There is an operator in OCL you may not know about yet: oclSingleton.

Use it like this: Class1.oclSingleton <and follow with operations or navigations on that single instance>

If Class1 does not have an instance at the time of executing the oclSingleton, one is created.

To use this, set the taggedvalue on Class1 “Eco.IsSingleton” to “true”.

It is a very practical design pattern to have a modeled Singleton class – this removes the need for other global variables – like logged-in user, etc. Since the Singleton class can have code-derived attributes and associations, you can pick up important info from the running environment and make it available in the model context. Try it – it will make your code less hairy.

oclSingleton is an operator that helps you implement the Singleton pattern (the pattern states that there is exactly one instance of this class - never more, never less).

If you mark a class with IsSingleton=true, you can use this oclSingleton operator to get access to the only object instance of this class.

Example:

Class: SysSingleton

Common expression: SysSingleton.oclSingleton.CurrentUser.IsAdmin -- this expression looks up the only SysSingleton instance that acts as a Global variable that points out the CurrentUser.