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04) The fflib_Application Class
Quality question... I mean honestly wtf is this thing? Lol, sorry, let's figure it out together. The fflib_Application class is around for two primary purposes. The first is to allow you an extremely abstract way of creating new instances of your unit of work, service layer, domain layer and selector layer in the Apex Common Library through the use of the factory pattern. The second is that implementing this application class is imperative if you want to leverage the Apex Mocks unit testing library. It depends on this Application Factory being implemented.
Most importantly though, if you understand how interfaces, inheritance and polymorphism work implementing this class allows you to write extremely abstract Salesforce implementations, which we'll discuss more in sections below
Ok, if we ignore the fact that this is required for us to use the Apex Mocks library, understanding the power behind this class requires us to take a step back and formulate a real world Salesforce use case for implementing it... hopefully the following one will be easy for everyone to understand.
Say for instance I have a decent sized Salesforce instance and our business has a use case to create tasks across multiple objects and the logic for creating those tasks are unique to every single object. Maybe on the Account object we create three new tasks every single time we create an account and on the Contact object we create two tasks every single time a record is created or updated in a particular way and we ideally want to call this logic on the fly from anywhere in our system. Well, no matter what we should probably place the task creation logic in our domain layer because it's relevant to each individual object, but pretend we have 35 places in the code that need to execute this logic and