October 20, 2020
Factory Pattern
Unity
Title: Creating Objects in Unity3D using the Factory Pattern By: Jason Weimann Youtube – Tutorial Description: Introductory tutorial to the concept of fatories and the factory pattern in OOP using Unity.
Overview
This tutorial quickly covers the basic idea behind using factories and the factory pattern in OOP through an example in Unity. The overall concept is pretty simple in that it focuses on using an object to create other objects, but they explore putting this idea to use as well as some of the pros and cons of its use.
Notes
Factory: in OOP, an object for creating other objects; a function or method that returns objects of a varying prototype or class (from Wikipedia)Some of the benefits noted are that this allows you to put all of your object creation into a single place and that you can extend it quite a bit without modifying the underlying logic (following the Open Closed Principle). You do not even need to know the exact type you are expecting to return with the factory since you can return interfaces or abstract classes as well to cover large groups of object types.
Basic Example
In the examples covered, their common practice is to create an abstract class for the type of objects they will want their factory to return. Then any of the objects you want that factory to be responsible for simply inherit from that abstract class. Then the factory class itself is created that uses some method to identify which specific objects to create and uses that information to return the desired objects.
Advanced Example
While not exactly advanced, this example is more in the direction of something that you would actually use. Again the abstract class is created to encompass the objects the factory will return, but this time a Name string with only a getter method is added to this class to help with identification and selection later in the process.
The factory class is much more involved for this example. They create a dictionary that uses the Name values as the keys and the various specific types of the different objects as the values. They then use an Assembly reference to get a reference to each of the different classes inheriting from the initial abstract class throughout the section of the project they are interested in. Using these together they fill the dictionary with all these type references by going through the list gotten from the Assembly reference and adding them to the dictionary.
They then have a method within the factory class to Get an object from it. This requires a string parameter (as this is what is used for the example as the keys for the dictionary created) and uses that to check if that option exists in the dictionary, and if so it creates and returns an instance of that specific type of object.
Final Example
This final example is closest to something they would actually use in a real world situation. The major difference being that they make the factory class into a static class. They use this approach because they do not want to create a new factory everytime they use it, and they did not want to use a singleton pattern because it does not need instantiated in Unity in anyway. This helps make it accessible for anything else to call into it.
While it is not a singleton in and of itself, it does use some similar redundancy approaches to make sure multiple are not created. It has an isInitialized check so that it is only created if it is used at all, and then afterwards it will not create another of itself. It is a bit over redundant, but it works for a quick coverage of the concept in this situation.
Extending the Pattern
One major benefit of this pattern over very basic ways of creating objects covered is how it follows the open closed principle. This is shown after the advanced example by adding another type of object for the factory to return after everything has been built. They create a new class of object again inheriting from that initial abstract class and show that the factory picks up on this and includes it in its options and creates the new object without ever accessing the factory class again. This was cleanly done by just adding its own new class and not modifying any of the previous classes.
Summary
While a very basic concept overall, breaking it down and covering its pros and cons did help me more clearly understand how to look at designing and developing systems to create objects in OOP. This also seems like a nice simple approach to starting to use systems for projects that are not very tiny in size at least, and could be more useful for larger projects when you have a better understanding of how to utilize it in a more complex manner. On a side note this also exposed me to the open closed principle which makes a lot of sense as something to strive for when looking to create more stable projects.