Improved Testability Leads to Better Design
Economic Testability is a simple concept but infrequently seen in practice. Essentially it boils down to recognizing that since code-testing should be a requirement and that we have in place some nice, economical, standard tools for testing (such as xUnit, Fit, etc.), then the products that we build should always satisfy the requirement of ease-of-test in addition to any other requirements. Happily the ease-of-test requirement reinforces rather than contradicts best practice.
Progressive testing is often stymied because certain necessary functions have not yet been implemented. For example, an object needs to be tested but it makes use of a yet-to-be-built object is a pretty common occurrence. A way around this is to allow the provision of the yet-to-be-built object via a parametrized constructor: When testing we can provide a test double object without changing the internals at all; in the production code we can provide the real (tested) object to deliver the required functionality. The API of the test double and the production object are identical and the object under test is unaware of whether it is running in a test or a production environment.
We can go further of course — a lot further. If we need to introduce some kind of logging trail for complex bug diagnosis, we can provide the logging logic via the plug-in technique outlined above, providing the logging object in the constructor parameters. In production, using the null object pattern (which will provide a "do-nothing" object with the same API as the logger), we can replace the logger with a harmless null-alternative. If it is possible that you need to dynamically enable logging in production mode, this technique makes it very simple. Then we have a production system that gracefully slips into logging mode when needed. It may be unfortunate that you should need to do this, but if you do, then you can look like a hero, i.e., a professional!
If you build your systems so that testing is made economic — while simultaneously of course preserving simplicity in the production model — the interfaces that you finally end up with are likely to be greatly improved over the one-environment system. Code which has to operate successfully and unchanged in two or more environments (the test and production environments) must pay more than lip-service to clean interfaces and maximum encapsulation if the task of embedding within the multiple environments is not to become overwhelming. The discipline needed leads to better design and more modular construction. It really is a win–win situation.
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