking about right, being responsible for adding a and b together could be a single responsibility. I'm not hating on the principle, just that it sounds like a simple rule but in practice is much more a great tool for reasoning about if something does too much, or how to split it when it is too large.

I guess for quite a few of the principles that is the key, knowing when to apply them and how. But also just using them as thought tools :D

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