Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process, Second Edition

23.4. Singleton (GoF)

The ServicesFactory raises another new problem in the design: who creates the factory itself, and how is it accessed?

First, observe that only one instance of the factory is needed within the process. Second, quick reflection suggests that the methods of this factory may need to be called from various places in the code, as different places need access to the adapters for calling on the external services. Thus, there is a visibility problem: how to get visibility to this single ServicesFactory instance?

One solution is pass the ServicesFactory instance around as a parameter to wherever a visibility need is discovered for it, or to initialize the objects that need visibility to it, with a permanent reference. This is possible ...