Step 4 – Practice!
This stage is harder than the last. You need to make sure:
Each child class should exactly reflect the abstract methods. If your calling code ever cares which sub-class it is calling by using strange parameters or converting the type then you are violating LSP – the Liskov substitution principle – The L of solid.
Each child class should have something relevant to do in the abstract classes. If it has methods that make no sense this is a violation of the interface segregation principle.
Step 5 – Finish SOLID
Read about the open-closed principle and the dependency inversion principle and try it in a couple of sections of code.
Open-closed basically means that you leave interfaces (abstract classes in LabVIEW). Then you can change the behavior by creating a new child class (open for extension) without have to modify the original code (closed to modification). This goes well with the dependency inversion principle. This says that higher level classes should depend only on interfaces (again abstract classes). This means the lower level code implements these classes and so the high-level code can call the lower level code without a direct dependency.
This goes well with the dependency inversion principle. This says that higher level classes should depend only on interfaces (again abstract classes). This means the lower level code implements these classes and so the high-level code can call the lower level code without a direct dependency. This can help in places where coupling is difficult to design out.
I leave these principles to the end because I think they are the easiest to write difficult to read code. I’m still trying to find a balance with these – following them wholeheartedly creates lots of indirection which can affect readability. I also think we don’t get as much benefit in LabVIEW with these since we don’t tend to package code within projects in the same way as other languages. (this maybe a good topic for another post!)
Step 6 – Learn some design patterns
This was obviously part of the point of this article. When I came back to design patterns after understanding design better and the SOLID principles it allowed me to look at the patterns in a different way. I could relate them to the principles and I understood what problems they solved.
For example, the command pattern (where you effectively have a QMH which takes message classes) is a perfect example of a solution to meet the open-closed principle for an entire process. You can extend the message handler by adding support for new message types by creating new message classes instead of modifying the original code. This is how the actor framework works and has allowed the developers to create a framework where they have a reliable implementation of control of the actors but you can still add messages to define the functionality.
Once you understand why these design patterns exist you can then apply some critical thinking about why and when to use them. I personally dislike the command pattern in LabVIEW because I don’t think the additional overhead of a large number of message classes is worth the benefit of being able to add messages to a QMH without changing the original code.
I think this will help you to use them more effectively and are less likely to end up with a spaghetti of design patterns thrown together because that is what everyone was talking about.
Urmm… so what do I do?
So I know this doesn’t have the information you need to actually do this so much as set out a program. Actually, all the steps still follow the NI course on OOP so you could simply self-pace this for general learning material.