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Layers, Levels, and DIP(2) These two dimensions are apparent in the work of Steve Mellor and Gregor Kiczales. Both of them have tried to address this problem in different ways. Gregor created the notion of ASPects, so that infrastructure could be woven into an application, and could "Depend" on that application instead of the app depending on it. Indeed, AOP is a very interesting mechanism for conforming to DIP. Mellor created "translation" which is really just AOP at a higher level of abstraction. He created the infrastructure independent from the application, and then wrote another program called a translator to bind the two together. In both cases the infrastructure is orthogonal to the rest of the app. The dilemma of level, and the two solutions, imply that we aren't thinking about this problem correctly. Infrastructure does not seem to be a layer at all. Somehow it is orthogonal to layers. Somehow it is both at a low level and at a high level. It's at a high level because it represents a policy for using the underlying platform. It's at a low level because it's a detail that most of the app wishes it didn't have to know about. Perhaps the diagram should be drawn like this: UI--->APP--->DOM\----------------------/AINF In other Words, the UI, APP, and DOM are traditional layers, with the dependencies pointing in the direction of abstraction. (Apparently this means the the greater the abstraction, the lower the level. I still don't understand why that is.) And these layers are somehow independent of the infrastructure, and the infrastructure somehow manages to make itself felt whether through AOP, or Translation, or some other mechanism. Without AOP, or Translation, we might draw the diagram this way: UI--->APP--->DOM V VVUIinf APPinf DOMinf\----------------------/AINF In other words, each of the layers uses an abstract interface that represents it's infrastructure needs. It does not know what infrastructure it will be using. It simply states it's needs through an abstract interface and eXPects the infrastructure to implement that interface and supply the required functionality.
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