Object Design Principles - Part 1

Singular Focus and Coupling

This will be the first installment of a multiple part posting on object design principles.  These are vital to keep in mind as we go through creation of large product platforms with many interaction services and objects that must be maintained and enhanced over time.

Object Responsibility

In my mind, the most vital question you can ask yourself at the outset of an object design is, "What is this object's core responsibility?"  Your entire design should flow from that very basic question. You should consistently look for the most granular object definition possible and work back from that.  It's really easy to get blinded by the vastness of the entire project and the complete set of user stories and completely misjudge where to segregate your logic.  Once you have defined the core responsibility and focus of an object, you can immediately start creating an inheritance path as well as supplemental interfaces to bring your object into full functionality.  This is the most vital step in ensuring a scalable and easily maintainable system long term.  Consider the following basic user requirement:  "I need to be able to view images and apply enhancements to them."  The first thing that comes to my mind is that although the user requirement is to always view images, every image view session will not ultimately result in an image enhancement session.  So, I immediately break into two objects, a viewer and an editor : viewer.  At its core, the viewer object should do nothing more than accept image data and render in a viewable state. The viewer should have no concept of any data or state outside of the image itself.  It should interact via an interface with a standardized image format so that it doesn't need to have any system context. 

The system also should have no specific knowledge of the viewer (more on that when we talk about coupling).  Then, the editor class should start by extending the known viewer class.  It should not extend any operation that pertains to the loading and displaying of the base image.  That aspect of responsibility should be a black box to the editor.  The editor should contain an interface for advertising its list of manipulations as well as a standardized message based interface for executing them.  This object is not responsible for a menu or toolbar system, it is simply an image manipulator requiring system input to launch an execution of its internally known action.  It also would not have any context for displaying an image history listing.  It would maintain a list of events, based on its internally known list of manipulations and allow an outside source to interact with that list via a standardized message or interface.

Loosely Coupled Objects

The main point of the granularity of the design listed above is to ensure a loosely coupled framework.  These controls and objects should all be interacted with via their interface over standardized messages.  If each object is singularly purposed and has a black box for its implementation, the only exposure and ultimate point for breakdown is the interface itself.  The loosely coupled system (or systems) interacts with itself based on standard messages or interfaces. The granularity, once released to production, allows for a dynamically upgradeable system with hot swap components.  In this scenario, we could easily swap out our image viewer or image editor in any given system with a very limited impact on the system as a whole. The change would be contained in a singularly purposed object that is loosely coupled with its housing system.  We would be able to quickly respond to a requirement for viewing a new image format without having to make changes to our image editor control.  Additionally, this transforms every control, service or object in every system into a plausible enterprise level base class by never assuming product knowledge or any aspect of external state.

By thoroughly investigating singular focus and coupling as part of every design, we can work toward a much more stable and dynamic system in the future.