Today we are going to talk about one of the most confusing topics of all and see if we can unravel the mess of Dependency Inversion, Inversion of Control and Dependency Injection.
It’s not completely important that we understand the specifics of each of these names, because many people end up using them interchangeably. It’s pretty unlikely that we are going to correct all that in this blog post.
What is important is that we understand the concepts and ideas behind each of these topics and understand what architectural problems each one is trying to solve.
Today, we are going to focus on Dependency Inversion and get a little bit into inversion of control.
Dependency inversion is the root
If you have the time to read through that article, do yourself a favor and do it. You don’t need to know much C++ to understand it, and it really sheds a light on the problem Bob was trying to solve.
Bob talks about this principle in his excellent book, “Agile Software Development, Principles, Patterns, and Practices, and Agile Principles, Patterns, and Practices in C#”
This principle is actually very simply stated:
- High-level modules should not depend on low-level modules. Both should depend on abstractions.
- Abstractions should not depend upon details. Details should depend upon abstractions.
I think this concept is easily misunderstood and misapplied, because the reason why we apply this principle is often neglected.
If you are familiar with IoC and Dependency Injection, you can see how both are based from this definition of Dependency Inversion.
What problem does dependency inversion solve?
Dependency inversion solves the problem of higher level modules being dependent on and coupled to the interfaces of lower level modules and their details.
Let me give you a real world example of a current problem that could be solved by dependency inversion.
Take a look around your house and count up all the devices that have batteries that must be charged somehow.
- Digital camera
- Cell phone
- Camcorder (flip cam)
- Wireless headphones
- Game controllers
What do all of these things have in common? They don’t have a charging interface in common. Some use micro-usb, some use mini-usb, some use their own funky plug.
So as a result, you can’t just have one thing that charged all your devices. You have to have a different thing for each device. Your home’s “charging mobile devices module” is dependent on the device. If you change your cell phone, you need a new charger. If you upgrade your camera, you need a new charger.
The dependency is going the wrong way. Lower-level appliances are defining the interface that your home has to use to charge them. Your home’s charging capability should define the interface the devices have to use. The dependency should be inverted. It would make your life a lot easier.
Let’s look at one more example of a place where I would bet dependency inversion is used. Now, I don’t know about Walmart’s IT structure, but I would venture to guess that when they receive invoices from all of their many distributors the invoices come in the file format that Walmart specifies and not the other way around.
I would bet that Walmart specifies the schema for all of the data it receives from its many business partners. Let’s assume they do. In this case they have inverted the dependency, actually they have inverted the control. Instead of their vendors controlling their interface, Walmart controls the vendors through their interface.
What this means is that every time the vendor changes their internal system they have to still conform to Walmarts interface instead of Walmart having to make changes to accommodate each vendors changes to their format.
To be continued
In my http://simpleprogrammer.com/2010/11/16/basic-to-basics-what-is-dependency-inversion-is-it-ioc-part-2/#mce_temp_url# I am going to actually give a code example of dependency inversion, address unit tests and finally answer the question of whether dependency inversion is the same thing as inversion of control.