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Dependency Inversion Principle (DIP) Comments

The entry point. Sometimes it’s main(). Sometimes it’s an extension point in some complicated framework. Most importantly, it is the border where Their Stuff runs Your Stuff. When I see entry points in other applications, I don’t see a lot of evidence of having thought about the design much. It becomes a kind of junk drawer for code. This happens especially when programmers are working in environments they don’t feel particularly comfortable in. (I don’t know where to put things, so I’ll put them here, since that seems to work.)

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Dependency Inversion Principle (DIP) Comments

You might have noticed dependency injection gaining popularity in recent years. You might also have noticed some notable figures (I’m thinking of Kevlin Henney) who appear to be bad-mouthing dependency injection, when really they are simply taking away what they feel is a veneer of mystery from the term.1 I must say that I appreciate their doing so without pretentious use of the annoying word “demystifying”. (Simply ignore my own “demystifying” article by most certainly not clicking here). While on a three-hour-long car trip it occurred to me—some would say much too late—that injecting dependencies, particularly through the constructor, acts precisely like partially applying functions. This leads to a couple of potentially useful conclusions:


  1. I consider it categorically unfair to interpret what Kevlin writes or Chris Oldwood writes as “bad-mouthing” the concept of dependency injection, but unfortunately, a careless reading of their work might suggest that in the mind of the careless reader. They mostly object to coining a new term for an old idea, with which I agree. Even so, dependency injection remains known as a term of art, so I just want to roll with it.

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Dependency Inversion Principle (DIP) Comments

This article describes an evolutionary design microtechnique. Specifically, it describes a weak signal that I use to guide myself towards more modular designs. I find it both simple and surprising: simple because it involves using a single value, but surprising because it involves a value that programmers largely recommend against using. I refer to the humble null.

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Comments

We recently ran a legacy code retreat based on your format. People were questioning the value of subclass-to-test in a real world scenario. The best reason I could come up with for myself was that it’s a smaller step to separating out responsibilities from a class.

How would you frame the value of subclass-to-test, and in which contexts would you use it?

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When clients ask me to help them with legacy code it generally takes less than 30 minutes for me to run into a debilitating constructor—a term I use to describe a constructor that does too much. You might think me melodramatic for calling it “debilitating”, but these constructors not only slow us down when we try to understand the code, but block our every effort to improve it. These constructors often hardwire a dependency either to some horrifying external resource (a database or a web service end point) or into a framework (even something as “simple” as threads or a lifecycle manager). These constructors also often grow to dozens, and in some extreme cases hundreds, of lines. These constructors kill code bases and, by extension, suck the life out of programmers.

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For years I’ve written about contract tests, and most notably have never had any clear examples of them to share with you. I’d like to change that today.

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Design credit: Shashank Mehta