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Poor Man’s DIP

Sometimes a lower-layer component needs to invoke a service on a higher-layer component. Consider, for example, a timer component (T) that periodically calls a handler function in a user-interface component (U). Component T is probably part of the OS kernel and thus clearly “lower” than component U.

In this setting, there is an upward dependency from T to U; such upward dependencies are undesirable, at least if they are bound at compile-time. Implemented naively, there is a hard-coded call to the UI component like this:

Dependency lines that point up in a component diagram are not just ugly: they denote that the lower-layer component cannot be independently reused and tested.

The classic dependency inversion principle (DIP) is usually applied to solve this problem: instead of having a hard-coded function call in the timer to the handling component, the timer calls back on a function pointer that is set to the timer-handling routine in the initialization code of the higher-layer component:

Note that there is still a T to U dependency, but now this dependency is only present at run-time, which is OK, as this doesn’t hinder reuse and testability. The U to T compile-time dependency is quite natural and doesn’t violate any design principles. So, the undesirable compile-time dependency has been successfully inverted. The classic DIP recipe looks like this:

1. In T export a callback interface
2. In U implement the callback interface
3. In U (or some init/startup code) register the implementation with T
4. In T call back on the interface

When you are working in a constrained environment like embedded systems, you often cannot afford the memory and performance overhead that accompanies such late (run-time) binding, so you might try what I call the “Poor Man’s DIP”: simply export a “callback interface” as a function prototype and “implement” it by defining the function in the upper-layer component:

This pattern gives you most of the advantages of the classical (run-time bound) DIP but doesn’t incur any overhead. It can (and should) be applied whenever there is a dependency from a lower-layer component to an upper-layer component that doesn’t need to change at run-time but stays fixed throughout the lifetime of the application.